Keywords: Fast luminosity monitoring using diamond sensors fast luminosity monitoring, diamond sensors, radiative Bhabha scattering, very high luminosity collider

SuperKEKB is a very high luminosity collider dedicated to the Belle II experiment, it consists of a Low Energy Ring (LER) of 4 GeV positrons and a High Energy Ring (HER) of 7 GeV electrons. The commissioning of this machine is split into three phases: phase 1 (single-beam phase) is dedicated to vacuum scrubbing, where beams circulate without focusing at the collision point. Phase 2, for which the major part of the Belle II detector will be installed, will enable the tuning of the final focus system to achieve a luminosity of 10 34 cm -2 s -1 . During phase 3, Belle II physics runs will start with an aimed luminosity up to 8×10 35 cm -2 s -1 .

In this context, the aim of my thesis is to develop and install a fast luminosity monitoring system, which is required for online correction of beam instabilities and maintenance of optimal luminosity. To reach the aimed relative precision of 10 -3 in 1 ms, the measurement will be based on the radiative Bhabha process at zero photon scattering angle, whose cross-section is large and well-known. These particles will be detected using diamond sensors, resistant to radiation and enabling very fast signal acquisition, to be placed outside of the beam-pipe and downstream of the interaction point.

Titre: Monitorage rapide de la luminosité au moyen de capteurs diamant pour SuperKEKB Mots clés: monitorage rapide de la luminosité, capteur diamant, Bhabha radiatif, collisionneur à très haute luminosité.

Résumé: SuperKEKB est un collisionneur à très haute luminosité construit pour l'expérience Belle II, constitué d' un anneau de basse énergie (LER) transportant des positrons de 4 GeV et d'un anneau de haute énergie (HER) où circulent des électrons de 7 GeV. Sa mise en service -ou commissioning-se déroulera en trois phases : La phase 1, durant laquelle des faisceaux circulent sans être focalisés au point de collision, a pour but de nettoyer la chambre à vide du gaz résiduel. La seconde phase, où le détecteur Belle II sera en partie installé, permettra le réglage du système de focalisation finale des faisceaux, jusqu'à atteindre une luminosité de 10 34 cm -2 s -1 . La troisième phase correspondra au démarrage de l'expérience Belle II avec une luminosité visée de 10 35 cm -2 s 1 à 8×10 35 cm -2 s -1 .

Dans ce cadre, ma thèse porte sur la conception et la mise en place d'un système permettant le monitorage rapide de la luminosité, système nécessaire pour pouvoir corriger en temps réel les instabilités des faisceaux et ainsi maintenir une luminosité optimale. Afin d'atteindre la haute précision relative souhaitée, de l'ordre de 10 -3 en 1 ms, la mesure sera basée sur le taux de comptage des particules issues de la diffusion Bhabha radiative à angle nul, processus bien connu et dont la section efficace est importante. Ces particules seront détectées au moyen d'un capteur en diamant, matériau résistant aux radiations et permettant une acquisition très rapide du signal, situé à l'extérieur de la chambre à vide et en aval du point d'interaction.

La première partie de cette thèse est consacrée à la recherche des localisations optimales pour le positionnement des capteurs diamants dans chacun des deux anneaux. Au moyen de simulations détaillées, nous avons étudié la dynamique des particules Bhabha lors de leur transport dans les anneaux ainsi que leur interaction avec la matière de la chambre à vide. Ces études ont permis d' une part d' identifier un emplacement à 11.9m dans le LER et un autre à 30 m dans le HER, et d'autre part de redéfinir pour l'une d' entre elle la géométrie locale du tube à vide.

La seconde partie, plus expérimentale, s'articule autour de la première phase du commissioning de SuperKEKB et des mesures réalisées au moyen des capteurs diamants que nous avons installés. Dans un premier temps, une étude détaillée des processus de perte single beam (Bremsstrahlung, effet Touschek, diffusion coulombienne) a été réalisée pour le LER en fonction des paramètres du faisceau et du collisionneur (courant, pression, taille transverse des faisceaux). Dans un deuxième temps les résultats de cette étude ont été comparés aux données que nous avons prises de février à juin 2016. Nous avons pu mettre en évidence un bon accord qualitatif et quantitatif entre nos simulations et nos mesures. Cela nous a permis d'estimer que le niveau de bruit de fond attendu dans le cadre des mesures pour le monitorage de la luminosité sera de plus de deux ordres de grandeurs inférieurs au taux du processus Bhabha radiatif à angle nul.

The first part of this work is dedicated to the investigation of the best locations for the diamond sensor positioning in both rings. Using detailed simulations, we studied the dynamics of Bhabha particles during their tracking in the rings and their interaction with the beam pipe material. This led to the identification of two positions, at 11.9 m in LER and at 30 m in HER, and to considering a new geometry for the vacuum pipe in the LER.

The second part is related to the phase 1 of the SuperKEKB commissioning and concerns the measurements performed with the diamond sensors that were installed. Single beam loss processes (Bremsstrahlung, Touschek, beamgas Coulomb scattering) were studied in detail with respect to the LER beam and ring parameters (current, pressure, transverse beam sizes). The results of this study were then compared to the data collected from February to June 2016. We found good qualitative and quantitative agreement between our simulations and measurements. From this we could estimate that the level of background to be expected during luminosity monitoring will be two orders of magnitude smaller than the rate of the radiative Bhabha scattering signal.

Introduction:

SuperKEKB est un collisionneur à très haute luminosité construit pour l'expérience Belle II, constitué d'un anneau de basse énergie (LER) transportant des positrons de 4 GeV et d'un anneau de haute énergie (HER) où circulent des électrons de 7 GeV. Sa mise en service -ou commissioningse déroulera en trois phases : La phase 1, durant laquelle des faisceaux circulent sans être focalisés au point de collision, a pour but de nettoyer la chambre à vide du gaz résiduel. La seconde phase, où le détecteur Belle II sera en partie installé, permettra le réglage du système de focalisation finale des faisceaux, jusqu'à atteindre une luminosité de 10 34 cm -2 s -1 . La troisième phase correspondra au démarrage de l'expérience Belle II avec une luminosité visée de 10 35 cm -2 s -1 à 8×10 35 cm -2 s -1 . SuperKEKB a adopté le schéma de collision "nano-beam" qui implique une focalisation très forte au point d'interaction (IP) pour obtenir des faisceaux de très petites tailles verticales. Parce que la taille des faisceaux est de quelques dizaines de nanomètres seulement, la luminosité est très sensible à la stabilité des éléments de focalisation finale dont les vibrations provoquent un mouvement d'orbite qui pourrait être 4 ou 5 fois plus grand que la taille verticale des faisceaux. De telles vibrations provoquent des oscillations verticales du faisceau à l'IP à des fréquences allant jusqu'à 100 Hz, alors que les mouvements horizontaux sont attendus à quelques Hz. Les différences d'orbite des deux faisceaux à l'IP conduisent à une dégradation de la luminosité. Pour parer à cela, un système de «dithering» est dédié à la rétroaction (feedback) horizontale consistant principalement en un lock-in amplificateur, des bobines de dithering et un moniteur rapide de luminosité. Le moniteur rapide de luminosité se compose de deux différents détecteurs, des capteurs monocristallin en diamant (sCVD) utilisés par notre groupe au LAL, et les détecteurs Cherenkov et scintillateur en coïncidence utilisés par groupe Zero Degree Luminosity Monitor (ZDLM) au KEK. Le moniteur de luminosité mesure la luminosité relative à 1 KHz et les corrections de feedback sont appliquées à l'échelle d'une seconde. Les deux détecteurs sont placés l'un à côté de l'autre, juste à l'extérieur du tube à vide dans les deux anneaux de SuperKEKB, et mesureront le signal du processus de Bhabha radiatif à angle nul. La précision relative requise sur la luminosité en phase 3 est de 10 -3 , avec moins de 1% de contamination par des bruits de fond.

Les positions des capteurs dans le LER et le HER:

Le choix des emplacements des capteurs dans chacun des anneaux a nécessité plusieurs étapes, de la génération des particules Bhabha et de leur dynamique (au moyen de GUINEAPIG++, outil de modélisation des interaction faisceau-faisceau) à leur transport dans les anneaux (en utilisant SAD), jusqu'à plusieurs dizaines de mètres en aval de l'IP. Les résultats ont montré que le meilleur emplacement pour positionner nos capteurs dans le LER se situe dans une section droite (drift) à 11.9m en aval de l'IP, le taux de Bhabha perdus y étant suffisant (potentiellement 4.7% de la section efficace totale) ainsi que l'espace libre disponible. L'interaction des particules perdues avec la matière de la chambre à vide a ensuite été simulé au moyen de GEANT4, pour pouvoir étudier et estimer le signal reçu par les capteur diamant. Dans le drift à 11.9m, la chambre à vide est un tube cylindrique de 3 mètres de long et de 6 mm d'épaisseur. Les positrons issus du processus Bhabha sortent de la chambre à vide avec un très petit angle (5 mrad), traversant 1.2 m de matériau, ce qui conduit à une grande absorption de la gerbe électromagnétique à la sortie du tube à vide et n'induit qu'un très faible signal dans le diamant.

Pour augmenter la gerbe électromagnétique et donc le signal dans les capteurs, l'inclusion d'une fenêtre à 45 o a été suggérée par les physiciens de KEK en charge du vide dans la machine. Les simulations de cette fenêtre avec GEANT4 ont montré une amélioration de la précision sur la luminosité d'un facteur 10 par rapport au design initial. Cependant, une telle fenêtre est coûteuse et peut provoquer des champs de sillage (wakefields) et entraîner des instabilités du faisceau. En conséquence, différents types de matériaux (Aluminium, Berilium, Titane ,Cuivre) ont été simulés avec GEANT4 à différentes épaisseurs. Les simulations ont révélé que la fenêtre donne la meilleure précision parmi tous les autres modèles, et les simulations de wakefields effectuées au KEK ont montré des instabilités suffisamment faibles. Il a donc été décidé d'installer la fenêtre dans le drift à 11.9 mètres dans le LER pour la phase 2.

Pour augmenter la gamme de luminosités sur lesquelles des mesures précises peuvent être effectuées, nous souhaitons également utiliser des capteurs diamant (DS) dans le HER. De la même manière que pour les positrons dans le LER, les électrons issus du processus Bhabha ont été générés et leur transport dans le HER simulés jusqu'à 70m. Contrairement à ce qui se passe dans le LER, la déflection des positrons Bhabha le long du HER n'est linéaire avec leur énergie. Les particules perdues sont distribuées de manières stochastiques de part et d'autres de la chambre à vide, et quelque soit le positionnement des capteurs, la fraction des électrons Bhabha interceptée dans le DS est trop faible pour atteindre la précision visée.

Une autre méthode consiste à mesurer le signal des photons issus du processus Bhabha considéré avec le DS dans le HER. Les photons Bhabha sont produits à l'IP à très petits angles, de l'ordre de 0.4 mrad. Du fait de leur neutralité électrique, ils ne sont pas déviés par les champs magnétiques des éléments optiques, et sortent à un endroit précis de l'anneau.

Des simulations détaillées sont actuellement effectuées dans le groupe LAL en considérant l'optique du HER et la géométrie de la chambre à vide. L'analyse préliminaire montre que le spot des photons atteint le front de l'antichambre au milieu d'un sextupôle (à 27.4 mètres en aval de l'IP) à un angle moyen de 5.9 mrad. En raison de leur distribution transverse initiale, le spot projeté des photons sur la chambre s'étalent sur 2 à 3 mètres de part et d'autre du point de sortie nominal situé à 27.4m en aval de l'IP. L'espace libre le plus proche de ce point pour y placer les moniteurs de luminosité se situe à 30m en aval de l'IP. Comme les études en cours le montrent, une fraction importante de la section efficace de Bhabha radiatif peut être détectée à cet endroit.

Les capteurs diamant

Le diamant possède de nombreuses propriétés intéressantes qui lui permettent d'être l'un des meilleurs candidats aux détecteurs de rayonnement à l'état solide. Il a la plus grande conductivité thermique parmi les matériaux solides en plus d'une isolation électrique élevée. Le diamant a une énergie de liaison très élevée qui le rend très insensible aux rayonnements. De plus, le diamant se caractérise par un large écart de bande qui réduit le courant de fuite et diminue ainsi le bruit.

De plus, le diamant dispose de fortes mobilités des électrons et des trous, ce qui se traduit par une collecte rapide des charges et donc par des signaux rapides, dans la gamme de quelques ns. La largeur du signal dépend de l'épaisseur du diamant, par exemple une largeur de signal de 4-5 ns correspond à un diamant de 500 μm d'épaisseur. L'énergie minimale nécessaire pour créer une paire électron-trou dans le diamant est de 13 eV. Un MIP (Minimum Ionizing Particle) créé 36 paires électron-trou par μm dans le diamant. Un MIP est une particule chargée avec un taux de perte d'énergie proche du minimum, et l'énergie d'un électron au MIP est d'environ 1.6 MeV. Le champ électrique de polarisation que nous appliquons pour collecter les paires électron-trou est E = 0.8 V/μm. Normalement, nous pouvons appliquer une tension de polarisation jusqu'à 1 V/μm, cependant, grâce à la très bonne qualité du diamant, l'efficacité de la collecte de charge atteint 100% avant cette tension de polarisation (à ≈ 0,2 V/μm). Pour la sécurité, nous choisissons d'appliquer une tension un peu inférieure à 1 V/μm, soit 0,8 V/μm. Un désavantage du capteur diamant est la difficulté d'avoir une grande zone active, ce qui restreint nos mesures à une partie de la gerbe dans le cas d'une chambre à vide sans fenêtre.

Le fonctionnement et la lecture du DS sont simples. Une particule chargée traverse le DS créant des paires électron-trou en fonction de son énergie. En appliquant une haute tension aux électrodes du diamant, des charges opposées dérivent vers les électrodes. Le signal de sortie résultant de la chute de tension est lue par un oscilloscope. Dans le cas d'un petit signal, par exemple une faible intensité des particules incidentes et/ou des DS très minces , l'amplification du signal est nécessaire. Le dépôt d'énergie dans le DS suit une distribution Landau.

Pour mieux comprendre le comportement du signal dans le DS, nous avons effectué des tests en salle blanche en utilisant des sources radioactives. La configuration expérimentale se compose de la source β 90 Sr, un DS de 140 μm (qui sera utilisé en phase 3 de SuperKEKB), un amplificateur de charge de CIVIDEC dont la constante de temps de mise en forme est d'environ 10 ns, un scintillateur, une alimentation de basse tension de 12V pour l'amplificateur de charge, une alimentation haute tension pour le DS et un oscilloscope pour lire le signal. Étant donné que l'amplificateur de charge est unipolaire, la haute tension (-100 V) sur le DS est appliquée pour polariser négativement et pour obtenir des impulsions négatives à sa sortie.

Un support mécanique spécial a été préparé pour les tests. Le capteur de 140 μm est connecté à l'amplificateur de charge alimenté avec 12 V et les deux sont posés sur le banc central du support mécanique. Sur la partie supérieure, il existe une ouverture spéciale pour les électrons émis par la source radioactive pour atteindre directement la surface 4x4mm 2 du diamant situé en dessous. Juste au-dessous du diamant, un scintillateur est utilisé pour le déclenchement, afin de collecter uniquement les signaux des MIP, c'est-à-dire que le MIP traversera le diamant en déposant une partie de son énergie, sans être absorbé, puis atteindra le scintillateur et donnera un signal de déclenchement. Les données sont collectées pendant généralement 10 heures puis analysées. En plus de la reconstruction de la distribution de Landau du maximum de signal pour les diamants de deux épaisseurs (140m et 500m), nous avons vérifié que la position en temps du MPV (Most Probable Value) du signal ne se déplace pas significativement et ne dépend pas de l'amplitude du signal. Ceci est très important parce que dans nos mesures, nous échantillonnons à 1 GSPS (Giga Sample Per Second), ce qui signifie chaque 1 ns.

La phase 1 du commissioning de SuperKEKB

Lors de la première phase du commissioning du SuperKEKB, où la machine a fonctionné sans système de focalisation finale et sans le détecteur Belle-II, les faisceaux de positrons et d'électrons ont été produits avec succès, accélérés, injectés et stockés dans les anneaux principaux (LER et HER) de SuperKEKB.

Les objectifs principaux de cette phase étaient de démarrer chaque système de hardware de la machine (linac d'injection, système magnétique, système RF dans les deux anneaux, système de vide, ...) et définir un logiciel d'opérations du faisceau basé sur SAD. Au cours de cette période de mise en service, le lavage sous vide a été effectué dans les deux anneaux à différents courants, à partir de courants très faibles (30 mA) jusqu'à des courants élevés (1 A), afin de nettoyer les chambres à vide des gaz résiduels. Le gaz résiduel provoque une perte de faisceau due à la diffusion des particules du faisceau sur les molécules de gaz, soit par Bremsstrahlung (perte d'énergie de la particule en émettant un photon), soit par diffusion Coulombienne (diffusion à grand angle). Le lavage sous vide s'effectue par la circulation des faisceaux dans chaque anneau: le champ électromagnétique des faisceaux interagi avec la matière du tube à vide et en arrache des molécules qui sont ensuite extraites de la chambre à vide par un système de pompes NEG. Le lavage sous vide à des courants élevés (500 mA à 1 A) a été demandé par la collaboration Belle II pour protéger le détecteur des rayons de haute intensité lors des prises de données qui commenceront pendant la phase 2.

Des études spéciales dans les deux anneaux ont été menées dans le cadre de BEAST II, telles que la mesure des bruits de fond issus de la diffusion Touschek, de ceux résultant de l'interaction du faisceau avec le gaz résiduel, des bruits d'injection et les études de collimation. L'étude des Touschek a été réalisée en modifiant la taille verticale des faisceaux, par l'induction d'un couplage entre les émittances horizontale et verticale. Les études du bruit de fond faisceau-gaz ont été effectuées en induisant des «bumps» de vide en amont de l'IP dans des sections particulières des anneaux. Ces «bumps» de vide sont le résultat de la désactivation de quelques pompes NEG à des emplacements sélectionnés. Pour étudier l'effet des collimateurs sur les pertes du faisceau, les paramètres de certains collimateurs (largeur) ont été modifié.

Un autre objectif essentiel de la phase 1 du commissioning était de comprendre les problèmes qui peuvent survenir lors de l'opération à courant élevé (le blow up de la taille verticale du faisceau, l'avortement du faisceau, le comportement non linéaire de la pression avec le courant). Le groupe de mise en service de SuperKEKB a également de nombreuses études optiques.

Notre projet en phase 1:

Pour obtenir la précision visée sur les mesures de luminosité, nos capteurs doivent être placés dans des endroits où la contamination par des bruits de fond émanant du faisceau (bruits de fond singlebeam) est minime (<1%). Par conséquent, l'étude des processus de perte single-beam est également importante pour notre projet.

Les capteurs diamant ont été installés aux emplacements que nous avons déterminés et nous ont permis durant la phase 1, l'acquisition de données permettant l'étude de plusieurs aspects:

• Vérifier le fonctionnement de nos capteurs et l'installation globale (électronique et mécanique) dans des conditions de fonctionnement de la machine.

• Mesurer les bruits de fond aux emplacements des capteurs choisis pour les mesures de luminosité dans les deux anneaux.

• Comparer les données à la simulation, et ainsi vérifier la fiabilité de nos simulations pour la phase 2.

• Une fois la fiabilité de la simulation confirmée, nous pouvons extrapoler nos simulations de bruits de fond à la phase 2 et estimer les fractions de signal de pertes single-beam et du processus de Bhabha radiatif dans nos capteurs.

Les bruits de fonds de Bremsstrahlung, la diffusion Touschek et la diffusion Coulombienne inélastique sont considérés comme des pertes single-beam, ils sont générés dans tout de l'anneau en raison de la diffusion des particules du faisceau sur le gaz résiduel ou de la diffusion des particules du faisceau les unes sur les autres. En plus d'être perdues à l'intérieur de la région d'interaction (IR) et d'engendrer des bruits de fond dans le détecteur, les particules provenant de pertes single-beam sont également perdues dans les anneaux, réduisant ainsi après un certain temps l'intensité et la qualité du faisceau. Dans des anneaux tels que SuperKEKB et en raison des courants très élevés et de leur très petites tailles, les faisceaux ont donc des durées de vie relativement courtes de l'ordre de 10 minutes. Les processus principaux déterminant la durée de vie des faisceaux sont l'effet Touschek et la diffusion Bhabha radiative à angle zéro à l'IP. Les faisceaux devraient donc être continuellement réinjectés pour maintenir leur intensité et donc maintenir une haute luminosité. Au cours de la phase 1 du commissioning de SuperKEKB, sans collision, les pertes principales étaient le Bremsstrahlung, la diffusion Touschek et la diffusion Coulombienne.

La diffusion Bremsstrahlung et les pertes résultantes sont proportionnelles au courant du faisceau et à la pression du gaz résiduel dans la chambre à vide. Pour l'effet Touschek, les pertes sont proportionnelles au carré du courant du faisceau et à l'inverse de la taille du faisceau. Comme pour Bremsstrahlung, la diffusion Coulombienne et les pertes résultantes sont proportionnelles au courant de faisceau et au niveau de vide.

Une version de SAD, écrite au KEK, a été utilisée pour simuler les pertes single-beam dans le LER. Les points de diffusion dans la version standard du code sont à l'entrée des quadrupôles, à l'exception des quadrupôles de focalisation finale pour les phases 2 et 3 et quelques aimants de courbure verticaux spéciaux . Le point d'observation est à l'entrée de chaque élément. Ainsi simulé, le spectre d'énergie des positrons Bremsstrahlung perdus dans le drift à 11.9 m, est discontinu et donc non physique. La raison de cette discontinuité est une résolution insuffisante des points de diffusion, qui ne permet pas d'obtenir une valeur correcte du taux de perte dans le drift.

Des modifications ont été apportées au code initial pour améliorer sa résolution, en ajoutant des points de diffusion à tous les éléments optiques et, pour les éléments longs, tels que les drifts et les aimants de courbure, en les découpant en tranches de longueurs égales. La diffusion des positons a ensuite été considérée à l'entrée de chaque tranche. Les aimants de courbure ont été découpés en petits tranches de ≈ 15 cm chacun, en gardant le même angle global de l'élément d'origine et les drifts ont été découpés en tranches de 10 cm chacune, de 50 mètres en amont à 13 mètres en aval de l'IP. De cette façon, nous avons récupéré tout le spectre d'énergie des positrons de Bremsstrahlung perdus dans le drift et obtenu une meilleure résolution sur les positions de diffusion. À l'aide de cette technique, le taux de perte obtenu dans le drift a été saturé tout en conservant le taux de perte dans l'ensemble de l'anneau: lorsque l'on augmente le nombre de positions de diffusion, nous augmentons la perte localement dans le drift alors que la perte totale globale dans toute l'anneau reste compatible dans les statistiques les erreurs.

Cette technique a ensuite été utilisée pour simuler et calculer les taux de pertes des diffusion de Bremsstrahlung, Touschek et Coulomb dans le drift, pour des pressions de vide, des courants et des tailles verticaux du faisceau différents. GEANT4 a été utilisé pour étudier les signaux de ces pertes dans le DS.

Pour pouvoir mesurer les pertes single-beam en phase 1, une installation expérimentale complète a été préparée au LAL et installée sur les deux anneaux de SuperKEKB, comprenant les supports mécaniques, les capteurs diamant et le système d'acquisition du signal.

Les supports mécaniques se composent de piliers dans chaque anneau. Chaque pilier prend en charge une plaque mobile avec deux capteurs diamant connectés chacun à un amplificateur de charge CIVIDEC et une plaque fixe avec les moniteurs ZDLM. La plaque mobile est associée à un moteur commandé à distance pour scanner dans la direction verticale sur une plage de 2.5 cm. Dans le HER, le pilier est installé à 30 mètres en aval de l'IP et les capteurs sont en face de l'antichambre dans le drift. Le pilier dans le LER est installé à 11.9 mètres en aval de l'IP et les capteurs sont en face à un tube de vide cylindrique normal dans le drift. En tout quatre capteurs diamant de 500 m ont donc été installés pour la phase 1 du commissioning.

Deux procédures d'acquisition de signaux ont été implémentées, soit par des enregistrements continus de 10 ms pouvant être traités à la volée toutes les 5-6 secondes soit jusqu'à une cinquantaine d'enregistrements de 410 ms dont le traitement a lieu en différé et dure quelques heures. Les données brutes acquises par un oscilloscope Keysight ont ensuite été converties en fichiers ".mat" et traitées avec MATLAB. Les fonctions de traitement ont été construites pour rechercher les formes d'onde des signaux des DS. Une fois les formes d'onde définies, les maxima et leurs positions dans le temps ont été extraites, ainsi que le FWHM de chaque forme d'onde. De plus, nous avons créé notre propre archive EPICS pour sauvegarder les enregistrements des paramètres de la machine (comme le courant, la pression, la taille du faisceau, l'heure ...) et nous avons fusionné les données du DS avec les données EPICS après chaque acquisition.

Pendant la phase 1 du commissioning, nous avons pris des données principalement pendant les phases de nettoyage sous vide dans les deux anneaux et pendant des études de bumps de vide et les variations de la taille du faisceau dans le LER.

Les données prises pendant des bumps de vide ont été utilisées pour calculer la fraction des pertes liées au Bremsstrahlung dans des sections où les bumps ont été effectués et pour les comparer à des simulations. Les comparaisons ont montré que les données correspondent aux simulations au erreurs statistiques près.

Une étude de Touschek a été lors de l'augmentation de la taille verticale du faisceau. Les données ont été collectées pour six courants de faisceau différents dans le LER, I = 180, 360, 540, 720, 900 et 1000 mA et avec cinq taille de faisceau à chaque fois. Leur analyse a permis d'extraire les contributions de Bremsstrahlung et de Touschek. L'ajustement des pertes dans le diamant en fonction de l'inverse de la taille vertical du faisceau (σ y -1 ) permet de séparer les deux contributions de la manière suivante. Pour σ y -1 proche de zéro, ce qui signifie que σ y est très important, les pertes de Touschek devraient disparaître et donc toutes les pertes dans le DS devraient être issues du Bremsstrahlung.

Les pertes de Bremsstrahlung extraites des ajustements ont été comparées aux simulations en fonction du produit de la pression de vide et du courant du faisceau. Les résultats ont montré que les données sont toujours supérieures d'un facteur à ceux prédits par la simulation. Ce facteur s'explique par le fait que la valeur de la pression mesurée par le CCG (Cold Cathode Gauge) devrait être multipliée par un facteur 3 pour représenter correctement la pression réelle de CO au centre de la chambre à vide.

Les pertes de Touschek extraites des ajustements ont également été comparées aux simulations. Nous observons un facteur de simulation/données = 1.7 ± 0.37. Nous avons étudié les raisons possibles de cette écart, et exclu des erreurs possibles concernant l'estimation des tailles longitudinale et verticale du faisceau, cohérente avec la valeur estimée à l'aide de la modélisation basée sur les mesures des fonctions optiques dans le LER. Une raison plausible pourrait être l'emittance horizontal, mais malheureusement il n'existe pas de mesures fiables de la taille horizontale du faisceau à SuperKEKB pour vérifier cette hypothèse. Nous ne pouvons pas non plus exclure complètement qu'une partie de cet écart vient de la simulation elle-même.

Le bon accord qualitatif et quantitatif entre nos simulations et les mesures nous a permis d'estimer le niveau de bruit de fond dans le DS pour la phase 2. Les estimations ont montré que le niveau de bruit de fond résultant des pertes d'un seul faisceau devrait être d'environ deux ordres de grandeur inférieur au taux du signal de Bhabha radiatif pour la phase 2. Cela satisfait les conditions spécifiées pour obtenir une haute précision relative sur la mesure de luminosité.

Conclusion:

Le travail de ma thèse a été utile pour trouver les meilleurs positions pour les moniteurs de luminosité dans les deux anneaux de SuperKEKB afin d'atteindre la précision visée sur la mesure de la luminosité. En outre, les simulations effectuées pour les pertes single-beam sont cohérentes avec les données prises au cours de la phase 1, ce qui nous a permis d'estimer le rapport entre le signal et le bruit de fond dans le DS attendu en phase 2, compatible avec la précision visée.

To my parents and siblings first … And to every marginalized and oppressed person in this world !

ل ل و و أ وإخوتي وأبي مي و أ إلى ! الرض هذه على مش و ومه مظلوم ل و ك وإلى

Many thanks to the LAL director, Achille Stocchi, for hosting me almost 3 years and a half and for giving me the chance to work on this very interesting project. I would like as well to deeply thank the referees Jerome Baudot and Jean Marie De Conto for reading my thesis report and for giving nice comments on my work. Many thanks to Yoshihiro Funakoshi, Uehara Sadaharu and Manuela Boscolo for being the members of my thesis committee.

I would like to enormously thank my supervisors Philip Bambade and Cécile Rimbault for every single advice and for every information, discussion and support they gave me. I thank them for believing in me and my work and for their trust. Thank you for giving me the chance to participate on very important conferences and summer schools and thus acquiring a lot of experience in life.

Many thanks to every person in our group who without their help in every manner, I couldn't have achieved what I have today. Thanks to Didier Jehanno, Viacheslav Kubytskyi, Yann Peinaud, Patrick Cornebise, Alex Blin and Frederic Bogard.

Part of this work has been funded by the P2IO LabEx (ANR-10-LABX-0038) in the framework "Investissements d'Avenir" (ANR-11-IDEX-0003-01) managed by the French National Research Agency (ANR), so many thanks to P2IO, as well many thanks to the Ecole doctorale PHENIICS for funding the other part, many thanks to Fabien Cavalier and Elias Khan.

Many thanks as well to all the people of the administrative service at LAL for helping me in all the administrative issues related to my thesis.

I will not forget the great support I got from Japanese Scientists, I would like to enormously thank Funakoshi san for giving me the great chance as a student to participate on the SuperKEKB commissioning and for letting me participate on operation shifts, as well for the very interesting discussions and for replying on all my questions. Thank you for believing in my abilities and inviting me to return to KEK to work on SuperKEKB, I believe I will always be in your debt. Many thanks to Uehara san for the nice collaboration and for the very interesting discussions. Many thanks to Ohnishi san for your code and for your enormous help. Sincere thanks to Iida san, Muira san, Ikeda san, Flannagan san, Masuzawa san and Koiso san.

In addition, I would like to thank every single person in the accelerator department for the nice moments we shared in the lab. Special thanks to Robert Chehab, Walid Kaabi, Hayg Guler, Cynthia Vallerand, Iryna Chaikovska, Mathilde Court, Mohamed El Khaldi, Cristelle Bruni, Sophie Chancé, Nicolas Delerue, Noureddine El Kamchi and Jean-Noel Cayla. Special thanks also to Marica Biagini for all the nice moments and for the physics and "non-physics" discussions.

Very special thanks to Catherine Bourge for being the big sister in everything and for supporting me during every nostalgic moment during more than 4 years. Thanks to Dominique Bony for the nice lunch moments together as well.

Many thanks to all my professors at the Master 2 "NPAC" and for all my professors at the Lebanese university. Thanks to the first teachers who inspired me and guided me on the way of success, thanks to my teachers at "Asaad Abboud school". Many thanks as well to my teachers at "Sohmor high school".

Far from the office and the laboratory, special people were surrounding me and giving me a positive energy everyday to start. I cannot forget the memories we shared, the laughters which filled our small houses and the dances and "dabki" we performed which brought joy to my heart. Special, kind and lovely thanks to Fatima Al Reda, Marwa Zaarour, Marwa Koumaiha, Zeinab Koumaiha, Mohammad Tarhini, Mohamad Ayoub, Elie Saikaly, Mostafa Al Reda and Mayla Salman.

Infinite thanks to the one who considered me and still considering me as his daughter, for the one who taught me a lot of valuable things in life, who guided me while far from all the people I love, thank you Tokio Ohska, thank you my Godfather, I love you. Sweet thanks as well to my Classical singing teacher, for the lovely Keiko Nishimura.

For the people who were my family in Japan and will always be, thank you for being you and for every moment we spent together and for every support. Thank you Umberto Tamponi, Monica Frattari, Maryam Salihi, Hulya Hatmacan, Manca Mrvar, Bianca Scavino, Martin Ritter and Torben Ferber.

Many special thanks to the amazing friends Nuria Fuster Martinez, Thomas Vinatier, Shan Liu, Ping Chen, Renjun Yang, Chengguo Pang, Luca Garolfi, Sarah Bira, Alexis Gamelin and Hadda Rachidi for the nice moments and for every support.

Thank you Georges Audi and Ghislaine Audi for the nice moments we shared. Special thanks to my aunt Inaam and her family and to my cousin Soha Salam and her husband Rayan Abdallah for being beside me in the very hard moments at the beginning of my thesis. Thank you my cousin Imad for the support during these last years.

Infinite, enormous and lovely thanks to the people who considered me like their daughter and hosted me in their house for several special family gatherings, to the people who I owe a lot, to the kindest Bernard and Isabelle Thomas and their nice family.

To the piece of my heart, to my soul, to every spot of life in me, to my "Gods", my parents Tarek and Fatima, you are everything and will always be. If I sacrifice my life, it will not be enough. I worship you and without you I have had never done any single step. To my sister Samah, you are the sunshine of my life, thank you for always being by my side. To my adorable brothers Maher and Ziad, you are my strength and you are my guards. To my adorable family, I say, I will always be me and I will always make you proud of me.

Introduction

Accelerators for high energy physics

We live in a universe that behaves under well defined laws that may include some exceptions. It is very important to understand our universe, study about its constituents, its history and to predict its future. Scientists around the world participated on the foundation and development of the Standard Model (SM), the theory which concerns the electromagnetic, weak, and strong nuclear interactions, as well as the classification of all known subatomic particles. The SM gained its credibility by the confirmation of the existence of the quarks, and afterwards by the discovery of all its particles, in particular, the top quark [START_REF] Abe | Observation of Top Quark Production in pp Collisions with the Collider Detector at Fermilab[END_REF] at the CDF (Collider Detector at Fermilab) and Dφ at Tevatron (1995), the tau neutrino at DONUT collaboration (Direct Observation of the NU Tau) at Fermilab (2000) [START_REF] Patzak | First direct observation of the tau neutrino[END_REF] and the Higgs Boson at ATLAS (A Toroidal LHC ApparatuS) [START_REF]A Particle Consistent with the Higgs Boson Observed with the ATLAS Detector at the Large Hadron Collider[END_REF] and CMS (Compact Muon Solenoid) [4] at LHC (Large Hadron Collider) at CERN (2012). However, the SM doesn't explain some phenomena like the gravitation theory, the neutrino oscillation, the expansion of the universe by dark matter, and questions about matterantimatter asymmetry. In addition, still a lot of information on the origin of the Higgs mechanism and its properties are missing. So, more investigation on the Higgs boson and physics expected beyond the Standard model needs to be done. This requires upgrades of already existing accelerators like the LHC and the construction of new accelerators with high luminosities and energies.

Accelerators for high energy physics are mainly of two kinds: circular and linear colliders. These colliders are called hadron colliders if they collide hadrons like protons, and are called lepton colliders if they collide electrons and positrons. The advantage of circular colliders in general is that the beams can be reused for several turns, however in the linear colliders beams collide once and are then dumped. In hadron circular colliders, very high energies can be reached by improving high field super conductive magnets to bend the proton beams for a given ring radius, however the beam energies in lepton circular colliders (e + e -circular collider) are limited by synchrotron radiation due to the small electron mass. Thus lepton linear colliders are considered to achieve higher energies by extending the length of the accelerating lines. Lepton colliders in general provide a clean environment of interactions at the Interaction Point (IP), which helps the physicists to perform cleaner analysis of the produced events compared to the huge hadronic showers produced from the interaction of hadrons. As well, the initial state of the interacting particles is generally well known in lepton colliders, for instance the knowledge of the energy and momentum, and also the spin in the case of linear colliders, can be used to constrain the reconstruction of the final state, taking into account the conservation laws governing the physics process under study.

The HL-LHC (High Luminosity Large Hadron Collider) aims at a peak luminosity up to 5×10 34 cm -2 s -1 and 14 TeV center of mass energy by reducing the beam sizes and upgrading the injector. The FCC-ee (e + e -Future Circular Collider) [5] is planned in a 100 km tunnel to be constructed in the Geneva area. It aims at a very high center of mass energy of 400 GeV. A 100 TeV FCC-pp (proton proton collider) with a peak luminosity of 5×10 34 cm -2 s -1 . The ILC is a 30 km long linear accelerator which aims to collide electrons against positrons at a luminosity of 10 34 cm -2 s -1 , with a 500 GeV up to 1 TeV center of mass energy. The CEPC [6] (Circular Electron Positron Collider) is a future circular e + e -collider ring of 50 Km circumference planned in China with a luminosity of 1.8×10 34 cm -2 s -1 with beam energies of 120 GeV, aiming at a clean study of the Higgs boson (125 GeV). The CEPC can later be upgraded to a proton-proton collider enabling to achieve very high energies and discover new physics beyond the Higgs. CLIC (Compact LInear Collider) [7] is a future e + e -linear collider project based in CERN, aiming at a nominal luminosity of 6×10 34 cm -2 s -1 by squeezing the beam sizes to 40 nm horizontally and 1 nm vertically, with center of mass energy up to 3 TeV.

High luminosity colliders

To achieve high precisions on the physics detected from particle collisions and to be able to see new particles, two very important parameters need to be optimised, the energy and the luminosity. High energies can be achieved by huge rings and long linear colliders (for lepton colliders). For future hadron colliders, the challenge is the development of very high field superconducting magnets. High luminosities require in general very small spot sizes at the IP and high currents.

SuperKEKB is an asymmetric B meson factory which aims to study the CP violation in the B meson sector and to search for new physics. It will collide electrons and positrons at a center of mass energy of 11 GeV, and it will achieve the highest ever luminosity in the history of high energy accelerators (L= 8×10 35 cm -2 s -1 ). This very high luminosity will be achieved thanks to a new scheme called the nano-beam scheme, where very strongly focused beams (σ y = 60 nm) collide at a large crossing angle of 83 mrad. The beam currents will reach 3.6 A in the positron ring and 2.6 A in the electron ring.

To be able to keep a very high luminosity operation, a dithering system is employed 1.2. High luminosity colliders to control the horizontal orbit of the beams at the IP. The dithering system consists of a set of magnets which will kick the beam at a frequency of 77 Hz. To monitor the effect of such kicks on the alignment of the beams and thus on the luminosity, a fast luminosity monitoring is required. The fast luminosity monitor is essential in the presence of dynamic instabilities, for feedback and optimisation.

At SuperKEKB, we perform fast luminosity measurements based on two technologies, the sCVD diamond sensors with our group at LAL and Cherenkov and Scintillator detectors with the ZDLM (Zero Degree Luminosity Monitor) group at KEK. We aim at a relative precision of 10 -3 in 1 ms and we measure the signals from the Bhabha process at zero photon scattering angle, which has a large cross section at the IP about 200 mbarn. The sensors are placed in both rings just outside the beam pipe, several meters downstream of the IP. For high quality measurements, the signals in our sensors should not be contaminated by background signals from single beam losses. As a result, detailed simulations of single beam losses had to be performed as well, optimising the sensitivity of our devices to the Bhabha process.

We successfully installed our diamond sensors in both rings and took data during the single beam commissioning phase of SuperKEKB. A quantitative and qualitative agreement between the measurements and simulations was observed. This validated our simulation, giving credibility to our predictions for the levels of background and Bhabha signals expected in our sensors during the second and third phases of the SuperKEKB commissioning.

This thesis is organised in seven chapters including the introduction:

Chapter 2 defines the SuperKEKB project, its design, goals and different parts including the Belle II detector. Chapter 3 defines the fast luminosity monitoring project, and describes the diamond sensors used and shows the results of characterisation tests performed in the clean room at LAL. As well, it describes in details the simulations performed to find optimal positions of the sensors in both rings and the results concerning the design of the vacuum chamber for phase 2. Chapter 4 describes the different commissioning phases of SuperKEKB with their goals, and describes some phenomena and studies performed during the single beam commissioning. Chapter 5 defines the processes of single beam losses at SuperKEKB and describes the simulations of these processes performed in phase 1. Chapter 6 describes the overall mechanical and electronic setup of our project during phase 1 with the data acquisition and analysis, and compares the simulation results to the measurements.

Chapter 1. Introduction

Chapter 7 summarizes all the results and discusses the future prospects.

Chapter 2

The SuperKEKB collider

Design and goals

The KEKB electron-positron collider built at KEK, in Japan, achieved the world's luminosity record (more than 2.11×10 34 cm -2 s -1 ) in 2009. The analysis of the large amount of data taken by the Belle detector (1.5 ab -1 ) from the collisions of the positron and electron beams has allowed the Kobayashi-Maskawa theory to be verified by observing the CP (Charge Parity) asymmetry in B mesons. The Kobayashi-Maskawa theory highlights differences between matter and anti-matter. At the beginning of the universe, equal quantities of matter and anti-matter were present, while almost no anti-matter is presently found in nature. This indicates that there are fundamental differences in the behaviour of matter and anti-matter. The Kobayashi-Maskawa theory alone cannot completely explain the deficit of anti-matter: More data and further research are needed. In this context, KEKB and the Belle experiment were upgraded to deliver higher luminosity and improve detection capabilities.

The upgraded machine, called SuperKEKB [START_REF] Ohnishi | Accelerator design at SuperKEKB[END_REF], consists of a 4 GeV positron ring and a 7 GeV electron ring and aims to reach a luminosity 40 times higher than that of KEKB (8x10 35 cm -2 s -1 ). It will be the highest luminosity particle collider that ever existed (fig. 2.1). It is an asymmetric collider with a center of mass energy equal to the Υ(4S) resonance energy to produce B meson pairs. With the asymmetric energies of the electron and positron beams, the B meson pairs are created with a Lorentz boost βγ of 0.425 which allows the measurement of the decay time of the B meson by measuring its travelled distance from the interaction point (IP). Belle II [9] is the upgrade experiment and it expects to start data taking in 2018. SuperKEKB will reuse some components of KEKB, while other components need to be either modified or developed. The accelerator design of SuperKEKB (fig. 2.2) consists of a Photocathode RF-Gun to generate an electron beam, a positron source to generate a positron beam from the incident electron beam, a linear accelerator for both beams, a positron damping ring, an injector linac which will inject the electron beam in the high energy ring (HER) and the positron beam in the low energy ring (LER).

SuperKEKB will be able to achieve a very high luminosity by increasing the beam currents (twice higher than at KEKB) and by using the nano-beam scheme which allows very strong focusing at the IP, to achieve very small vertical beam sizes (σ y = 60 nm). The table 2.1 shows the comparison of some parameters between KEKB and SuperKEB. The very strong focusing at SuperKEKB will be achieved thanks to a final focus system (FF) [START_REF] Ohuchi | Design of the superconducting magnet system for the SuperKEKB interaction region[END_REF] which includes superconducting magnets near the Interaction Point (IP) for each beam. The system of magnets is split into two cryostat boxes (fig. 2.3), and the closest magnets (in the LER) are at ± 935 mm from the IP.

SuperKEKB started its single beam commissioning in February 2016 and completed it in June 2016 (see chapter 4) during which both electron and positron beams were successfully injected and stored without collisions. During this commissioning, tuning of optics elements and parameters was performed and single beam background losses were measured by the BEAST II (see section 4) detectors and by the fast luminosity monitors (see chapter 3 ) is designed with a very high focusing RF field to control the space charge effect from high beam charge. These high focusing fields are generated by seven QTW-SIDE cavities (quasi-travelling wave) [START_REF] Natsui | Quasi-travelling wave side coupled RF gun commissioning for SuperKEKB[END_REF] which are characterised by narrow gaps and responsible of keeping the beam size.

The choice of the photocathode material is essential to produce high charge beams for a long-term operation like that at SuperKEKB. The photocathode which will be used at SuperKEKB is an Iridium Cerium cathode (r.h.s of fig. 2.4) thanks to its reasonably high QE (Quantum Efficiency=1.54×10 -4 at 266 nm) and long lifetime.

To achieve high repetition rates of a double-electron bunch at SuperKEKB (50 Hz), a solid state laser system based on Neodymium-doped (Nd:YAG) and Ytterbium-doped (Yb:YAG) crystals [START_REF] Zhou | Neodymium and Ytterbium hybrid solid laser of RF gun for Su-perKEKB[END_REF] with emission wavelength of 1064 nm and 1030 nm respectively, is employed to prevent the thermal increase due to the increase of the source pump repetition rate.

On the other hand, the 10 nC primary beam used to produce the positron beam is generated by a thermionic gun, the beam line of which is merged to that of the RF gun by two vertical bends [START_REF] Satoh | Commissioning status of SuperKEKB injector linac[END_REF].

Positron source

The positron source upgrade [START_REF] Kamitani | SuperKEKB positron source construction and status[END_REF] at SuperKEKB allows the increase of the positron bunch intensity from 1 nC to 4 nC and the achievement of very small beam emittance by introducing a damping ring. The 3.5 GeV accelerated double-bunch electron beam (10 nC each) hits a 14 mm thick amorphous tungsten target and produces positrons by a cascade shower. To protect the target from very high energy deposition due to the high intensity beams with very small spot sizes (0.4 mm), a beam spoiler [START_REF] Zang | SuperKEKB positron source target protection scheme[END_REF][17] is introduced 3 m before the target to increase the beam sizes up to 0.7 mm.

After being produced, the positrons are focused by a flux concentrator [START_REF] Zang | Design optimization of flux concentrator for SuperKEKB[END_REF] which is After the target, both secondary electrons and positrons are produced. After the LAS, the electron bunch is separated from the positron bunch by 175 ps due to the phase slippage. To be able to get pure positron beam, an electron/positron separator chicane is introduced after the LAS where secondary electrons are absorbed by a 60 mm thick tungsten-copper alloy block and positron beam continues.

Damping ring

As a result of very high electron/positron beam currents (2.6A/3.6A) at SuperKEKB and the adoption of the nano beam scheme which results in very small vertical beam sizes (60 nm), the Touschek lifetime in the LER becomes very short (≈ 600 sec). In order to maintain the same beam current, a bunch intensity of about 8 nC needs to be continuously injected at a repetition frequency of 25 Hz. However, the generated and accelerated positron beam has a relatively large emittance and energy dispersion, larger than the aperture of the LER. Thus a damping ring (DR) [START_REF] Kikuchi | Design of positron damping ring for Super-KEKB[END_REF] is needed to reduce the emittance before injection.

The DR has a circumference of ≈135 meters and a physical aperture of 34 mm. A pre-accelerated 1.1 GeV positron beam (emittance=1.7µm.rad) is injected to the DR passing through a transport line LTR (Linac To Ring) which contains an ECS (Energy Compression System) to compress the energy spread from 1.67% to 0.5%, to be within the energy acceptance of the DR. To achieve a short damping time, an optic design called "Reverse-bend FODO" [START_REF] Kikuchi | Reverse-bend FODO lattice applied to damping ring for SuperKEKB[END_REF] is used with relatively low field. The physical transverse emittances at the exit of the DR are ε x =41.4 nm and ε y =2.07 nm and the energy spread is 5.5 × 10 -4 . After being damped, the positron beam exits the damping ring via the RTL (Ring To Linac) where its bunch length is compressed by the BCS (Bunch Compression system). After that, the positron beam is accelerated from 1.1 GeV to 4 GeV and reaches the injection point with horizontal emittance of 11.8 nm and vertical emittance of 0.86 nm. The layout of the damping ring with the LTR and RTL is represented in fig. 2.7.

Injection

The injector linac [START_REF] Satoh | Commissioning status of SuperKEKB injector linac[END_REF] (fig. 2.8) is 600 meters long, it starts from the RF photocathode, passing by the positron source and ending up at the injection point. This linac provides the positron beams for the LER (Low Energy Ring) and the electron beams for the HER (High Energy Ring) of SuperKEKB, and the PF-AR (Photon Factory Advanced Ring) and PF (Photon Factory) electron rings. In order to maintain a constant luminosity at SuperKEKB, simultaneous injections are needed among the mentioned rings. Consequently, SuperKEKB uses a continuous injection "top-up injection". The multi-turn injection scheme adopted at SuperKEKB, consists of a septum magnet where an orbit bump in the horizontal plane is provided by two kickers. During injection, two kicker units move the circulating beam to the septum and the injected beam is steered to minimize the coherent oscillation. The injected beam performs betatron oscillations around the circulating beam. These betatron oscillations are damped by synchrotron radiation and the bunch-by-bunch feedback system. This is called betatron phase space injection. The transverse damping time is 43 ms for LER and 58 ms for HER. The maximum injection rate is 25 Hz during continuous injection.

Since the injection is continuous and simultaneous among four rings, the injection ring needs to be changed frequently and the RF injection bucket needs to be controlled [START_REF] Kaji | Upgrade of event timing system at SuperKEKB[END_REF]. At SuperKEKB, the positron injection process cannot be finished before the next pulse is launched since the positron beam is damped for 40 ms before injection. Thus two processes go in parallel, the electron beam is injected while the positron beam is damped.

Concerning the bucket selection, a system that controls the injection into RF buckets to satisfy the requested fill pattern is controlled by the machine operator. This control is accomplished by introducing a delay time to the linac. Normally, the linac is synchronised to the revolution signal (the signal of one turn inside one ring, at SuperKEKB the frequency of one revolution is about 100 KHz).The delay time introduced to the linac corresponds to the desired time position of the injector RF bucket.

Vacuum system for both rings

In the upgrade from KEKB to SuperKEKB, there were many challenges in relation to the upgrade of the vacuum system [START_REF] Suetsugu | Present Status of the SuperKEKB Vacuum System[END_REF], trying to use as much as possible the systems of KEKB. At SuperKEKB, 93% of the beam pipes are renewed for the LER and 82% of the beam pipes are reused for the HER. The main goal of the vacuum system is to achieve ultra-high vacuum (P≈ 10 -7 Pa with beams) in order to maintain small beam emittance and reduce the background noise in the Belle II detector due to the interaction of beam particles with the remaining gas molecules (for example: beam-gas Bremsstrahlung and Coulomb scattering).

In the presence of very high currents, the electron cloud effect [START_REF] Susaki | Electron cloud instability in SuperKEKB Low Energy Ring[END_REF][26] (see chapter 4) becomes very important in the LER and induces instabilities for the beam. In addition, the beam pipes are heated intensively due to the strong synchrotron radiation (SR). To cope with these effects, new beam pipes with antechambers were installed in the LER (fig. 2.9). By putting the NEG pumps and SR mask in the antechambers, the impedance of the beam pipe is reduced. As a result, new flanges and bellows were developed for these antechambers. To reduce the effect from the electron cloud, different measures were considered like roughening the antechamber wall, introducing solenoid fields and Tin coatings [START_REF] Shibata | SuperKEKB Vacuum System[END_REF].

On the other hand, the energy reduction from 8 GeV to 7 GeV from KEKB to Su-perKEKB for the electron beam makes the SR power tolerable with normal copper beam pipes. Consequently, the beam pipes from KEKB were reused in SuperKEKB, except in the wiggler section where the SR is stronger and where the normal copper beam pipes were replaced by beam pipes with antechambers.

Figure 2.9 -The concept of antechamber on l.h.s and the antechambers for wigglers on r.h.s at SuperKEKB

Nano-beam scheme

The luminosity L can be represented in equation 2.1, where N 1 and N 2 are the number of particles in the two colliding bunches, f is the repetition frequency (colliding frequency), and σ x and σ x are the horizontal and vertical beam sizes respectively. This formula shows clearly that the luminosity is proportional to the beam currents and inversely proportional to the transverse beam sizes. Thus a high luminosity needs high beam currents and small beam sizes. The design luminosity at SuperKEKB is 8×10 35 cm -2 s -1 and the machine parameters are shown in the Table . 2.2.

L = f N 1 N 2 4πσ x σ y (2.1)
To achieve very small vertical beam size (σ y = β y ε y ), the vertical β function must be Parameter LER HER N 3.6×10 7 2.6×10 7 σ x (µm) 10 11 σ * y (µm) 0.94 0.059 Table 2.2 -The machine parameters for a very high luminosity at SuperKEKB strongly squeezed at the IP. But in the presence of relatively long beam sizes (5 mm and 6 mm), the hourglass effect [START_REF] Venturini | Hourglass effect and the measurement of the transverse size of colliding beams by luminosity scans[END_REF] is an important limitation. It leads to the loss of luminosity for β y ≤ σ z . In order to prevent the hourglass effect, beams are set to collide at a large Piwinski angle with very small spot sizes. The Piwinski angle is defined in eq. 2.2, where θ c is the total crossing angle, σ x and σ z are the horizontal beam size and the bunch length respectively. By colliding the two beams at an angle, the effective bunch length, representing the length of the overlap region, becomes much shorter. To satisfy the hourglass requirement (eq. 2.

3) where φ is the half crossing angle i.e to be able to reduce the vertical β function, the horizontal spot size should be small and the crossing angle should be large. This is called the nano beam scheme (fig. 2.10) which was originally proposed in the context of the design of the SuperB collider [START_REF]SuperB Technical Design Report[END_REF]. SuperB was a high luminosity e -e + collider aiming at the study of B meson physics, like SuperKEKB.

It was supposed to be built near Rome, but was cancelled by the Italian government in 2012. The nano beam scheme is adopted at SuperKEKB where the vertical β functions will be ultra squeezed with superconducting focusing magnet systems at the IP, to attain vertical beam sizes of 60 nm, and the beams will collide at a large crossing angle of 83 mrad.

θ p = θ c σ z 2σ x (2.2) β * y ≥ d = σ * x sinφ (2.3)
Figure 2.10 -The nano beam scheme at SuperKEKB

Belle II

Due to high currents and small beam sizes at SuperKEKB, an increase of the beam background level (10-20 times higher than KEKB) at the detector level is expected. In addition, the high luminosity will increase the event rate by 10 times which requires faster triggers, data acquisition and computing system. For all these reasons, the Belle detector is being upgraded to the Belle II detector (fig. 2.11).

Belle ii

Physics

The Belle II detector will start collecting data during first physics runs in 2018. It aims to accumulate 50 ab -1 of e + e -collision events by the middle of the next decade. Belle II will study the decays of the B meson to probe new physics scenarios, for instance through the search for new CP-violating phases, new physics with an extended Higgs sector (charged Higgs particle), also new physics directly at the tree-level such as right-handed currents. Unlike at hadron colliders like LHC, the clean environment at SuperKEKB allows the study of physics phenomena involving measurements of decays with multiple undetected particles like neutrinos. At SuperKEKB, B mesons are produced in pairs, so the momentum reconstruction of one of two B mesons by the full reconstruction of its daughter particles, allows the knowledge of the momentum of the other B meson even if it decays into undetectable particles.

At Belle II, the physics program is very wide and includes the study of several decays, and each decay corresponds to the probe of one or more physical phenomena. For example, the study of the CP violation in the decay modes of B 0 → φK 0 and B → K * γ allows the probe of new physics in the loop diagrams. The study of the decays B → D τ -ν and B → τ ν allows the search of the charged Higgs boson. The semileptonic B decays allow to clarify the experimental situation of the CKM matrix element |V ub |. In addition, the abundant production of τ + τ -pairs at SuperKEKB allows the search for lepton-flavour-violating (LFV) τ decays with one of the cleanest environments to probe new physics.

Sub detectors

The Belle II detector consists of several sub detectors [START_REF] Wang | The Belle II experiment and SuperKEKB upgrade[END_REF], and each sub detector has its defined role. Here is a list of the Belle II sub detectors with a brief definition of their tasks.

• The vertex detector (VXD) detects the decay point of the B meson. It is made up of two parts, the inner layer is the Pixel detector (PXD) and it consists of two DEPFET layers (DEPleted p-channel Field Effect Transistor). The outer part is the Silicon Vertex Detector (SVD) and is made up of four layers of DSSD (Double Sided Strip Detectors).

The VXD is the closest detector to the IP.

• The CDC (Central Drift Chamber) is the main tracking device for charged tracks. Its smaller cell sizes compared to Belle improves the momentum and dE/dx resolution.

• TOP (Time of Propagation) sub detector is used for the particle identification in the barrel region of Belle II. It consists of 16 modules, each module consists of two quartz bars, one mirror, one prism and an array of photo-detectors. The TOP detector measures

Chapter 2. The SuperKEKB collider

Cherenkov photons generated from charged tracks.

• ARICH (Aerogel Ring Imaging Cherenkov) is used for particle identification in the forward cap.

• ECL (Electromagnetic Calorimeter) is used to detect photons and π 0 from the B meson decay. It consists of 8736 CsI(Tl) crystals with typical dimension of 6×6×30 cm 3 .

• KLM (K L -Muon detector) detects the K L and muons, it consists of a large area of thin planar detectors interleaved with the iron plates of the 1.5 T solenoid's flux return yoke.

BEAST II

The goals of the BEAST II (Beam Exorcism for a STable experiment project) [START_REF] Tanaka | BEAST II Technical Design Report DRAFT For use in US Belle II Project TDR[END_REF][32] is to estimate, measure, mitigate and protect against beam backgrounds. In the presence of very high currents, very small beam sizes and high luminosity, the level of backgrounds

BEAST ii

increases in the interaction region (IR) and thus in the Belle II detector. These backgrounds may damage the sub detectors of Belle II and affect the precision of its measurements. The backgrounds considered are divided into two types, backgrounds from single beam processes like beam-gas Bremsstrahlung, Coulomb scattering, Touschek scattering, synchrotron radiation and injection noise, and backgrounds from non interesting collision events at the IP like radiative Bhabha scattering and 4-fermion final state QED processes.

BEAST II started its commissioning along with the phase 1 commissioning of SuperKEKB (see chapter 4). It aims to validate the MC (Monte Carlo) simulation of the backgrounds by comparing expected levels to measurements in dedicated sensor devices. It serves also to provide information on beam losses to SuperKEKB operators. The detector components of BEAST II measure the background levels by measuring the X-rays, fast/thermal neutrons, charged particles, etc ... BEAST II consists of seven sub detectors (fig. 2.13).

In the phase 1 commissioning of SuperKEKB, our fast luminosity monitors measured particle losses (see chapters 5 & 6) from single beam processes which are correlated to what was measured by BEAST II detectors. The same simulation tools were used to estimate the loss rates which enabled a cross check of the reliability of the simulation code. Data from our diamond sensors were integrated in the BEAST II ntuples to provide a feedback of beam losses at different locations in the rings. Chapter 3

Fast Luminosity Monitoring

Motivations And Specifications

To achieve very high luminosity, and as mentioned and explained in chapter 2, Su-perKEKB will adopt the nano-beam scheme which implies very strong focusing at the IP resulting in very small vertical beam sizes. Because the beam sizes are tiny, the luminosity is very sensitive to the stability of the final focusing elements whose vibrations cause an orbit motion which could be 4 or 5 times larger than the vertical beam sizes. Such vibrations cause vertical oscillations of the beam at the IP at frequencies up to 100 Hz, whereas horizontal movements are expected at few Hz. The resulting orbit differences between the two beams at the IP lead to a luminosity degradation.

Methods and Techniques

Orbit feedbacks in both horizontal and vertical planes are needed to minimise the losses [START_REF] Funakoshi | Interaction Point Orbit Feedback System at SuperKEKB[END_REF].

The vertical orbit feedback is performed using four BPMs (Beam Position Monitors) around the IP to detect the vertical orbit offset. A dithering system is dedicated for the horizontal feedback, it mainly consists of a lock-in amplifier, dither coils (symmetric coils mounted on the normal cylindrical beam pipe and asymmetric coils mounted on Antechamber beam pipe (fig. 3.1)) and a fast luminosity monitor.

Eight sets of dither coils are placed in the LER. Each set consists of two pairs of coils, one kicks the beam horizontally at 77 Hz and the other gives a vertical kick to the positron beam in order to correct the x-y coupling caused by the misalignment of the horizontal coils. The fast luminosity monitor consists of two different detectors, single crystalline diamond sensors (sCVD) (l.h.s of fig. 3.2) used by our group at LAL, and Cherenkov and Scintillator detectors (r.h.s of fig. 3.2) used by the ZDLM (Zero Degree Luminosity Monitor) [START_REF] Hirai | Real-time luminosity monitor for a B-factory experiment[END_REF] group at KEK. The luminosity monitor measures the relative luminosity at 1 KHz and feedback corrections are applied on the second time scale. Both detectors are placed next to each other, just outside the beam pipe in both rings of SuperKEKB, and will measure the signal from the radiative Bhabha process at zero photon scattering angle.

Fast enough luminosity measurements are needed as input for luminosity optimisation and feedback in the presence of such dynamic imperfections. A relative precision of 10 -3 in 1 ms over all 2500 bunches in phase 3 (L= 8×10 35 cm -2 s -1 ), with less than 1% contamination from non-luminosity scaling effects, is specified. In parallel, average single bunch luminosities are very important for beam dynamics studies, related for instance to the continuous top-up injection scheme used at SuperKEKB.

Evaluation of dithering algorithm

A dithering system kicks the beam in the horizontal direction at 77 Hz (T ≈ 13 ms). In order to see the effect of such a kick on the luminosity performance of the SuperKEKB, fast luminosity measurement is required. In this case, we perform our fast luminosity monitoring using diamond sensors along with Cherenkov and Scintillation detectors. In order to reconstruct the modulation in the luminosity from the horizontal dithering, considering the conditions of the sampling theorem [35] which states that sampling frequency should at least be twice faster than the frequency of the signal itself, we choose to perform luminosity measurements each 1 ms (at 1 KHz)(≈ 13 times the dithering frequency).

In an attempt to estimate the minimum rate of luminosity events in the luminosity monitors required to give an efficient feedback on the luminosity, a Monte-Carlo simulation using Mathematica [36] was performed by S. Uehara at KEK. In the following, the main features of this simulation and its results are briefly summarized [37].

The dithering frequency "f 0 =77Hz" modulates the orbit of one of the two beams as "r=sin(2πft)". The Luminosity depends on this parameter with a Gaussian function given in equation 3.1 and represented in fig 3 .3. Dithering quantities "p" and "q" represent the amplitude of dithering (the kick of the magnets) and the shift of the operation point from luminosity peak respectively. If |q| >p, the operation point of the dithering system goes up and down on a side of the slope and thus we lose a quite large percentage of the luminosity, on the other hand if |q| <p, it goes up and down across the peak with negligible luminosity losses. If we are at the top of the luminosity profile, the FFT (Fast Fourier Transform) of the luminosity measurement profile (fig. 3.4) gives a peak at 2×f 0 (l.h.s of fig. 3.5), whereas as we go very far from the peak luminosity, the peak of the FFT is at f 0 (r.h.s of fig. 3.5). Our aim is to evaluate the measurement rate (precision) needed to sufficiently decrease the peak at f 0 while increasing the peak at 2×f 0 to ensure maximal luminosity. The model of the lock-in amplifier [38] is employed, where the measured signal is decomposed into its Fourier components at the base frequency f 0 and the double frequency 2×f 0 . The components at the base frequency are expressed as N i=1 cos(2πf 0 t i ) and N i=1 sin(2πf 0 t i ) whereas they are expressed as N i=1 cos(4πf 0 t i ) and N i=1 sin(4πf 0 t i ) at the double frequency, where t i is each L measurement at a rate of 1 KHz and N is the total number of L measurements in a period of time sufficient to reconstruct the f 0 and 2f 0 base and double frequencies via FFT. It is typically 1 second since feedback corrections are applied every second. The simulation considered different values of "p" at different counting rates. As a conclusion, a precision of 1.6×10 -3 over 1 ms (event rate of 400 KHz in 1 ms) was found to be sufficient to control the horizontal orbit every 1 second based on the dithering feedback. Furthermore, the baseline frequency (l.h.s of fig. 3.6) information provides much better precision to pick up the true peak position than the double frequency (r.h.s of fig. 3.6).

L(t) = exp(- (q + p × sin(2πf t)) 2 2 ) (3.1)

Our Project

Our group at LAL contributes to the orbit feedback project at SuperKEKB by supplying fast luminosity measurements using diamond sensors (DS) as input. We choose the DS thanks to its radiation hardness and relatively fast signals. The DS will measure signals from radiative Bhabha at zero photon scattering angle, by installing them just outside the beam pipe, downstream of the IP, at locations with event rates consistent with the aimed precision (Table . 3.1), and small enough contamination by backgrounds from single-beam particle losses. The number of Bhabha particles produced at the IP is proportional to the luminosity as given by equation 3.2, where N is the number of Bhabha particles produced at the IP, σ is the radiative Bhabha cross section at the IP in cm 2 (0.15 10 -24 cm 2 ), L is the luminosity in cm -2 s -1 and T is the time in s. We aim at a precision of 10 -3 in 1 ms. The fraction of the cross section which must be detected in the sensors to achieve the specified precision is shown in Table . 3.1. For the very high luminosity L=8×10 35 cm -2 s -1 , the simulation results reported in section 2 of this chapter show that a precision of 10 -3 is needed for the dithering feedback in the horizontal plane. However, for a lower luminosity (L=10 34 cm -2 s -1 ) the horizontal beam sizes are twice larger, which means that the precision on the luminosity measurement to control the horizontal orbit could be looser by some factor, which can be determined by the same simulation.

N = σ × L × T (3.2) Luminosity (cm -2 s -1 )

CVD diamond sensors

Diamond detectors are fabricated by the CVD (Chemical Vapour Deposition) process.

In this process, a small amount of hydrocarbon gas such as methane is mixed with molecular hydrogen and oxygen gas. The mixture is then ionised so that carbon based radicals settle down on a substrate, normally silicon or molybdenum, and link together with σ-type bonds forming a diamond lattice (fig. 3.7) [START_REF] Friedl | Diamond Detectors for Ionizing Radiation[END_REF]. The diamond sensor (DS) has many interesting properties which allow it to be one of the best candidates for solid state radiation detectors. It has the highest thermal conductivity among solid state materials in addition to a high electrical insulation. A detailed explanation of the diamond's properties, characterisation, functionality and types is found in [START_REF] Friedl | Diamond Detectors for Ionizing Radiation[END_REF]. Here, a few properties and advantages of the DS over other semiconductor materials are reviewed. A comparison among diamond, silicon and germanium is presented in (Table . 3.2).

The diamond has a very high binding energy which makes it very insensitive to radiation. As well, diamond is characterised by a large band gap which reduces the leakage current and thus decreases the noise. However, the large band gap in the diamond sensor also results in a weaker signal compared to Si and Ge for instance, but in our project, we are able to measure the signal from one electron using a 500 µm for a low luminosity, however for a higher luminosity, we will use a 140 µm with a window design (explained later in this chapter) which will increase the signal. In addition, diamond has high electron and hole mobilities which results in fast charge collection and thus in fast signals, in the range of few ns. The width of the signal depends on the thickness of the diamond, for example a signal width of 4-5 ns corresponds to a 500 µm diamond. The minimum energy needed to create an electron/hole pair in the diamond is 13 eV. One MIP (Minimum Ionising Particle) creates 36 e -/h pairs per µm in the diamond. The bias field which we choose to apply to collect e --h pairs is E=0.8 V/µm. Normally we can apply a bias voltage until 1 V/µm, however, thanks to the high quality DS, the charge collection efficiency reaches 100% long before this bias voltage (at ≈ 0.2 V/µm). For security, we chose to apply a voltage bias a bit smaller than 1 V/µm, let it be 0.8 V/µm. One disadvantage of the diamond sensor is the difficulty of having large active area, which constraints our measurements to a part of the shower, however this is not a critical problem in the case of the window (see later in the chapter) where we will be able to achieve our aimed precision by increasing the electromagnetic shower.

Property

Operation

The operation and the readout of the DS is simple and is presented in fig. 3.8. A charged particle traverses the DS creating e -/h pairs depending on its energy. A high voltage is applied at the electrodes of the diamond, opposite charges drift to the electrodes. The output signal resulting from the voltage drop is read by an oscilloscope. Amplification of the signal is necessary in the case of small signal, i.e low intensity of incident particles and/or very thin DSs. The energy deposition in the DS is a Landau distribution, as confirmed by performing a GEANT4 simulation using a 500 µm DS with a beam of incident electrons of 3 GeV (fig. 3.9). These electrons are considered as MIPs. A MIP is a charged particles with an energy loss rate close to the minimum, and the energy of a MIP electron is around 1.6 MeV.

Characterisation with radioactive sources

To better understand the behaviour of the signal in the DS, we performed tests in the clean room at LAL using radioactive sources. [START_REF] Natsui | Quasi-travelling wave side coupled RF gun commissioning for SuperKEKB[END_REF], and an oscilloscope to read the signal. Since the charge amplifier is unipolar, the high voltage (-100 V) on the DS is applied to polarize negatively and to obtain negative pulses at output of the DS.

A special mechanical stand shown in fig. 3.10 was prepared for the tests on the DS. The 140 µm sCVD is connected to the charge amplifier which is powered with 12 V and both are laid on the middle bench of the mechanical stand. On the top part, there is a special opening for the electrons emitted by the radioactive source to hit straight the 4x4 mm 2 diamond below. Just below the diamond, a scintillator is used to trigger, in order to collect signals from MIPs only, i.e the MIP will traverse the diamond depositing some of its energy, without being absorbed, and will then hit the scintillator and give a signal to trigger. The 90 Sr undergoes a cascade of two β decays (fig. 3.13) resulting in electrons with a whole spectrum of energies. One MIP produces 36 e -/h per µm in the DS, and since the energy to produce one e -/h pair is 13 eV, then an electron with an energy E e - < E min =65.52 KeV (13 eV × 36 e -/h × 140 µm) will be absorbed in the 140µm thick DS. So the scintillator trigger makes sure that E e -is greater than E min .

Two types of trigger are used, one is a trigger on all the events including signals from the MIPs and noise from the amplifier, and the second one is the multistage trigger. In the multistage trigger, a window of 6 ns is considered, this allows selecting only the events in the DS whose rising edge arrives 6 ns before the rising edge of the signal in the scintillator. The window of 6 ns is chosen after taking into account the delay of the signal in the scintillator due to the difference in the cable lengths connecting both the scintillator and the diamond to the oscilloscope, and the fact that the electron passes the DS before hitting the scintillator. In addition to the Landau reconstruction, we need to verify that the position in time of the MPV of the signal doesn't move significantly and doesn't depend on the magnitude of the signal. This is very important because in our measurements we are sampling at 1 GSPS (Giga Sample Per Second), which means each 1 ns. To be precise in the reconstruction of the shape of the signal amplified by the charge amplifier and to determine the maximum which represents the total charge collected in the diamond during the acquisition time, this maximum should move with an RMS less than 1 ns. For this study, we use a 500 µm sCVD biased with -200 V and we trigger using a multistage trigger at 6 ns using the same already described experimental setup. To make sure that there are no other reasons behind the relatively large RMS of the position in time of the signal from the diamond other than that of the scintillator, we need also to check that there is no correlation between the maximum of the signal and its position in time. On l.h.s of fig. 3.21, we can see the maximum of the signal in the sCVD as a function of its position in time, and the plot shows no correlation between these two parameters which is a very good sign. As well, r.h.s of fig. 3.21 shows the maximum of the signal in the sCVD as a function of its width, also showing no significant correlation. Data from LEETECH @ PHIL Diamond sensors are not only tested in the clean room but also at PHIL (PHoto Injector at LAL) [START_REF] Brossard | PHIL Accelerator at LAL -Diagnostic status[END_REF], where a new facility named LEETECH (Low Energy Electron TECHnique) [START_REF] Attie | LEETECH facility as a flexible source of low energy electrons[END_REF] is installed. PHIL (fig. 3.22) is a photo-injector followed by a six-meter long beam line which delivers electron bunches up to 5 MeV with an intensity of 10 8 to 10 10 particles per bunch and at a repetition frequency of 5 Hz. LEETECH (fig. 3.23) is a facility for detector R&D, which uses the electron beam of PHIL to deliver mono-chromatic low energy electrons with adjustable energy and intensity using a bending magnet with adjustable field in addition to collimators at its entrance and exit.

Among the tested detectors are a 500 µm CIVIDEC DS and another 500 µm DS proto-typed and fabricated at LAL [START_REF] Kubytskyi | DIAMOND SENSOR RESOLUTION IN SIMULTANEOUS DETECTION OF 1,2,3 ELECTRONS AT THE PHIL PHOTOINJECTOR FACIL-ITY AT LAL[END_REF]. The data from the CIVIDEC DS were used to analyse the distribution of the position in time of the maximum of the signal. Unlike the tests with a radioactive source in which just one electron hits the DS at once, the tests at LEETECH correspond to 1,2,3,... electrons hitting the DS depending on the magnetic field and the collimator aperture. The distribution of the maximum of the signal of the RMS of the distribution of the difference is 1 ns, which is less than the RMS (1.5 ns) given by the same distribution when using the scintillator instead of the laser as a triggering apparatus. This confirms that the position in time of the maximum of the signal in the DS depends mainly on the position of the signal of the trigger.

Radiative Bhabha at zero photon scattering angle

To measure luminosity, we need to collect events produced at the IP with rates proportional to the luminosity. We choose to measure events from a well known physical process: radiative Bhabha at vanishing photon scattering angle (fig. 3.26), which has a large cross section (≈ 200 mbarn). The considered process is one of the main sources of particle losses in a very high luminosity e + e -collider such as SuperKEKB. The particles of the electron and positron beams scatter through the exchange of a quasi-real photon at almost zero angle. The virtual photon interacts with a particle of the other beam and loses energy by emitting a real photon. The particle which emits the virtual photon, called spectator, keeps its energy and stays in the beam, however, the scattered particle loses part of its energy and will be deflected downstream of the IP at different locations, mainly after bending magnets, and will exit the vacuum chamber (fig. 3.27). The diamond sensors placed just outside the vacuum pipe measure the signal from the charged secondary particles, resulting from the interaction of the lost Bhabha particles with the material of the vacuum pipe. GUINEA-PIG is a program to simulate beam-beam interactions in high-energy e + e - linear colliders, written in C language by D. Schulte [START_REF] Schulte | [END_REF]. It includes pinch effect, beamstrahlung, pair creation and calculation of hadronic backgrounds. It was upgraded to GUINEA-PIG++ [START_REF] Meur | [END_REF] at LAL, which is its C++ version. This code is very important for the R&D of future linear colliders. It is able to simulate the interaction of an electron beam with a positron beam and predict the luminosity of the collision in the presence of pinch and disruption effects resulting from the very dense charge distributions in such colliders.

The input file of GUINEA-PIG++ contains the parameters of both beams, including the energies, intensities, β functions, emittance and bunch length. Normally, the code is prepared to simulate head on collisions, but in the case of SuperKEKB, the collision is at an angle. This is taken into account by replacing the total intensities by the intensities within the overlap region, and replacing the bunch length (σ z ) by the effective bunch length (σ zef f = σx sinθ , where θ is the crossing angle (2θ=83 mrad)). The output files contain the energies and angular distributions of the incident beams, the Bhabha particles (electrons and positrons) and the Bhabha photons. The energy of the Bhabha particles extends from very low energy to the beam energy (4 GeV for e + and 7 GeV for e -) (fig. 3 ticles with the beam energy will bend on the correct trajectory, high energy particles will bend less and take the longest trajectory inside the magnet, whereas low energy particles will bend more (fig. 3.31). In the case of Bhabha positrons, the magnitude of the energy deviation due to the emission of a photon can be large and results in the loss of the particle.

At the IP, the produced Bhabha particles are still inside the beam, the separation of the Bhabha particles from the beam particles is clearer after bending magnets, we can see that for example in fig. 3.32 after tracking the beam positrons (blue) and Bhabha positrons (red) to the exits of the third and fourth bending magnets. I tracked the Bhabha positrons to various locations downstream of the IP, after bending magnets, after quadrupoles and in drifts. The physical aperture of the beam pipe is 40 mm starting from ≈4 m downstream from the IP. In the following, I show few plots of the tracked Bhabha positrons after bending magnets. In fig. 3.33, we can see the horizontal position distribution of the Bhabha positrons as a function of their energies after the third and fourth bending magnets. We can see a very nice linearity in the E-x plane, in which low energy particles go close to the edge of the vacuum chamber and high energy particles stay close to the center of the vacuum chamber. It is clear from the plots that the low energy side is at +x and it is where we have installed our DS. Fig. 3.34 represents the en-Figure 3.31 -Accelerated beam in a bending magnet ergy spectrum of the Bhabha positrons after the third and the fourth bending magnets, the green represents the particles that have been already lost before the magnets, the blue represents the particles intercepted in the DS and the red represents the particles that remain in the beam pipe. We can see that after the fourth bending magnet we don't detect any Bhabha positron in the DS, this is explained by the fact that the fourth bending magnet is a very big magnet (Table . 3.3) and far from the IP, which means that very few low energy particles will reach it and will get lost inside it. On the other hand, after the third bending magnet 72 particles out of 10000 produced Bhabha positrons are intercepted in the DS. This fraction is close to the fraction required to attain the specified precision, but there is no adequate geometry and not enough space for the equipments as shown in fig. 3.35. Fig. 3.35 shows the β functions and dispersion functions in the horizontal and vertical planes of the LER lattice 27 meters downstream of the IP. second bends (fig. 3.36). This is mainly due to the fact that the first bending magnet is a vertical bend. This strange tail extends in the +x and -x direction and thus a negligible number of Bhabha positrons is detectable. As a result, bending magnets are not good candidate locations for the DS and we need to search for other candidates. I tracked in The DS is perfectly placed in the middle of the distribution x-E at 13.9 m shown on the l.h.s of fig. 3.37 and detects ≈ 4.7% of the total radiative Bhabha cross-section (r.h.s of fig. 3.37). In addition, the location has a 3 meter drift with enough space for the DS, the Cherenkov and Scintillator detectors, with their mechanical setup and cables. SAD doesn't take into account the beam pipe, its material and geometry, only the tracking in the optics of the accelerator, thus for a precise estimate of the signal in the DS we need to perform further simulations using GEANT4 [48].

Simulation of signal in the sCVD

After choosing the best candidate of the DS, it is very important to study the signal considering the lost Bhabha positrons themselves, their energies and exiting angles, the geometry of the beam pipe including its material and thickness. For this reason, a detailed simulation in GEANT4 is considered. The beam pipe at 13.9 meters is a 3 m long cylindrical Copper chamber of 6 mm thickness. The simulation uses the output of the tracking with SAD after the quadrupole QKBLP as its input. The input Bhabha positrons are defined by their energies (l.h.s of fig. 3.38), their positions and their horizontal angle (r.h.s of fig. 3.38). The Bhabha positrons hit the 6 mm Cu pipe at a very small angle of 5 mrad, which would lead then to cross 1.2 m of the material, corresponding to ≈ 80 radiation lengths (X 0 =1.436 cm). Most of the particles produced in the showers induced by the interaction of the scattered positrons with the beam pipe are hence absorbed, and the probability for having charged secondaries exiting the beam pipe is 3), where N is the total number of detected Bhabha positrons, 4.7% is the fraction of the total Bhabha cross section exiting the 3 m drift, L is the luminosity (10 34 cm -2 s -1 at phase 2 & 8×10 35 cm -2 s -1 at phase 3), σ is the total cross section of the radiative Bhabha process and f is the fraction of exiting Bhabha particles hitting the DS and resulting in a detectable signal. In the simulation, a Bhabha positron is considered detectable if it deposits an energy of at least 50 KeV in the 500µm DS, corresponding to 2.43 mV or 3σ of the charge amplifier noise (see section 4.2.1.1 of this chapter). In the case of the parallel orientation, and due to their angular distribution after the 6 mm Cu tube, the secondaries may cross less than 500 µm and thus deposit less energy than the normal Landau distribution of the 500µm DS, or may cross more than 500 µm and deposit higher energies and thus appears in the very end of the extended tail of the distribution.

P = 1 √ N ; N = 4.7% × L × σ × f ; f = N diamond N exiting (3.3)
The fractions of detected exiting Bhabha positrons are the same for parallel and perpendicular sensors as a function of the distance from the beam pipe (fig. 3.42). It can also be

Design of new vacuum chamber insertion

Since neither the normal beam pipe nor a beam pipe as thin as 1 mm allows the aimed precision to be achieved, K. Kanazawa from the vacuum group at KEK suggested a special insertion with a window at 45 • in the drift where we will place the DS.

Simulation of window at 45 •

The suggested window (fig. 3.44) starts at 81.5 cm from the center of the QKBLP. It is a 1.5 cm deep inclination at 45 • of 6 mm thick Copper. It is 48 cm long with symmetric edges to reduce the beam pipe impedance, in order to limit beam instabilities due to wakefields. Such a window permits the exiting Bhabha positrons to traverse much less material than the normal beam pipe. Particles exiting the window at 5 mrad will cross 8.5 mm Cu only and thus most of the electromagnetic shower will be able to exit. Since the distance crossed by the charged Bhabhas is very short compared to the shower maximum In order to increase the shower and thanks to the design of the window, we can put a radiator of a heavy material with a thickness equal to its shower maximum, at the exit of the window (l.h.s of fig. As mentioned before, the radiator will increase the shower and consequently increase the The simulation of the suggested window proves clearly the improvement of the precision on the luminosity. With the window and the radiator, we should achieve the aimed precision. On the other hand, such a design has a cost besides introducing some potentially detrimental wakefields affecting the stability of the beams. A less expensive solution could be using another material for the beam pipe at a suitable thickness. For quantitative assessment of the expected precisions, GEANT4 was used to compare performances with a regular cylindrical beam pipe, considering also different materials (Cu, Al, Be, Ti), Design phase 2 phase 3 design (10 34 cm -2 s -1 ) (10 35 cm -2 s -1 ) (8×10 35 cm -2 s -1 ) cylinder 6 mm Cu positrons along the 3 m long drift as a function of its thickness, as well as the signal in the DS and thus the precision on luminosity as a function of the different orientations of the DS with respect to the vacuum chamber (0 • ,45 • and 90 • ).

6.3×10 -2 2×10 -2 7×10 -3 cylinder 1 mm Cu 3.6×10 -2 1.1×10 -2 4×10 -3 window at 45 • 6×10 -3 1.89×10 -3 6.7×10 -4 window + radiator 3.7×10 -3 1.2×10 -3 4×10 -4
The secondaries exit the thickness of the beam pipe uniformly over the 3 m drift, this means that wherever the DS is placed in the drift, we will get the same result. The average value of the number of secondaries per incident particle that exit the beam pipe changes with the material of the vacuum chamber and its thickness. When the distance travelled by the incident Bhabha inside the thickness of the beam pipe is close to the shower maximum of the material, the number of secondaries per incident particle is large and vise versa. This is simulated and shown in the plots in Appendix A.

The secondaries exit the beam pipe with an energy profile ranging around few tens of MeV. They exit with different angular profiles depending on the material of the beam pipe and its thickness, this is because of the different paths traversed by the incident particle. The different number and angular profiles of the secondaries result in variations in the number of secondaries in the DS depending on its orientation. This affects the precision on the measured luminosity. The results of the simulations for the different materials (Cu, Al, Be, Ti) are represented in Appendix A along with the comparison with the design of the window at 45 • . The decision on the design of the local vacuum chamber is discussed as well.

The results showed that among all the different materials/thickness/geometries of the beam pipe and different orientations of the DS, the window at 45 • is the best choice to achieve the specified precisions in phases 2 and 3 (fig. 2021). Thus, the window will be installed in the LER for phase 2.

Case of HER

The HER (High Energy Ring) is the electron ring with a beam energy of 7 GeV. We aim to use diamond sensors in the HER as well as in the LER to increase the range of luminosities over which precise measurements can be performed. The HER was studied in the same way as the LER to find the best possible locations for the DS. Bhabha electrons were generated using GUINEA-PIG++ and then tracked using SAD.

Difficulties and restrictions

Bhabha electrons are tracked to different locations after bending magnets and in the drifts to 24 meters downstream of the IP (fig. 3.50). The list of the first four bending magnets downstream of the IP in the HER is given in table 3.6. Unlike the LER which shows a nice linearity of the energies of the tracked Bhabha positrons with respect to their transverse positions, the HER shows strange distributions in the E-x plane (fig. This strange distribution in the E-x plane is mainly due to the fact that the second, third and fourth magnets are vertical bends, in addition to the existence of sextupoles to correct the chromaticity of the final focusing quadrupoles. The disadvantage of such distributions is that the Bhabha electrons will be lost on both sides of beam pipe (low energy side (LES)(+x) and high energy side (HES) (-x)). In the case of installing the DS on the LES to prevent the hits of high energy photons, the intercepted fractions of Bhabha electrons in the DS will be very low, in the range of 0.5% of the total Bhabha cross section. These fractions are insufficient to attain the aimed precision. Thus we tried to search for another method to measure the luminosity in the HER.

Method implemented

Since that Bhabha electrons don't satisfy the conditions for our fast luminosity monitoring, we may think about measuring the signals from the Bhabha photons. The energy of Bhabha photons in the HER varies from very low energy to 7 GeV as shown on l.h.s of fig. 3.54. They are produced at the IP with a very small transverse angular distribution with an RMS of 0.4 mrad (r.h.s of fig. 3.54). The advantage of the photons is that they are neutral particles and will not be deflected by the magnetic fields of the optical elements, so they continue straight and exit the beam pipe at well defined distance from the IP.

Detailed simulations are currently being performed in the LAL group considering the lattice of the HER and the geometry of the beam pipe. The preliminary analysis shows that the γ spot hits the antechamber edge in the SLYTRE sextupole (at 27.4 meters downstream of the IP) (fig. 3.55) at an average angle of 5.9 mrad. Due to the transverse angular distribution of the photons, some photons will hit the vacuum chamber several meters on both sides of the nominal exit point at 27.4 meters downstream from the IP (fig. 3.57). The closest free space for the DS is just after the exit of the sextupole SuperKEKB single beam commissioning

Introduction

The SuperKEKB commissioning is split in three phases and each phase has its defined goals. The single beam commissioning took place starting from February until June 2016. This commissioning went in parallel with the BEAST II commissioning to study the backgrounds from single beam losses in the interaction region. Our fast luminosity monitors (diamonds + ZDLM monitors) also took data during that period.

General goals and achievements

The single beam commissioning of SuperKEKB, which represented the phase 1 of the commissioning, took place without the final focus system and without the Belle II detector, during which both positron and electron beams were successfully produced, accelerated, injected and stored in the main rings (LER and HER) of SuperKEKB.

The main goals of this phase [START_REF] Funakoshi | Overview of Phase 1 Commissioning[END_REF] were to start each hardware system of the machine (injector linac, magnet system, RF system in both rings, vacuum system, ...) and to establish the beam operation software based on SAD. During this period of commissioning, vacuum scrubbing was performed in both rings at different currents, starting from very low currents (30 mA) to high currents (1 A), in order to clean the vacuum chambers from residual gases. The residual gas causes beam loss due to the scattering of the beam particles on the gas molecules, either by Bremsstrahlung (energy loss of the particle by emitting a photon) or Coulomb scattering (scattering at large angle). Vacuum scrubbing (fig. 4.1) is performed by circulating the beams in each ring, the interaction of the electromagnetic field of the beams with the material of the pipes leading to the extraction of molecules off the surfaces of the vacuum chambers, and by using the NEG pumps these residual molecules could be removed from the vacuum pipes. Vacuum scrubbing at high currents (500 mA to 1 A) was demanded by BelleII to protect the detector from high beam backgrounds during the physics runs which will start during phase 2.

During phase 1, BEAST II (beam background group) performed background studies with all its sub-detectors. Special studies in both rings were dedicated to background studies of BEAST II (Touschek study, beam-gas backgrounds, injection background and collimation studies). The Touschek study was performed by changing the vertical beam size, using the ECK (Emittance Control Knob). Changing the setting of the ECK means changing the field of the skew quadrupole component of the sextupoles, which induces a coupling of the horizontal emittance into the vertical emittance all over the ring and thus results in the change of the vertical beam size. The beam-gas background studies were performed by inducing vacuum bumps in both rings in sections upstream of the IP. These vacuum bumps are the result of deactivating a few NEG pumps at selected locations. To study the effect of collimators (fig. 4.2) on the beam losses, parameters of some collimators (collimator widths) have been modified each time. From these studies, the BEAST II group aimed to test its detectors under beam conditions, and to compare the data with the simulation, in order to validate the simulation in view of extrapolating it to phases 2 and 3. Another main goal of the phase 1 commissioning was to understand the problems

Next stages of SuperKEKB commissioning

The phase 1 commissioning was successfully accomplished and finished in June 2016.

For the phase 2 commissioning, which is planned to start in the end of 2017 and beginning of 2018, the last final focus quadrupoles (QCS) will be installed in the IR along with the Belle II detector without its vertex detector. An increase of the background level mainly due to Touschek is predicted due to focusing to small beam sizes at the IP. This could be dangerous for the vertex detector which is the closest to the IP among the Belle II sub detectors, for this reason it will not be installed until the background level is confirmed and assured to be under the threshold that could damage the vertex detector. During this phase, the commissioning of the damping ring will take place in order to achieve a small emittance positron beam before injection into the LER. Background studies with the BEAST II detectors will be performed mainly to give feedback on the radiation levels in order to protect the Belle II detector. Luminosity tuning will start and the target luminosity is 10 34 cm -2 s -1 .

During phase 3, the Belle II detector will be fully installed and the physics run will resume, this time with the vertex detector. The beam currents will be increased in both rings and the luminosity tuning will continue to achieve a luminosity of 10 35 cm -2 s -1 and up to 8×10 35 cm -2 s -1 .

Goals for our fast luminosity project

Our project is to perform fast luminosity monitoring as part of the dithering feedback system in the horizontal plane at SuperKEKB. We aim at a precision of 10 -3 in 1 ms by measuring signals from the radiative Bhabha process at zero photon scattering angle with our diamond sensors in both rings. Our sensors need to be placed at locations where contamination from single beam backgrounds is minimal (< 1%). Therefore studying the single beam loss processes is important for our project as well.

Our diamond sensors were installed and took data during phase 1 commissioning. We aimed from this data acquisition to address several aspects:

• To check the operation of our sensors and overall installation (electronics and mechanics) under machine conditions.

• To measure the backgrounds at the locations of the sensors chosen for the luminosity measurements in both rings.

• To compare the data to the simulation, and thus to check the reliability of our simulations for phase 2.

• Once the reliability of the simulation is confirmed, we can extrapolate our background simulations to phase 2 and estimate the fractions of signal from single beam losses and

Beam Loss Monitors 63

from the radiative Bhabha process in our sensors.

The simulation was also used to compute the amount of Bhabha signal in the sensors, this is the detected fraction of the Bhabha cross section, which determines the precision to be expected for the luminosity measurement in different beam pipe geometries, materials and window designs. These predictions will also be validated once the simulations are confirmed.

During phase 2, the window at 45 • will be installed in the drift at 11.9 meters in the LER. We will continue the backgrounds measurements before the luminosity tuning starts. The main aim of our project in this phase is to start the fast luminosity measurements with a modification of the data acquisition system which will enable us to monitor the train and the bunch integrated luminosities in real time, and thus provide input to the feedback system based on the horizontal dithering. We will pursue the fast luminosity measurements during the phase 3 as well.

Beam Loss Monitors

The BLMs (Beam Loss Monitors) at SuperKEKB are in charge of the measurement of the particle loss levels at a number of locations along both rings, in order to provide a protection of the hardware systems and the Belle II detector. The loss monitor system is a very important part of the abort system at SuperKEKB. Due to higher currents and smaller beam sizes compared to KEKB, the simulated beam losses are 50 times higher at SuperKEKB. These losses are mainly concentrated in the IR and around collimators. The beam loss monitors used at SuperKEKB are the PIN photodiodes and the ion chambers [START_REF] Ikeda | Beam loss monitor at SuperKEKB[END_REF].

PIN diodes

The PIN diodes (PD) used at SuperKEKB are 2.65×2.65 mm 2 , and they are placed mainly around the collimators (l.h.s of fig. 4.5) to detect the losses in all directions. They are characterised by high speed and high radiant sensitivity, their interlock signal is fast enough to protect the collimators. They are also installed near the bellows (r.h.s of

Ion chambers

The Ion Chambers (IC) at SuperKEKB are 5 m long FC-20D coaxial cables (fig. 4.6) and they are placed on the outer wall of the tunnel. The inner and outer conductors of the IC are separated by an air gap. The loss signal from the IC is logged at 1 Hz in a 16 bit ADC. The IC is sensitive to very small loss rates of 0.1 mA/s.

During the phase 1 commissioning of SuperKEKB, I participated in operation shifts on the machine in the KEKB control room, and assisted with the beam loss monitors. My task was to integrate the beam loss monitors, considering their type and position, in the lattices of both rings of SuperKEKB. The aim of such a work, which was done in the framework of the SAD software, was to enable monitoring through EPICS (Experimental Physics and Industrial Control System) the values of each channel of the beam loss system at a given frequency (1 Hz for example), to quickly provide information about the position of the beam loss abort signal.

Vacuum scrubbing

Since a large part of the beam pipes has been replaced by new one, especially in the LER, vacuum scrubbing was a major task during the phase 1 commissioning [START_REF] Suetsugu | First commissioning of the SuperKEKB vacuum system[END_REF], to increase the beam lifetime and decrease the beam background in the Belle II detector during the physics run of phases 2 and 3. The vacuum scrubbing took place successfully, with vacuum pressures of less than 1×10 -7 Pa achieved in both rings. The stability of the new vacuum components (antechambers, bellows, flanges, ...) was checked and confirmed. No extra heating and abnormal pressure rise were observed in these components.

Electron cloud

The synchrotron radiation emitted by the positron beam in the LER generates photons which are intercepted by the material of the vacuum chamber. In that process, electrons can be extracted by photo-emission. Such electrons can then be attracted and interact with the positron beam, later hitting the chamber and extracting more electrons. In the presence of multi-bunch operation with narrow bunch spacing, these electrons accumulate in the vacuum chamber and build up the electron cloud. The electrons of the electron cloud thus create a current along with the passage of the positron beam. The antechambers and the TiN coatings (fig. 4.7) were observed to have an important effect on decreasing the electron current in the LER compared to other types of beam pipes.

Non-linear pressure rise with beam current

One problem encountered during the vacuum scrubbing in phase 1 commissioning was a non-linear pressure rise [START_REF] Suetsugu | First commissioning of the SuperKEKB vacuum system", presentation TUOCB01, IPAC 2016[END_REF] against the beam current in the LER, starting from a beam current of 500 mA (fig. 4.8). This behaviour is similar to the behaviour of the electron current in the region of aluminium parts without the TiN coatings. The reason behind this non linear behaviour of the pressure can be explained by the multipacting of the electrons in aluminium bellows. In the LER, the aluminium bellows are 0.2 m long and are placed each 3 m on average, so they represent more or less 5% of the ring circumference. When the positron beam passes by the bellows, the electrons of the electron cloud are accelerated by the field in the bellows. These accelerated electrons hit the vacuum chamber thus introducing secondary electrons. When the transit time of the electrons crossing the vacuum pipe is comparable to the bunch spacing, multipacting occurs [START_REF] Wei | Electron-cloud effects in high-intensity proton accelerators[END_REF] thus leading to a non-linear vacuum pressure rise.

Countermeasures

As an attempt to mitigate the non-linear rise of the vacuum pressure with respect to beam current, a magnetic field of axial direction was applied with solenoids (l.h.s of fig. 4.9) or permanent magnets (r.h.s of fig. 4.9) at nine aluminium bellows, which represent a 30 m long section. The pressure rise in that section was observed to relax ( l.h.s of fig. 4.10). In the maintenance period of 2-4 June, 800 permanent magnets were installed around most of the aluminium bellows in the arc sections of the LER, as a result the pressure rise was observed to relax all over the ring (r.h.s of fig. 4.10). However, the non-linear behaviour of the pressure remains at a high beam current (> 700 mA). To deal with this problem, permanent magnets were installed afterwards in part of a drift region.

Description of the phenomenon

In the LER, when the positron beam passes through the electron cloud, the electrons oscillate and this oscillation induces a short range wake field. These wake fields are responsible for strong head-tail instabilities which result in the vertical beam size blow up [START_REF] Flanagan | Observation of vertical betatron sideband due to electron clouds in the KEKB LER[END_REF]. This phenomenon has a great effect on reducing the luminosity of the affected bunches, thus it must be controlled.

To decrease the density of the electron cloud, normal cylindrical chambers are replaced by ante chambers. In the presence of ante chambers, the interaction between the positron beam and the emitted electrons is reduced since it is hard for the electric field to enter the chamber slot.

During the phase 1 commissioning of SuperKEKB, beam size blow up was observed at an early stage [START_REF] Ohmi | oral presentation "Instabilities simulation and observation[END_REF]. A dedicated study of this phenomenon was performed as a function of different parameters as the beam current and the fill pattern (bucket spacing), each time the vertical beam size was monitored by the SRM (Synchrotron Radiation Monitor) [START_REF] Fukuma | Beam instrumentation for the SuperKEKB rings[END_REF]. The beam current threshold of the beam size blow up varies for different fill patterns (l.h.s of fig. 4.11).

Figure 4.10 -The pressure rise against the beam current, at the 30 m bellow section before and after applying the solenoidal magnetic fields on l.h.s and in two arc sections before and after the installation of 800 permanent magnets around the aluminium bellows in the arc section on r.h.s [START_REF] Suetsugu | First commissioning of the SuperKEKB vacuum system", presentation TUOCB01, IPAC 2016[END_REF]

Effect of permanent solenoids

One important motivation of the installation of the permanent solenoids at the Aluminium bellows was to see their effect on the beam size. Beam size blow up study was performed after the installation of the solenoids as well. It was observed that the solenoids had a great effect to raise the blow up threshold by a factor of 1.5 for the different bunch spacing patterns (r.h.s of fig. 4.11) [START_REF] Funakoshi | Overview of Phase 1 Commissioning[END_REF]. Thus it was confirmed that the beam size blow up is mainly due to the electron cloud, which is dense at the Aluminium bellows due to the multi-pacting effect.

For the high currents planned in SuperKEKB during phase 3, where the bunch spacing will be reduced to 4 ns, more solenoids will need to be installed in order to control the electron cloud density and thus to control the beam size blow up for the high luminosity operation.

Figure 4.11 -The vertical beam size in the LER as a function of the total beam current for different bucket spacing, before the installation of permanent solenoids at the Al bellows on l.h.s and after the installation of permanent solenoids at the Al bellows on r.h.s [START_REF] Funakoshi | Overview of Phase 1 Commissioning[END_REF] Chapter 5

Single Beam Losses at Phase 1

Single beam loss processes

In SuperKEKB at very high luminosity, the beam backgrounds in the Belle II detector (fig. 5.1) are estimated to be much higher than at KEKB. These backgrounds deteriorate the detector resolution and analysis performance at phases 2 and 3 of the SuperKEKB project. The major sources of these backgrounds are synchrotron radiation (SR), scattering of the beam on residual gas (Bremsstrahlung and inelastic Coulomb scattering), Touschek scattering, radiative Bhabha scattering and electron-positron pair production via the two photon process e + e -→e + e -e + e -[57] [58] [START_REF] Nakayama | SuperKEKB background simulation, including issues for detector shielding[END_REF]. The main reasons behind these high backgrounds are the high currents (3.6 A in the LER and 2.6 A in the HER) and very small beam sizes in SuperKEKB compared to KEKB. To cope with the various kinds of backgrounds different measures will be considered. Movable horizontal and vertical collimators are to be installed along the rings to stop the deviated particles before they reach the detector. The inner surface of the Beryllium beam pipe are coated with gold to absorb the SR before it reaches the inner detector. Backgrounds from Bremsstrahlung, Figure 5.1 -Backgrounds at the interaction region (Belle II) [START_REF] Nakayama | SuperKEKB background simulation, including issues for detector shielding[END_REF] Touschek scattering and inelastic Coulomb scattering are considered as single beam losses, they are generated overall the ring due to scattering of the beam particles on residual gas or scattering of the beam particles upon each other. In addition to being lost inside the interaction region (IR) and resulting in backgrounds in the detector, particles from single beam losses are lost all over the rings which reduces the beam intensities after a certain time. In rings such as SuperKEKB and due to the very high currents and the very small beam sizes, beams have therefore relatively short lifetimes of the order of 10 minutes. The main processes determining the beam lifetimes are the Touschek large angle intra-beam Coulomb scattering and the zero angle radiative Bhabha scattering at the IP. The beams should thus be continuously re-injected to maintain their intensity over time. During the single beam commissioning of SuperKEKB during phase 1, the main single beam losses were the beam gas Bremsstrahlung, the Touschek scattering and the Coulomb scattering.

Beam-Gas Bremsstrahlung

The beam-gas Bremsstrahlung (fig. 5.2) is the deceleration of the charged particles of the beam when interacting with the electromagnetic field of the atomic nucleus of the residual gas atoms in the vacuum chamber. The charged particles lose energy by emitting a photon, and can exit the beam pipe after a certain time. The energy spectrum of the scattered Bremsstrahlung positrons is similar to that of the radiative Bhabha process, with a difference that the Bhabha spectrum is produced at the IP only, whereas the Bremsstrahlung positrons are generated over all the ring. The beam-gas Bremsstrahlung and resulting losses are proportional to the beam current and residual pressure in the vacuum chamber. The beam currents at SuperKEKB are twice higher than at KEKB while the vacuum level, except at the interaction region, is expected to be the same, thus the loss rates are expected to be a bit higher at SuperKEKB. During phase 1 commissioning, the vacuum level in both rings ranged between 10 nTorr and less than 1 nTorr (1 Torr = 133.322 Pa).

Touschek scattering

In a storage ring, the electrons in the beam are subject to quantum excitations as they pass through the magnetic fields of the different magnets, where the discrete emission of photons results in a random walk of the beam particles diluting their trajectories and thereby enhancing the beam emittance. The beam emittance reaches an equilibrium value after beam injection when the radiation damping [60] due to synchrotron radiation balances the quantum excitations. Having acquired transverse velocities, particles in the same bunch can scatter upon each other in a process called Touschek scattering [START_REF] Duff | Single and multiple Touschek effects[END_REF] [START_REF] Lee | Accelerator Physics" second edition, Chapter 4 "Physics of electron storage rings" section II "Beam lifetime[END_REF]. Extrapolated simulations showed that the Touschek backgrounds in SuperKEKB will increase by a factor of 20 compared to that at KEKB. Since the horizontal emittances are larger than the vertical emittances, we assume that the initial momenta of the scattering particles are in the horizontal direction. We consider the Touschek scattering of the two particles in the center of mass system (CMS)(fig. 5.3), where p 1,init =(p x ,0,0) and p 2,init =(-p x ,0,0) are the initial momenta of the two particles expressed in the x, s and z coordinate system. Fig. 5.3 represents the particle scattering where θ is the scattering angle, and χ is the angle between the momentum of the scattered particle p 1,scatt and the s axis. The scattering momentum of the other particle isp 1,scatt . The momentum vector of the scattering particle is expressed as p 1,scatt =(p x sinχ cosφ, p x cosχ, p x sinχ sinφ). Thus the momentum transfer to the longitudinal plane in the CMS is ∆p cms given in eq. 5.1, it is maximum when χ is zero.

∆p cms = γ × p x × cosχ (5.1)
In the laboratory frame, the maximum momentum transfer represented in eq. 5.1 transforms to ∆E in eq. 5.2 where γ is the Lorentz factor. γ is calculated as given in eq. 5.3 where E is the energy of the particle, m is the mass of the electron and c is the speed of light. In SuperKEKB, γ is 7843 and 13725 in the LER and HER respectively. The horizontal momentum is calculated as given in eq. 5.4, where ε x is the horizontal emittance and β x is the horizontal β function at the scattering position. In the Touschek process, one particle loses and the another gains energy. The particles are lost if their relative energy variation ∆E exceeds the energy acceptance of the machine. The energy acceptance of SuperKEKB is 1-1.5%, thus if a positron gains or loses more than 0.06 GeV or an electron gains or loses more than 0.1 GeV, they will be lost from the beam. The loss rate of Touschek is proportional to the square of the beam current and the inverse of the beam size. ∆E = γ × p x (5.2)

E = γmc 2 (5.3) p x p = ε x β x (5.4)

Coulomb scattering

Coulomb scattering is the scattering of the charged particles of the bunch on the nucleus or the atomic electrons of the residual gas atoms. Coulomb scattering changes the direction of the beam particle (momentum) and thus the particle will be lost due to this transverse kick. The Coulomb scattering and the resulting losses are proportional to the beam current and the vacuum level (similar to Beam-gas Bremsstrahlung). Unlike Bremsstrahlung, simulations reveal that the Coulomb scattering rate is ≈100 times higher at SuperKEKB than at KEKB due to smaller beam pipe aperture at the interaction region and larger β functions at SuperKEKB [START_REF] Nakayama | Beam background and MDI design for SuperKEKB/BELLE-II[END_REF].

Simulation of single beam losses in phase 1

A version of SAD was used to simulate the single beam losses (Beam-gas Bremsstrahlung, Touschek scattering and Coulomb scattering), consisting of two main parts, one which generates the single beam backgrounds and tracks them and produces files with the tracking results, and the other which analyses the tracking files and calculates the loss rates [START_REF] Ohnishi | Private Communication[END_REF]. Each main part calls a group of libraries responsible for different tasks (definition of the physical apertures, definition of the physics of the backgrounds, e.g. cross sections or cuts in the phase space, definition of the mechanism of tracking, especially the scattering and observations points, definition of the calculation process of the loss rates, ... ). This code was used to calculate the loss rates of the three different single beam backgrounds in the drift where we place our diamond sensors in the LER and to get the dynamics of the tracked particles of the three processes (energies, positions in the vacuum chamber, angular distributions, etc ..) after the quadrupole QKBLP just before the drift, to be used then to study the signal in the DS using GEANT4. In chapter 3, I showed that the optimal position of the DS in the LER is at 13.9 meters downstream of the IP. However, GEANT4 showed a uniform loss of the Bhabha particles all over the drift and thus the diamond sensors will be placed at 11.9 meters downstream of the IP. Below, I will discuss the simulations for the three different single beam loss processes.

Beam-Gas Bremsstrahlung

Modifications were performed to the code to obtain reliable results in the context of our study.

Case a: The scattering points in the standard version of the code are at the entrance of quadrupoles, except the final focus quadrupoles for phases 2 and 3 and some special vertical bending magnets (only 6 of them in the lattice). The observation point is at the entrance of each element. As a first attempt to study the lost particles in the drift of the DS, the code was used without modification. We expected having a whole spectrum of Bremsstrahlung positrons lost in the drift since Bremsstrahlung scattering takes place not only at the IP but in the entire ring. The energy spectrum of the lost Bremsstrahlung positron in the case of no modifications in the code is presented on l.h.s of fig. Case b: Small modifications to the code were implemented including scattering at

The Touschek process

In the Touschek process, one particle loses energy and the another one gains energy and the energy variation corresponds to the relation given in eq. 5.2, which relates directly to the β function and the emittance. The Touschek scattered positrons which are lost in the drift are scattered at a maximum distance of 20 meters upstream from the IP, and they can lose or gain a maximum of 7% of their energy (l.h.s of fig. 5.15). The particles which gained energy exit the beam pipe on the high energy side (-x) with a very shallow horizontal angle, whereas the particles which lost energy exit the beam pipe on the low energy side (+x) (r.h.s of fig. 5.15).

The theoretical estimations of the energy variation of the particles due to Touschek scattering (eq. 5.2) are consistent with what we get from the simulation. The profile of the horizontal β function is given in fig. 5.16 between 50 meters upstream of the IP and the IP. The percentages of the energy variations calculated from the horizontal β function at the scattering position and the horizontal emittance of the beam (ε x = 1.95 × 10 -9 m) are shown in fig. 5.17, where energy losses between 4% and 7% are estimated in the region from 20 to 5 meters upstream of the IP, consistent with the simulation.

Since the Touschek particles exit the beam pipe on both sides in the horizontal plane, they will produce charged secondaries mainly in the same plane after having traversed the material of the beam pipe (l.h.s of fig. 5.18). Each exiting charged secondary corresponds to one incident particle, nevertheless some incident particles are absorbed totally in the After having calculated the signal in the DS, the losses from Touschek scattering for different beam currents could be calculated. The results show a quadratic behaviour

Coulomb

Coulomb scattering is an elastic scattering between the particles of the beam and the nuclei of the residual gas atoms. Beam particles will get a transverse kick while keeping their initial energies. The Coulomb scattered positrons at positions up to ≈ 80 meters upstream of the IP will be lost in the drift due to their transverse angles in the horizontal and vertical planes (fig. 5.21). The Coulomb scattered positrons are lost mainly in the vertical plane (fewer particles are lost in the horizontal plane), and this can be observed clearly from the distribution of the charged secondaries which are the result of the in-teraction of the lost incident positrons with the material of the 6 mm Cu cylinder beam pipe (fig. 5.22). The transverse offset of particle at a given loss position (B) due to a kick at an emission point (A) is proportional to the transfer matrix elements R12 for the horizontal plane and R34 for the vertical plane, between A and B, which can be expressed as √ β A β B sin(φ AB ), where β A and β B are the transverse β functions at the emission and loss positions respectively. Due to the fact that β x and β y are about equal at the loss position (in the drift) (fig. 5.23), then the reason why Coulomb scattered positrons are lost in the vertical plane is that β y is larger than β x at the positions of scattering. Fig. 5.24 shows the distribution of the scattering positions of the Coulomb positrons lost in DS in the drift, it shows as well a peak at ≈ 80 meters upstream of the IP where the β y is larger than β x . Since our DS is placed in the horizontal plane at the low energy side (+x) and the Coulomb loss is mainly in the vertical plane, GEANT4 simulations reveal that we will have no signal from Coulomb scattering in the DS. Measurements of Single Beam Loss at Phase 1

S_scattering(m) -80 -60 -40 - 20 0 angle_x(mrad) -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 h1

Experimental configuration

To be able to measure the single beam losses at phase 1, a complete experimental setup was prepared and installed in both rings of SuperKEKB, consisting of:

• Mechanical setup • Sensors and readout

Mechanical setup

The mechanical setup consists of pillars in each ring. Each pillar supports a movable plate with two sCVDs each connected to a CIVIDEC charge amplifier, and a fixed plate with the ZDLM monitors. The movable plate is associated to a remotely controlled motor to scan along the vertical direction over a range of 2.5 cm. In the HER, the pillar is installed 30 meters downstream of the IP and the sensors are facing the exit of the antechamber in the drift (fig. 6.1). The pillar in the LER is installed at 11.9 meters downstream of the IP and the sensors are facing a normal cylindrical beam pipe in the drift (fig. 6.2). Both pillars are fixed on the ground with screws and contain one box each for cable interconnections.

Sensors and readout

For phase 1, four sCVDs connected to CIVIDEC charge amplifiers were installed, two of them in the LER (l.h.s of fig. 6.3) and two others in the HER (r.h.s of fig. 6.3). The signals from the sCVDs were sent to rack F-8 on the 2 nd floor of the Electronic Hut (E-Hut) of the Belle II experiment, by rigid half inch heliax cables connected at different stages with special connectors (fig. 6.4).

During phase 1, the Belle II detector with its E-Hut were rolled out from the interaction region (fig. 6.5) with the entire detector 13 meters to the side of the IP. Additional cables were used to connect the sensors to the electronic rack in the E-Hut. During phases 2 & 3, the Belle II detector will be rolled in and these extra cables will be removed. The electronic rack for phase 1 (fig. 6.6) was equipped with a special "RF SYNCHRO" crate to synchronize the future DAQ sampling clock to the RF clock, a 10 bit Keysight oscilloscope (2.5 GHz BandWidth and up to 20 GSPS sampling), a PC serving as gateway to the Linux operated Belle-II DAQ computers, a Windows based server on which analysis using MATLAB was performed, a high voltage power supply for the diamond sensors (400 V) and a low voltage power supply for the charge amplifiers (12 V).

Two signal acquisition procedures were implemented, either by continuous 10 ms records that could be processed on-the-fly every 5-6 seconds or up to fifty 410 ms long records spaced by few seconds, where the processing took place afterwards and lasted for a few hours. The raw data acquired by the Keysight oscilloscope were in ".H5" binary data format. These data were saved first in the hard disk of the oscilloscope and then copied to folders on the server. The copied ".H5" files were afterwards converted to ".mat" files and processed using MATLAB. The processing functions were built to search for the waveforms of the DS signals. Once the waveforms were defined, the maxima and their positions in time were extracted, as well as the FWHM of each waveform. In addition, Figure 6.2 -The pillar in the HER at 11.9 meters downstream of the IP holding the two sCVDs and the ZDLM monitors we created our own EPICS archive to save records of the machine parameters (like beam currents, vacuum pressure, beam size, time, ...) and we merged the data from the DS to the EPICS data after each acquisition. The time reference we used was given by the Belle II DAQ network (bdaq network). For our data to become accessible to the BEAST II working group of the Belle-II collaboration, we created a table with all the data recorded in phase 1. A subset of this table is processed to obtain a ROOT n-tuple and is included in the BEAST II ROOT n-tuples.

Single beam losses in the sCVD in phase 1

We aim from the measurement of the single beam losses in phase 1 to verify the simulations by comparing their results to the data we acquire. The verification of the simulation is necessary to extrapolate to phase 2, in order to have an idea about the amount of single beam losses we will have in the DS compared to the radiative Bhabha signal, as well as Figure 6.3 -The two sCVDs connected to a charge amplifier each to be installed in the LER on l.h.s and in the HER on r.h.s confirm the detector acceptance and efficiency.

During the phase 1 commissioning, we took data mainly during vacuum scrubbing in both rings. On the oscilloscope, we connected two channels to signals from diamond sensors in both rings, e.g. in combinations such as: (LER1,LER2), (LER2,HER2), (HER1,HER2), where LER and HER correspond to the sCVDs installed in the LER and HER, and 1 and 2 correspond to the sCVD closest and farthest from the IP, respectively. On the third channel of the oscilloscope we connected the revolution signal on which we trigger. The revolution signal is a signal given by the accelerator which corresponds to one turn of the beam in the ring (≈ 3 km). The frequency of the revolution signal in SuperKEKB is about 100 KHz.

Losses as a function of different parameters

During the acquisition of signals from both rings, we noticed that we have much more signal in the LER than in the HER for similar beam currents. This can be clearly seen At SuperKEKB, a top-up injection is considered where the beam is injected continuously at a frequency of 50 Hz. One of the important background sources is thus the injection noise [START_REF] Funakoshi | presentation "some considerations on beam background after beam injection[END_REF]. The injected particles oscillate at an amplitude larger than that of the stored particles. If the injected particle oscillates outside the dynamic aperture, it will be lost after 10 to 1000 turns at some point in the ring. During phase 1, the injection noise was detected in our diamond sensors after each injection and it represented a large signal since several injected particles may be lost at once at the same place. This injection noise could be very high and cause a significant dead time in the acquisition of some of the Belle II sub-detectors, by causing electronics channels to go into saturation or large event sizes.

In HER, diamond sensors are located at 30m downstream of the IP, on the high energy side, to measure the Bhabha photons produced at the IP. The longitudinal extent of the spot size of the Bhabha photons hitting the inner wall of the beam pipe comes from the angular distribution of the beam particles (0.4 mrad). During phase 1, we mainly detected Bremsstrahlung photons which were produced in a region delimited by the last and the first bending magnets around the IP. This effective region represents 21.5 meters. Since the beams were not focused, the angular distribution of the photons is determined by the Bremsstrahlung process, whose characteristic emission angle is of the order of 1/gamma (0.07 mrad) [START_REF] Taylor | The Schiff angular bremsstrahlung distribution from composite media[END_REF]. Only the photons produced in the straight section at an angle larger than 4.5 times the characteristic angle can reach the sensors at 30 m, as well as a few photons produced in the bends. A preliminary study has estimated that the fraction of Bremsstrahlung reaching the HER 30 m section represents 10 -4 of the amount of Bremsstrahlung produced in all the HER. In LER the fraction of the Bremsstrahlung losses at 11.9 m section represents ≈ 6 x 10 -3 of the total Bremsstrahlung losses in the ring. From this we can expect a large factor (>50) between LER and HER signal rates.

Figure 6.7 -An image of the oscilloscope during the acquisition with (LER2(yellow), HER2(green))

Signals in the sCVD

1 MIP corresponds to 2.9 fC generated charge in the 4x4 mm 2 metallised part of the 500µm thick sCVD. The signal is then amplified by the charge amplifier (Gain 4 mV/fC) to 11.6 mV. The energy deposited in the sCVD follows a Landau distribution, and this was simulated with GEANT4 (l.h.s of fig. 6.8) and confirmed by data (r.h.s of fig. 6.8).

However, there has been observed a difference between the simulation and the data at the level of the Landau fit. The limits of the Landau fit were considered from 6 mV to 200 mV in both cases, and the MPV (Most Probable Value) parameter was set to 8 mV. After performing the fit, the MPV of the Landau distribution in the measurements (11.6 mV) (r.h.s of fig. 6.8) was observed to be 1.26 times greater than that in the GEANT4 simulations (9.2 mV). Therefore, to have a reasonable comparison between the data and the simulation, the factor 1.26 has been taken into account to define the thresholds.

The separation between the noise from the charge amplifier in the measurements was observed at 6 mV. Thus the threshold taken for the data during analysis was 6 mV, and the threshold for the simulation was taken at 4.75 mV.

Losses as function of current and pressure

The data analysis consisted in finding the number of peaks during a certain acquisition time which represents a sequence of sub acquisitions of 10 ms each, considering the threshold at 6 mV, and plotting this number as a function of different beam parameters (current, vacuum pressure,vertical beam size).

After analysis of the data, we can see that the losses (l.h.s of fig. 6.9) in the sCVD clearly follows the beam current profile (r.h.s of fig. 6.9) during the phases of injection and storage of the beam. The variation of the total loss rate in the sCVD per 10 ms given as a function of the beam current (l.h.s of fig. 6.10) shows a quadratic behaviour due to Touschek losses (chapter 5) and the variation as a function of vacuum pressure shows a linear behaviour(r.h.s of fig. 6.10). Figure 6.9 -Loss rate in the sCVD in 10 ms on l.h.s and beam current in the LER on r.h.s as function of time Figure 6.10 -The loss rate in 10 ms in the sCVD in LER as a function of beam current on l.h.s and as a function of vacuum pressure on r.h.s

Data from Cherenkov in LER

To compare the data from the sCVDs to that from the ZDLM monitors, we acquired data from the Cherenkov sensor using our Keysight oscilloscope. Since we have two channels dedicated for detector signals, we connected one sCVD in the LER to channel 1, and the Cherenkov detector in the LER to channel 2 of the oscilloscope. The threshold considered for the data taking and the analysis of the Cherenkov signals was 30 mV. The distribution of the peak values of the collected events is presented in fig. 6.11. The data was taken during the vacuum scrubbing in the LER, where the average current was 703 mA and the average vacuum pressure was 7.6 nTorr. From the loss distributions shown in fig. 6.12, we can observe clearly that the losses in the Cherenkov detector (1541 in 10 ms, red) are ≈ 11 times higher than those in the sCVD (136 in 10 ms, blue) under the same beam conditions. This factor is smaller than the ratio of the active areas of the sensors, but must also depend on the distances of the sensors from the beam pipe and the efficiency of the chosen threshold to define signals. Figure 6.12 -The loss distributions in 10 ms in the sCVD (blue) and Cherenkov detector (red) for an average beam current of 703 mA and average vacuum pressure of 7.6 nTorr in the LER

Losses from vacuum bumps in the LER

Data were also acquired during dedicated BEAST II studies. On the 23 rd of May, a study was dedicated to the effects of vacuum bumps in the LER, during which several vacuum bumps were introduced in different sections around the IR. I analysed the data trying to separate the contributions from the losses due to Touschek and Bremsstrahlung in the sCVDs, but I noticed that the data taken during this study are not easy to analyse because the exact profile of the pressure along the section around the IP is not well known.

The map of the vacuum sections is given in fig. 6.13. During the study of the vacuum bumps, each time the NEG (Non-Evaporable Getters) in some defined subsections are heated to a certain temperature, to increase the vacuum pressure locally. Before each vacuum bump study, the beam is aborted and vacuum workers go into the tunnel to unplug the NEGs belonging to the corresponding subsections.

The distances from the IP of the subsections in which vacuum bumps took place are shown in table. 6.1. The bumps took place in the D02 section upstream of the IP in different subsections. The average pressure in the D02 section during the vacuum bumps in the subsections (L26,L25) and (L24,L23) is shown on l.h.s of fig. 6.14, and the profile of the losses in the sCVD (LER2) resulting from vacuum bumps is shown on r.h.s of fig. 6.14. We can see that the vacuum bumps are clearly correlated with losses in the sCVD and this confirms that our sensors in the drift at 11.9 meters downstream from the IP are sensitive to Beam-gas Bremsstrahlung scattering up to 35 meters upstream of the IP (see fig. 8a I used data from the first two bumps to calculate the fraction of losses in the sCVD from the scattering regions where the vacuum bumps were performed and compare them Figure 6.14 -The profile of the average pressure at D02 section on l.h.s and the profile of the losses in the sCVD (LER2) on r.h.s during the successive vacuum bumps in the subsections (L26,L25) and (L24,L23) of the D02 section to the simulations. The fraction calculated is defined as f and is represented in equation 6.1, where loss bump represents the value of the loss in the sCVD at the maximum of the vacuum bump (r.h.s of fig. 6.14) and loss all-sections represents the total loss simulated in the sCVD when pressure is equal to that of the maximum of the bump everywhere in the ring. For each vacuum bump study, the bump was performed at two consecutive subsections. So to be able to calculate precisely the fraction given in equation 6.1, I needed to consider the average value of the vacuum bumps at the consecutive subsections (fig. 6.17, 6.18). The results are represented in table 6.2 and the fractions given by the data and that given by the simulation match for the two vacuum bumps at the two different positions. Table 6.2 -Comparison between data and simulation of the fraction of losses in the sCVD coming from the different sections during the vacuum bump

Losses for different vertical beam sizes

Another BEAST II study performed during the phase 1 commissioning of SuperKEKB was called Touschek study. Several days were dedicated to this study, both for the LER and HER. We were taking data in both rings but only data from the LER are analysed.

The aim of this study is to extract the contribution of the Touschek losses to the total losses by changing the vertical beam size σ y . The beam size was increased by varying the value of the ECK (Emittance Control Knob), which consists in changing the field of the skew quadrupole component of the chromaticity correcting sextupoles, inducing a coupling of the horizontal emittance into the vertical plane in the ring, thus increasing the vertical beam size.

During the study, beam loss data were collected for six different beam currents in the LER, I = 180, 360, 540, 720, 900 and 1000 mA (l.h.s of fig. The data from the sCVD (LER2) were analysed during this study. Losses observed in the sCVD result from both Beam-gas Bremsstrahlung and Touschek processes. For fixed values of the current (which also maintains more or less constant vacuum pressure) and varying the vertical beam size, the contribution of the Beam-gas Bremsstrahlung can be expected to remain constant while that of Touschek scattering should vary. Fitting the losses in the sCVD as a function of the inverse of the vertical beam size (σ -1 y ) allows separating both contributions in the following way. For σ -1 y close to zero, which means σ y is very large, the losses from Touschek scattering should vanish and thus all the losses in the sCVD should be from Beam-gas Bremsstrahlung. The contributions from Bremsstrahlung at each beam current are thus extracted from the y-intercept of the fits represented in fig. 6 Touschek scan from 21 June @ I=720 mA Figure 6.21 -Fit of the losses in the sCVD as a function of the inverse of the vertical beam size at 540 mA on l.h.s and at 720 mA on r.h.s

Losses from beam-gas Bremsstrahlung

The losses from Bremsstrahlung extracted from the fits (table. 6.3) are compared to the simulation as a function of the product of the beam current and the vacuum pressure in the LER and are represented on l.h.s of fig. 6.23. The fit of the losses from Bremsstrahlung is linear in both data and simulation (l.h.s of fig. 6.23), but we can see that there is a factor between the two fits, where the data is always greater than the simulation by about 3 (r.h.s of fig. 6.23). This factor is explained by the fact that the measured value of the pressure from the CCG (Cold Cathode Gauge) should be multiplied by a factor 3 to properly represent the actual CO equivalent pressure in the center of the vacuum chamber [66]. 6.4 -The slopes of the fit of the relative losses in the DS as a function of relative inverse vertical beam size, along with their errors for six different beam currents in the LER tween data and simulations, the slopes of the fits in figs. 6.20-fig. 6.22 were extrapolated quadratically to the same current (1000 mA). The weighted arithmetic mean of the extrapolated slopes was then computed along with the errors, according to eq. 6.2 and eq. 6.3, respectively. The result of this calculation is Touschek data = (1530±172)× 1 σy .

A simulation was performed at a beam current of 1000 mA, for different beam sizes.

To vary the beam size, the value of the x-y coupling was changed which will vary the value of the vertical emittance and thus the vertical beam size. The result of the losses from Touschek scattering as a function of the inverse vertical beam size at 1000 mA is shown in fig. 6.28. The result of the fit gives Touschek simu =(2623±508.6)× 1 σy .

We observe a factor of simu data =1.7±0.37. Trying to investigate the origins of this factor, the longitudinal beam size used in the simulation (σ z =6 mm) was checked to be the same as that measured in SuperKEKB. Another possible source of discrepancy could be the vertical beam size itself, but it is believed that there is no significant error in the measurement of the vertical beam size, because it is consistent with the estimated value using the modelling, based on the level of residual mismatch observed in the measurements of optical functions in the LER.

The Touschek losses are proportional to the product of the inverse transverse beam sizes ( 1 σx × 1 σy ), where σ x = √ ε x × β x and σ y = ε y × β y , ε x and ε y are the transverse emittances and β x and β y are the transverse β functions. In the simulation, the transverse emittance ratio k= εy εx was varied, thus the simulation results are proportional to 1 εx . Another possible explanation for the factor of 1.7 between data and simulation could therefore be that the horizontal emittance in the simulation is 1.7 times smaller than in reality in the LER ring. Reliable measurements of the horizontal beam size at SuperKEKB are unfortunately not available to check this hypothesis. We can also not completely exclude that some part of this discrepancy comes from in the simulation itself. The good qualitative and quantitative agreement between our simulations and the measurements enabled us to estimate the level of background in the DS in phase 2. The same simulations performed for phase 1 have been carried for phase 2, considering the LER lattice with the final focus system. During phase 2, the collimators will be used to mitigate backgrounds before they reach the Belle II detector.

Since we are interested in the ratio of the radiative Bhabha signal to the background signal in the DS in phase 2, the normal beam pipe of phase 1 was considered instead of the window at 45 • , which will be installed in the drift at 11.9 meters in the LER for phase 2.

Beam-gas Bremsstrahlung

For the simulation of Beam-gas Bremsstrahlung using the SAD code, a vacuum pressure of 1 nTorr and a total beam current of 1 A were considered, which correspond more or less to SuperKEKB parameters foreseen in the LER during phase 2. A Bremsstrahlung loss rate of 660 particles per second in the DS was estimated. This loss rate corresponds to ≈ 68% of the loss rate in the DS in phase 1 under the same vacuum pressure and beam current. This is due to the different optics in the IR with the final focus insertion.

Touschek scattering

With the phase 2 lattice and a beam current of 1 A in the LER, the Touschek loss rate in the DS was estimated to be 118 particles per second, this loss rate corresponds to ≈ 4.9% of the Touschek loss rate in the DS in phase 1 for the same beam current.

Signal to Noise in the LER in phase 2

The loss rate of the radiative Bhabha in the DS in phase 2 for L=10 34 cm -2 s -1 with the normal beam pipe is estimated to be 225000 particles per second. The total loss rate of Touschek scattering and beam-gas Bremsstrahlung is 778 particles per second. This means that the background level resulting from single beam losses is expected to be around two orders of magnitude smaller than the rate of the radiative Bhabha scattering signal during luminosity monitoring. This satisfies the conditions specified to achieve a high relative precision on the luminosity measurement.

Chapter 7

Conclusions and perspectives

Fast luminosity monitoring is essential in the presence of dynamical imperfections, for feedback and optimization. The aim of this thesis was to design and to implement an instrument for the fast luminosity measurement at SuperKEKB to correct many instabilities of the beam and thereby to optimize it. The target relative precision is 10 -3 in 1 ms. The process studied is the radiative Bhabha scattering at zero photon scattering angle, which has a large cross-section. The technology used is based on diamond, which is radiation hard and provide a fast signal, located just outside the vacuum chamber downstream from the IP where the counting rate is sufficient to obtain the required precision and where the contamination from background processes is minimal.

Generation of the Bhabha particles has been performed using GUINEA-PIG++ and then tracked at several locations in the LER and HER of SuperKEKB using SAD. Thanks to the tracking results, the optimal position of the diamond sensors in the LER at which high Bhabha event rates could be detected, was chosen to be in the drift at 11.9 meters downstream from the IP to measure the signals from the interaction of the Bhabha positrons with the material of the beam pipe. On the other hand, for the HER, good candidate positions to measure the Bhabha electrons were not found. Therefore, studies in the HER indicate that it is better to measure signals from the Bhabha photons, and for this purpose the position of the diamond sensors was chosen to be at 30 meters downstream from the IP. GEANT4 simulations have been performed considering different designs of the vacuum chamber in the LER at 11.9 meters, and results show that the normal 6 mm thick cylindrical beam pipe should be replaced by a window at 45 • , for better detection of the electromagnetic shower and to achieve the aimed precision of 10 -3 in 1 ms.

Characterisation tests of a thinned diamond sensor (4x4 mm 2 , 140µm) and of a charge amplifier (with shaping time ≈ 10 ns) took place in the clean room at LAL using radioactive sources. The data from the tests have been analysed with MATLAB and root. Analysis results are consistent with the parameters of each of the tested elements.

A study of the single beam loss processes during phase 1 of the single beam commissioning at SuperKEKB has been performed. Simulations of beam-gas Bremsstrahlung, Touschek and Coulomb scattering were performed using a SAD code written at KEK.

Several modifications have been applied on the code to ensure reliable results. Afterwards, the losses from the three processes have been studied and calculated as a function of several parameters (beam current, vacuum pressure, beam size) in the LER. Simulation results indicate that at 11.9 meters in the LER, the diamond sensor is only sensitive to losses from beam-gas Bremsstrahlung and Touschek scattering. The signal in the diamond sensor from these two processes has been then studied by GEANT4.

The mechanical setup has been installed in December 2015 in both rings. Two diamond sensors connected to charge amplifiers were installed in each ring in the horizontal plane on the low energy side. Tests with radioactive sources were performed in situ to check the entire setup. By the end of February 2016, a complete readout system was installed and data taking lasted to the end of the phase 1 commissioning of SuperKEKB (end of June 2016). Data were acquired during vacuum scrubbing and also during dedicated studies of the single beam losses. Along with that, I participated in operational shifts in the control room of SuperKEKB, where interesting beam dynamics studies took place.

Data from single beam losses at phase 1

The analysis of the data taken during phase 1 has been performed using MATLAB, the results of the analysis are summarised in the following:

• Vacuum bumps in the LER: During vacuum bumps performed in the vacuum section upstream from the IP, loss bumps in the diamond sensors were observed and analysis results are consistent with the simulation, on the sensitivity of the diamond sensor at 11.9 meters to the scattering positions.

• Touschek study in LER: During this study, six beam currents have been considered and the vertical beam size was varied about five times for each beam current. From the analysis of data taken during this study, the contributions of the beam-gas Bremsstrahlung and the Touschek scattering to the total loss rate could be extracted. The results of beam-gas Bremsstrahlung is consistent with the simulation, whereas concerning Touschek scattering, a factor of 1.7 larger rate was observed in simulation compared to data. Parameters like the vertical emittance and the longitudinal beam size, which could affect the Touschek rate, may explain this factor, have been checked in the simulation and appear to be consistent with the data. Only the horizontal emittance could not be confirmed experimentally. The qualitative and quantitative agreement of our simulations and the measurements of single beam losses in phase 1 enabled us to estimate that the background level from the single beam losses will be two orders of magnitude smaller than the rate of radiative Bhabha signal during luminosity monitoring in phase 2. It also confirmed the modelling of the efficiency of the detector, thereby validating the estimated signal rates and luminosity precisions.

Vacuum chamber design and installation

A 6 mm thick Cu window at 45 • will replace the normal 6 mm thick Cu cylindrical beam pipe for the data acquisition of the phase 2 of SuperKEKB. The installation of the window will start by the end of 2017. This new design requires some modifications to adapt the mechanical setup used during phase 1.

Precision on luminosity

Thanks to the window at 45 • and a radiator, we should be able to achieve a precision of 3.7×10 -3 for phase 2 at L=10 34 cm -2 s -1 and a precision of 1.2×10 -3 for phase 3 at L=8×10 35 cm -2 s -1 .

Readout electronics

For phases 2 and 3, the oscilloscope will be replaced by a 10 bits ADC (Analog-to-Digital Converter) connected to an FPGA programmed to calculate the train integrated luminosity and the bunch integrated luminosity, and followed by a 16 bits DAC to provide these luminosities as analog output for feedback use.

In phase 2, for L=10 34 cm -2 s -1 , there won't be a luminosity signal each bunch crossing in the DS, thus the 500 µm DS with the charge amplifier used in phase 1 will be fast enough to do fast luminosity monitoring. However, in phase 3, for a luminosity up to 8×10 35 cm -2 s -1 , the DS will have signals from successive bunches, separated only by 4 ns. In this case, different schemes could be adopted for the fast luminosity monitoring. Since we have two channels in each ring, we may use one channel for train luminosity (L τ ) monitoring, and the second for bunch luminosity (L B ) monitoring.

For L τ monitoring, two schemes could be considered, either keeping the same scheme as in phase 2 with the 500 µm DS and the charge amplifier (σ ≈ 10 ns) while considering a different data processing in the presence of pile-up, or using a thinner DS (140 µm) connected to either a faster charge amplifier (σ ≈ 2-3 ns) or to a voltage amplifier which gives a signal as fast as the DS (σ ≈ 2-3 ns). The voltage amplifier has a higher noise compared to charge amplifier, however this should not be a problem for phase 3 since several MIPs will be intercepted in the DS at once, resulting in larger signals.

For L B monitoring, it is not necessary to have a precision as high as for L τ monitoring. In this case, considering the same scheme as phase 2 could be good enough by moving one channel away from the window, to decrease its signal level, which is acceptable given the reduced precision requirement.

Concerning the data collected in phase 1 in HER, it will be analysed to calculate the rates of the single beam losses in the DS for different beam parameters. For phase 2, a GEANT4 simulation should be performed considering the geometry of the antechamber where we place the DS in the HER (30 meters downstream of the IP) to study the signals of the radiative Bhabha photons in the DS and define the reachable precision. On the other hand, a detailed simulation of the single beam losses during phase 2 should be performed considering the geometry of the window at 45 • in the LER.

From the experience during phase 1, data acquisition codes for phase 2 using the FPGA should be written and tested in advance before the start of the phase 2 commissioning, not to lose time when the beams are circulating and the collisions are performed. Once the shape of the output signals is confirmed, processing and analysis codes should be prepared to enable an online analysis of the data to be sent to the lock-in amplifier each 1 ms to enable a feedback based on the dithering algorithm.

Aluminium

The secondaries exit the beam pipe uniformly whatever the thickness of the beam pipe. For a thickness of 6 mm (fig. 8.14a), the number of exiting charged secondaries increases progressively until 50 cm from the exit of the quadrupole QKBLP where it stabilizes, the reason lies behind the absorption of the shower due to the shallow exiting angle of the Bhabha positrons and the large thickness of the beam pipe.

Precision of L evaluation

The best precision is obtained also for a 1 mm thick Titanium beam pipe and with a DS at 90 • (2% in phase 2 (l.h.s of fig. 8.17) and 6.3×10 -3 in phase 3 (r.h.s of fig. 8.17)). The precision is 5 times better in the case of the window and the radiator compared to 1 mm thick Ti beam pipe.

List of Tables

Figure 2 . 1 -

 21 Figure 2.1 -Luminosity at SuperKEKB compared to other colliders

Figure 2 . 3 -

 23 Figure2.3 -The final focus Superconducting magnet system at SuperKEKB[START_REF] Ohuchi | Design of the superconducting magnet system for the SuperKEKB interaction region[END_REF]

Figure 2 . 4 -

 24 Figure 2.4 -The RF gun on l.h.s and the photocathode on r.h.s, at SuperKEKB

Figure 2 . 5 -Figure 2 . 6 -

 2526 Figure 2.5 -The scheme of positron source on l.h.s and the large aperture S-band accelerating structure at SuperKEKB on r.h.s

Figure 2 . 7 -

 27 Figure 2.7 -The layout of the damping ring at SuperKEKB

Figure 2 . 8 -

 28 Figure 2.8 -The layout of the SuperKEKB injector linac

Figure 2 . 11 -

 211 Figure 2.11 -The layout of the Belle II detector

Figure 2 . 12 - 1 Figure 2 . 13 -

 2121213 Figure 2.12 -The layout of the BEAST II in the interaction region of SuperKEKB at phase 1

Figure 3 . 1 -Figure 3 . 2 -

 3132 Figure 3.1 -Example of Ante-chamber asymmetric dithering coil at SuperKEKB

Figure 3 . 3 -Figure 3 . 4 -Figure 3 . 5 -Figure 3 . 6 -

 33343536 Figure 3.3 -The dithering parameters on the luminosity profile

Figure 3

 3 Figure 3.7 -The diamond lattice

Figure 3 . 8 -Figure 3 . 9 -

 3839 Figure 3.8 -The readout circuit on l.h.s and the operation scheme of the diamond sensor on r.h.s

Figure 3 . 10 -Figure 3 . 12 - 64 Figure 3 . 14 -

 31031264314 Figure 3.10 -The experimental setup of the tests on the diamond sensor in the clean room

Figure 3 . 15 -

 315 Figure 3.15 -Landau reconstruction of 140 µm sCVD on l.h.s and noise reconstruction of the charge amplifier on r.h.s in case of triggering on all events in the DS

Figure 3 .Figure 3 . 19 -

 3319 Figure 3.18 -Landau reconstruction of the signal from 500 µm sCVD on l.h.s and the width of the signal of the 500 µm sCVD on the r.h.s in the case of multistage trigger

Figure 3 . 20 -

 320 Figure 3.20 -Difference of the position of the maximum of the diamond and scintillator

Figure 3 . 21 -

 321 Figure 3.21 -The maximum of the signal as a function of its position in time in the sCVD on l.h.s and as function of its width on r.h.s

Figure 3 .Figure 3 . 23 -

 3323 Figure 3.22 -The PHIL accelerator design

Figure 3 . 25 -

 325 Figure 3.25 -Difference of the time position of the diamond and laser signals

Figure 3 .

 3 Figure 3.26 -Radiative Bhabha process at zero photon scattering angle

Figure 3 . 29 -

 329 Figure 3.29 -Angular distributions of Bhabha positrons in the horizontal plane on l.h.s and in the vertical plane on the r.h.s

Figure 3 .

 3 Figure 3.30 -Angular distributions of Bhabha electrons in the horizontal plane on l.h.s and in the vertical plane on r.h.s

Figure 3 . 3 -

 33 Figure 3.32 -Transverse distribution in x and y of beam positrons (blue) and Bhabha positrons (red) after third and fourth bending magnets

Figure 3 .Figure 3 . 34 -

 3334 Figure 3.33 -The horizontal position distribution of Bhabha positrons as a function of their energies, after third and fourth bending magnets

Figure 3 .

 3 Figure 3.35 -Beta functions and dispersion functions over first 27 m downstream the IP for the LER

Figure 3 .

 3 Figure 3.36 -The horizontal position distribution of Bhabha positrons as function of their energies after first and second bending magnets

Figure 3 .

 3 Figure 3.37 -The x-E distribution of the Bhabha positrons on l.h.s and the energy distribution of Bhabha positrons which are already lost (green), detectable in the DS (blue) and still remaining in the beam pipe (red) on r.h.s, in the drift at 13.9 meters

Figure 3 .

 3 Figure 3.38 -Energy spectrum on l.h.s and exiting angle distribution on r.h.s of the lost Bhabha particles in the drift after QKBLP

Figure 3 .

 3 Figure 3.39 -The 3D plot of the charged secondary particles resulting from the interaction of the exiting Bhabha positrons with the 6 mm thick Cu cylindrical beam pipe

Figure 3 .

 3 Figure 3.40 -The transverse angular distribution of the exiting charged secondaries from the 6 mm Cu beam pipe

Figure 3 . 41 -Figure 3 . 42 -Figure 3 . 43 -Figure 3 . 44 -Figure 3 . 45 -

 341342343344345 Figure 3.41 -Illustration of an electromagnetic shower of a Bhabha positron exiting the 6 mm Cu beam pipe and of the parallel and perpendicular positioning of the DS (violet)

Figure 3 . 46 -

 346 Figure 3.46 -The development of the shower in the radiator on l.h.s and the simulated window with an iron radiator at its shower maximum on r.h.s

  3.4 shows the precision on luminosity for different designs of the vacuum chamber and at different phases of SuperKEKB. The table shows that the window with the radiator is the best candidate to attain the aimed precisions at different phases.

Figure 3 .Figure 3 . 49 -

 3349 Figure 3.48 -The energy of the incident particles giving signal in the centred 5x5 mm 2 DS (blue) and with no signal (red) in the case of the window with a radiator

Figure 3 . 50 -Figure 3 . 51 -Figure 3 . 52 -Figure 3 . 53 - 3 . 4 .

 35035135235334 Figure 3.50 -The β and dispersion functions of the lattice of the HER up to 24 meters downstream of the IP E-x distribution of the bhabha electrons after bending magnets

Figure 3 . 54 -Figure 3 . 55 -

 354355 Figure 3.54 -The energy spectrum of Bhabha photons in the HER on l.h.s and their horizontal angle distribution on r.h.s

Figure 3 . 56 -Figure 3 . 57 -

 356357 Figure 3.56 -The lattice of the HER where the γ spot hits, showing the free spaces for the luminosity monitors

Figure 4 . 1 -

 41 Figure 4.1 -History of the vacuum scrubbing in SuperKEKB during phase 1, in the LER on l.h.s and in the HER on r.h.s

Figure 4 . 2 -

 42 Figure 4.2 -The position of collimators in SuperKEKB rings during phase 1

Figure 4 . 3 -

 43 Figure 4.3 -The physical vertical emittance in the LER during phase 1 after several corrections[START_REF] Funakoshi | Overview of Phase 1 Commissioning[END_REF]

Figure 4 . 4 -

 44 Figure 4.4 -Loss monitor system at SuperKEKB

Figure 4 . 5 -Figure 4 . 6 -

 4546 Figure 4.5 -PIN diodes at SuperKEKB, around the collimator [50] on l.h.s and near a bellow [50] on r.h.s

Figure 4 .

 4 Figure 4.7 -The electron current measured in the LER as a function of the beam current for different types of the beam pipe[START_REF] Suetsugu | First commissioning of the SuperKEKB vacuum system[END_REF]

Figure 4 . 8 -

 48 Figure 4.8 -The non-linear pressure rise at nine different gauges at the arc section as a function of beam current in LER

Figure 4 . 9 -

 49 Figure 4.9 -A solenoid magnet on l.h.s and a permanent magnet on r.h.s at an aluminium bellow at SuperKEKB

Figure 5 . 2 -

 52 Figure 5.2 -Bremsstrahlung scattering of a charged particle in the field of the nucleus of an atom

Figure 5 . 3 -

 53 Figure 5.3 -Touschek scattering in the CMS[START_REF] Lee | Accelerator Physics" second edition, Chapter 4 "Physics of electron storage rings" section II "Beam lifetime[END_REF]

  5.4. The spectrum is discontinuous, and the reason behind this discontinuity is the poor resolution of scattering points. The r.h.s of fig. 5.4 shows the energy of the lost particles in the drift as a function of their scattering position. The discontinuity of the scattering positions on

Figure 5 . 4 -

 54 Figure 5.4 -The energy spectrum of Bremsstrahlung positrons lost in the drift on l.h.s and their energy as a function of their scattering position on r.h.s in case a (scattering at the entrance of quadrupoles and special vertical bends)

Figure 5 . 5 -

 55 Figure 5.5 -The LER lattice from 50 meters upstream to 11 meters downstream from the IP

Figure 5 . 6 -Figure 5 . 8 -Figure 5 . 9 -

 565859 Figure 5.6 -The energy spectrum of Bremsstrahlung positrons lost in the drift on l.h.s and their energy as a function of their scattering position on r.h.s in case b (scattering at the entrance of all elements)

Figure 5 . 10 -

 510 Figure 5.10 -The energy of the lost Bremsstrahlung positrons in the drift as a function of their loss position on l.h.s and as a function of their horizontal exiting angle on r.h.s

Figure 5 . 11 -

 511 Figure 5.11 -The 3D distribution of the charged secondaries exiting the 6 mm thick Cu beam pipe on l.h.s and their energy spectrum on r.h.s

Figure 5 . 13 -

 513 Figure 5.13 -Histogram of number of secondaries deposited in the DS by each incident particle

Figure 5 . 14 -

 514 Figure 5.14 -The behaviour of the losses detected in the DS as a function of the product of beam current and vacuum pressure

Figure 5 . 15 -

 515 Figure 5.15 -The energy of the lost Touschek positrons in the drift as a function of their scattering position on l.h.s and as a function of their exiting horizontal angle on r.h.s

Figure 5 . 16 -

 516 Figure 5.16 -The β function in the LER between 50 m upstream the IP and the IP

Figure 5 . 17 -

 517 Figure 5.17 -The percentage of the energy variation due to Touschek scattering computed from eq. 5.2, between the scattering regions extending 50 meters upstream the IP and the IP

Figure 5 . 18 -

 518 Figure 5.18 -The 3D distribution of the charged secondaries resulting from the interaction of the incident lost positrons with the 6 mm Cu cylinder beam pipe on l.h.s and their number per incident particle on r.h.s

Figure 5 . 19 -

 519 Figure 5.19 -Histogram of number of secondaries deposited in the DS by each incident particle

Figure 5 . 20 -

 520 Figure 5.20 -The behaviour of the loss from Touschek scattering in 10 ms in the DS as a function of the beam current

Figure 5 . 21 -Figure 5 . 23 -Figure 5 . 24 -

 521523524 Figure 5.21 -The horizontal exiting angles on l.h.s and the vertical exiting angles on r.h.s, of the lost Coulomb scattered positrons in the drift

Figure 6 . 1 -

 61 Figure 6.1 -The pillar in the HER at 30 meters downstream of the IP holding the two sCVDs and the ZDLM monitors

Figure 6 . 4 -Figure 6 . 5 -

 6465 Figure 6.4 -Half inch heliax cables with the connectors on l.h.s and connections being done at B4 level in Tsukuba hall on r.h.s

Figure 6 . 6 -

 66 Figure 6.6 -Our electronic rack in the E-Hut of the Belle II detector of the SuperKEKB experiment

Figure 6 . 8 -

 68 Figure 6.8 -Landau distribution of deposited charge of incident particles from simulations in GEANT4 on l.h.s and from data on r.h.s

Figure 6 . 11 -

 611 Figure 6.11 -The distribution of the peak values of the signals collected by the Cherenkov sensor

Table 6 . 1 -

 61 in chapter 5). Another vacuum bump was performed between 35 Gauges Distance upstream of the IP (meters) Distances upstream of the IP of subsections where vacuum bumps took place in the D02 section and 70 meters upstream of the IP in the LER (l.h.s of fig.6.15). During this vacuum bump we did not notice any increase in losses in the sCVD. As can be seen on r.h.s of fig.6.15, sCVD losses are just similar to the beam current profile in the LER (fig.6.16). This confirms the simulation result in fig.8bin chapter 5, which shows that our sCVDs are not sensitive to Beam-gas scattering occurring beyond ≈ 23 meters upstream of the IP.

Figure 6 . 13 -

 613 Figure 6.13 -The vacuum sections overall the LER at SuperKEKB

17 Figure 6 . 15 -

 17615 Figure 6.15 -The profile of the average pressure at D02 on l.h.s and the profile of the losses in 10 ms in the sCVD on r.h.s during the vacuum bump between 35 m and 70 m upstream from the IP in the LER

Figure 6 . 16 -Figure 6 . 17 -Figure 6 . 18 -

 616617618 Figure 6.16 -The beam current profile in the LER during the vacuum bump between 35 and 70 meters upstream from the IP

  .20, fig. 6.21 and fig. 6.22.

Figure 6 . 19 -

 619 Figure 6.19 -Beam current on l.h.s and vertical beam size in the LER on r.h.s during the Touschek study

Figure 6 . 24 -

 624 Figure 6.24 -The relative loss in the sCVD as a function of the relative vertical beam size for the beam currents of 180 mA on l.h.s and 360 mA on r.h.s

Figure 6 . 25 -

 625 Figure 6.25 -The relative loss in the sCVD as a function of the relative vertical beam size for the beam currents of 540 mA on l.h.s and 720 mA on r.h.s

Figure 6 . 26 -

 626 Figure 6.26 -The relative loss in the sCVD as function of the relative vertical beam size for the beam currents of 900 mA on l.h.s and 1000 mA on r.h.s

6. 3 .Figure 6 . 27 -

 3627 Figure 6.27 -The relative loss from Touschek as a function of the relative inverse vertical beam size from simulations at I= 1000 mA

Figure 6 . 28 -

 628 Figure 6.28 -The simulated losses from Touschek scattering as a function of the inverse vertical beam size at I= 1000 mA

7. 2 .

 2 Implications for phase 2 113 7.2 Implications for phase 2 7.2.1 Signal to background ratio during luminosity monitoring

(Figure 8 . 12 -

 812 Figure 8.12 -Histograms of the number of charged secondaries in the DS per incident exiting Bhabha positron for different orientations of the DS at 1 mm thick Copper beam pipe

Figure 8 . 16 -

 816 Figure 8.16 -Dynamics of the secondaries exiting 6 mm thick Titanium beam pipe

Figure 8 . 17 -

 817 Figure 8.17 -Precision in 1 ms on the luminosity as a function of the thickness of the beam pipe for DS at 0 • (blue), DS at 45 • (red) and DS at 90 • (black) compared to the window + radiator (red spot) in phase 2 on l.h.s and in phase 3 on r.h.s

2 . 11

 211 The layout of the Belle II detector . . . . . . . . . . . . . . . . . . . . . . 2.12 The layout of the BEAST II in the interaction region of SuperKEKB at phase 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13 The list of sub detectors of BEAST II [32] . . . . . . . . . . . . . . . . . 3.1 Example of Ante-chamber asymmetric dithering coil at SuperKEKB . . . 3.2 Single crystalline diamond sensor (sCVD) on l.h.s and the Cherenkov and Scintillator detectors on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 The dithering parameters on the luminosity profile . . . . . . . . . . . . 3.4 The luminosity signal in the luminosity monitor from Monte-Carlo simulation at a given counting rate and dithering with modulation amplitude "p" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 The FFT of the square root of the measured luminosity profile with a peak at the double frequency (2×f 0 ) on l.h.s and a peak at the baseline frequency (f 0 ) on the r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 The profile of the signal size at p=0.2 and rate=400 KHz per 1 ms of the baseline frequency on l.h.s and the double frequency on r.h.s . . . . . . . 3.7 The diamond lattice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 The readout circuit on l.h.s and the operation scheme of the diamond sensor on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Landau distribution of energy deposited by MIPs in 500 µm diamond sensor simulated by GEANT4 . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 The experimental setup of the tests on the diamond sensor in the clean room 3.11 140 µm CIVIDEC diamond sensor on l.h.s and the CIVIDEC charge amplifier on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.12 Low voltage supply for the charge amplifier on l.h.s and high voltage supply for the diamond sensor on r.h.s . . . . . . . . . . . . . . . . . . . . . . . 3.13 The 90 Sr decay scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.14 Screenshots from the oscilloscope, pink: 140µm diamond and yellow: scintillator, diamond signal in the tail of Landau on l.h.s and diamond signal close to the maximum of Landau on r.h.s . . . . . . . . . . . . . . . . . 3.15 Landau reconstruction of 140 µm sCVD on l.h.s and noise reconstruction of the charge amplifier on r.h.s in case of triggering on all events in the DS 3.16 Signal and noise amplitude distributions in the 500µm diamond sensor [41] 3.17 Landau reconstruction of 140 µm sCVD on l.h.s and noise reconstruction of the charge amplifier on r.h.s in the case of multistage trigger . . . . . . 3.18 Landau reconstruction of the signal from 500 µm sCVD on l.h.s and the width of the signal of the 500 µm sCVD on the r.h.s in the case of multistage trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.19 Position of the maximum of the signals in the 500 µm sCVD on l.h.s and in the scintillator on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.20 Difference of the position of the maximum of the diamond and scintillator 3.21 The maximum of the signal as a function of its position in time in the sCVD on l.h.s and as function of its width on r.h.s . . . . . . . . . . . . . 3.22 The PHIL accelerator design . . . . . . . . . . . . . . . . . . . . . . . . . 3.23 The LEETECH facility design . . . . . . . . . . . . . . . . . . . . . . . . 3.24 The maximum of the signal in the sCVD on l.h.s and the position in time of the maximum of laser signal at PHIL on r.h.s . . . . . . . . . . . . . . 3.25 Difference of the time position of the diamond and laser signals . . . . . 3.26 Radiative Bhabha process at zero photon scattering angle . . . . . . . . . 3.27 Deflected Bhabha particles after optical elements downstream of the IP at SuperKEKB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.28 The energy distribution of Bhabha positrons on l.h.s and of Bhabha electrons on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.29 Angular distributions of Bhabha positrons in the horizontal plane on l.h.s and in the vertical plane on the r.h.s . . . . . . . . . . . . . . . . . . . . 3.30 Angular distributions of Bhabha electrons in the horizontal plane on l.h.s and in the vertical plane on r.h.s . . . . . . . . . . . . . . . . . . . . . . 3.31 Accelerated beam in a bending magnet . . . . . . . . . . . . . . . . . . .3.32 Transverse distribution in x and y of beam positrons (blue) and Bhabha positrons (red) after third and fourth bending magnets . . . . . . . . . . 3.33 The horizontal position distribution of Bhabha positrons as a function of their energies, after third and fourth bending magnets . . . . . . . . . . . 3.34 Energy distribution of Bhabha positrons which are already lost (green), detectable in the DS (blue) and still remaining in the beam pipe (red) after the third bend (BLC1LP2) and fourth bend (BLC2LP) . . . . . . . List of Figures 3.35 Beta functions and dispersion functions over first 27 m downstream the IP for the LER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.36 The horizontal position distribution of Bhabha positrons as function of their energies after first and second bending magnets . . . . . . . . . . . 3.37 The x-E distribution of the Bhabha positrons on l.h.s and the energy distribution of Bhabha positrons which are already lost (green), detectable in the DS (blue) and still remaining in the beam pipe (red) on r.h.s, in the drift at 13.9 meters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.38 Energy spectrum on l.h.s and exiting angle distribution on r.h.s of the lost Bhabha particles in the drift after QKBLP . . . . . . . . . . . . . . . . . 3.39 The 3D plot of the charged secondary particles resulting from the interaction of the exiting Bhabha positrons with the 6 mm thick Cu cylindrical beam pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.40 The transverse angular distribution of the exiting charged secondaries from the 6 mm Cu beam pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.41 Illustration of an electromagnetic shower of a Bhabha positron exiting the 6 mm Cu beam pipe and of the parallel and perpendicular positioning of the DS (violet) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.42 The fraction f of exiting Bhabha positrons with a detectable signal in the DS for a parallel sensor (blue) and perpendicular sensor (red) as a function of the distance from the beam pipe . . . . . . . . . . . . . . . . . . . . . 3.43 Precision on luminosity in 1 ms in phase 2 (L=10 34 cm -2 s -1 ) on l.h.s and in phase 3 (L=8 x 10 35 cm -2 s -1 ) on r.h.s . . . . . . . . . . . . . . . . . . 3.44 The design of the window at 45 • suggested by K. Kanazawa . . . . . . . 3.45 The number of exiting charged secondaries per 1000 incident positrons crossing a given thickness of Cu beam pipe . . . . . . . . . . . . . . . . . 3.46 The development of the shower in the radiator on l.h.s and the simulated window with an iron radiator at its shower maximum on r.h.s . . . . . . 3.47 The energy spectrum of the particles exiting the window and the radiator 3.48 The energy of the incident particles giving signal in the centred 5x5 mm 2 DS (blue) and with no signal (red) in the case of the window with a radiator 3.49 Histograms of the number of charged secondaries in the DS per incident exiting Bhabha positron for a window with radiator on l.h.s and for a window without a radiator on r.h.s . . . . . . . . . . . . . . . . . . . . . 3.50 The β and dispersion functions of the lattice of the HER up to 24 meters downstream of the IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.51 The energies of the Bhabha electrons as a function of their transverse position x after first bending magnet (l.h.s) and second bending magnet (r.h.s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.52 The energies of the Bhabha electrons as a function of their transverse position (x) after third bending magnet (l.h.s) and fourth bending magnet (r.h.s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.53 The energies of the Bhabha electrons as a function of their transverse position x in the drift at 20 m (l.h.s) and at 15.6 m (r.h.s) . . . . . . . . 3.54 The energy spectrum of Bhabha photons in the HER on l.h.s and their horizontal angle distribution on r.h.s . . . . . . . . . . . . . . . . . . . . 3.55 The distance from the IP at which the photons exit the HER beam pipe 3.56 The lattice of the HER where the γ spot hits, showing the free spaces for the luminosity monitors . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.57 The distances at which the Bhabha photon spot hits the vacuum chamber in the HER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 History of the vacuum scrubbing in SuperKEKB during phase 1, in the LER on l.h.s and in the HER on r.h.s . . . . . . . . . . . . . . . . . . . . 4.2 The position of collimators in SuperKEKB rings during phase 1 . . . . . 4.3 The physical vertical emittance in the LER during phase 1 after several corrections [49] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Loss monitor system at SuperKEKB . . . . . . . . . . . . . . . . . . . . 4.5 PIN diodes at SuperKEKB, around the collimator [50] on l.h.s and near a bellow [50] on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 The cross section of the ion chamber at SuperKEKB [50] . . . . . . . . . 4.7 The electron current measured in the LER as a function of the beam current for different types of the beam pipe [51] . . . . . . . . . . . . . . 4.8 The non-linear pressure rise at nine different gauges at the arc section as a function of beam current in LER . . . . . . . . . . . . . . . . . . . . . 4.9 A solenoid magnet on l.h.s and a permanent magnet on r.h.s at an aluminium bellow at SuperKEKB . . . . . . . . . . . . . . . . . . . . . . . . 4.10 The pressure rise against the beam current, at the 30 m bellow section before and after applying the solenoidal magnetic fields on l.h.s and in two arc sections before and after the installation of 800 permanent magnets around the aluminium bellows in the arc section on r.h.s [52] . . . . . . 4.11 The vertical beam size in the LER as a function of the total beam current for different bucket spacing, before the installation of permanent solenoids at the Al bellows on l.h.s and after the installation of permanent solenoids at the Al bellows on r.h.s [49] . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Backgrounds at the interaction region (Belle II) [59] . . . . . . . . . . . . 5.2 Bremsstrahlung scattering of a charged particle in the field of the nucleus of an atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Touschek scattering in the CMS [62] . . . . . . . . . . . . . . . . . . . . List of Figures 5.4 The energy spectrum of Bremsstrahlung positrons lost in the drift on l.h.s and their energy as a function of their scattering position on r.h.s in case a (scattering at the entrance of quadrupoles and special vertical bends) . 5.5 The LER lattice from 50 meters upstream to 11 meters downstream from the IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 The energy spectrum of Bremsstrahlung positrons lost in the drift on l.h.s and their energy as a function of their scattering position on r.h.s in case b (scattering at the entrance of all elements) . . . . . . . . . . . . . . . . 5.7 Example of slicing of bending magnets: horizontal (yellow) and vertical (red), and drifts (grey) between IP and the drift (included) . . . . . . . . 5.8 The energy spectrum of Bremsstrahlung positrons lost in the drift on l.h.s and their energy as a function of their scattering position on r.h.s in case c (scattering at all the elements after slicing the bending magnets and the drifts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9 The total loss from Beam-gas Bremsstrahlung overall the ring as a function of the scattering resolution on l.h.s and the percentage of loss in the drift with respect to the resolution of 10 cm as a function of scattering resolution on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 The energy of the lost Bremsstrahlung positrons in the drift as a function of their loss position on l.h.s and as a function of their horizontal exiting angle on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.11 The 3D distribution of the charged secondaries exiting the 6 mm thick Cu beam pipe on l.h.s and their energy spectrum on r.h.s . . . . . . . . . . 5.12 The horizontal angular distribution of charged secondary particles exiting the 6 mm Cu cylinder beam pipe on l.h.s and their number per incident particle on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.13 Histogram of number of secondaries deposited in the DS by each incident particle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.14 The behaviour of the losses detected in the DS as a function of the product of beam current and vacuum pressure . . . . . . . . . . . . . . . . . . . . 5.15 The energy of the lost Touschek positrons in the drift as a function of their scattering position on l.h.s and as a function of their exiting horizontal angle on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.16 The β function in the LER between 50 m upstream the IP and the IP . . 5.17 The percentage of the energy variation due to Touschek scattering computed from eq. 5.2, between the scattering regions extending 50 meters upstream the IP and the IP . . . . . . . . . . . . . . . . . . . . . . . . . 5.18 The 3D distribution of the charged secondaries resulting from the interaction of the incident lost positrons with the 6 mm Cu cylinder beam pipe on l.h.s and their number per incident particle on r.h.s . . . . . . . . . .8.4 Histograms of the number of charged secondaries in the DS per incident exiting Bhabha positron for different orientations of the DS at 5 mm thick Copper beam pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 8.5 Dynamics of the secondaries exiting 4 mm thick Copper beam pipe . . . 120 8.6 Histograms of the number of charged secondaries in the DS per incident exiting Bhabha positron for different orientations of the DS at 4 mm thick Copper beam pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 8.7 Dynamics of the secondaries exiting 3 mm thick Copper beam pipe . . . 122 8.8 Histograms of the number of charged secondaries in the DS per incident exiting Bhabha positron for different orientations of the DS at 3 mm thick Copper beam pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 8.9 Dynamics of the secondaries exiting 2 mm thick Copper beam pipe . . . 124 8.10 Histograms of the number of charged secondaries in the DS per incident exiting Bhabha positron for different orientations of the DS at 2 mm thick Copper beam pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 8.11 Dynamics of the secondaries exiting 1 mm thick Copper beam pipe . . . 126 8.12 Histograms of the number of charged secondaries in the DS per incident exiting Bhabha positron for different orientations of the DS at 1 mm thick Copper beam pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 8.13 Precision in 1 ms on the luminosity as a function of the thickness of the beam pipe for DS at 0 • (blue), DS at 45 • (red) and DS at 90 • (black) compared to the window + radiator (red spot) in phase 2 on l.h.s and in phase 3 on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 8.14 Dynamics of the secondaries exiting 6 mm thick Aluminium beam pipe . 129 8.15 Precision in 1 ms on the luminosity as a function of the thickness of the beam pipe for DS at 0 • (blue), DS at 45 • (red) and DS at 90 • (black) compared to the window + radiator (red spot) in phase 2 on l.h.s and in phase 3 on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 8.16 Dynamics of the secondaries exiting 6 mm thick Titanium beam pipe . . 131 8.17 Precision in 1 ms on the luminosity as a function of the thickness of the beam pipe for DS at 0 • (blue), DS at 45 • (red) and DS at 90 • (black) compared to the window + radiator (red spot) in phase 2 on l.h.s and in phase 3 on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 8.18 Dynamics of the secondaries exiting 6 mm thick Beryllium beam pipe . . 133 8.19 Precision in 1 ms on the luminosity as a function of the thickness of the beam pipe for DS at 0 • (blue), DS at 45 • (red) and DS at 90 • (black) compared to the window + radiator (red spot) in phase 2 on l.h.s and in phase 3 on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 List of Figures 8.20 Precision in 1 ms on the luminosity as a function of the thickness of the beam pipe for the four different materials, Cu (blue), Ti (black), Al (red), Be (magenta) and window+radiator (red spot), for a DS at 0 • , in phase 2 on l.h.s and in phase 3 on r.h.s . . . . . . . . . . . . . . . . . . . . . . . . 8.21 Precision in 1 ms on the luminosity as a function of the thickness of the beam pipe for the four different materials, Cu (blue), Ti (black), Al (red), Be (magenta) and window+radiator (red spot), for a DS at 45 • , in phase 2 on l.h.s and in phase 3 on r.h.s . . . . . . . . . . . . . . . . . . . . . . 8.22 Precision in 1 ms on the luminosity as a function of the thickness of the beam pipe for the four different materials, Cu (blue), Ti (black), Al (red), Be (magenta) and window+radiator (red spot), for a DS at 90 • , in phase 2 on l.h.s and in phase 3 on r.h.s . . . . . . . . . . . . . . . . . . . . . .

2. 1

 1 Comparison between achieved parameters at KEKB and design parameters at SuperKEKB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 The machine parameters for a very high luminosity at SuperKEKB . . . 3.1 Minimum required fractions of Bhabha cross-section in the sensors . . . . 3.2 Properties of different solid state detectors at T=300 K . . . . . . . . . . 3.3 The list of the first four bending magnets downstream of the IP in the LER 3.4 Precision on luminosity at different phases of SuperKEKB and different designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Characteristics of the different materials considered for the design of the vacuum chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 The list of the first four bending magnets downstream of the IP in the HER 6.1 Distances upstream of the IP of subsections where vacuum bumps took place in the D02 section . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Comparison between data and simulation of the fraction of losses in the sCVD coming from the different sections during the vacuum bump . . . . 6.3 Beam-gas Bremsstrahlung in 10 ms in the DS for different beam currents and vacuum pressure in the LER . . . . . . . . . . . . . . . . . . . . . . 6.4 The slopes of the fit of the relative losses in the DS as a function of relative inverse vertical beam size, along with their errors for six different beam currents in the LER . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

  ... نفس آخر تى ت ح ا ا شكر الحكاية ل ت وك البداية أنتم الساس، أنتم ، روح ت ال من أغلى يا وإخوتي وأبي مي ت أ صحيح هو العكس ن ن ولك نجاح ن ال نتيجة هو سعادة ن بال شعور ن ال ن ن أ ناس ن ال ن ن "يظ " سعادة ن بال شعور ن ال نتيجة هو نجاح ن ال Accelerators for high energy physics . . . . . . . . . . . . . . . . . . . . . 1.2 High luminosity colliders . . . . . . . . . . . . . . . . . . . . . . . . . . . Design and goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Accelerator layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evaluation of dithering algorithm . . . . . . . . . . . . . . . . . . . . . . 3.3 Our Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Position of the diamond sensor in both rings . . . . . . . . . . . . . . . . 3.4.1 Radiative Bhabha at zero photon scattering angle . . . . . . . . . 3.4.2 Case of LER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2.1 Generation of Bhabha positrons with GUINEA-PIG++ 3.4.2.2 Particle Tracking . . . . . . . . . . . . . . . . . . . . . . 3.4.2.3 Simulation of signal in the sCVD . . . . . . . . . . . . . 3.4.3 Design of new vacuum chamber insertion . . . . . . . . . . . . . . 3.4.3.1 Simulation of window at 45 • . . . . . . . . . . . . . . . .

	Contents 2.1 Chapter 1 1 Introduction 1.1 2 The SuperKEKB collider
	Albert Einstein

الفقي إبراهيم

" Money won't create success, the freedom to make it will" Nelson Mandela "If you can't explain it simply, you don't understand it well enough" 2.2.1 Electron source . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Positron source . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Damping ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Vacuum system for both rings . . . . . . . . . . . . . . . . . . . . 2.2.6 Nano-beam scheme . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Belle II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Sub detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 BEAST II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Fast Luminosity Monitoring 3.1 Motivations And Specifications . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Methods and Techniques . . . . . . . . . . . . . . . . . . . . . . . 3.2 3.3.1 CVD diamond sensors . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1.1 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1.2 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1.3 Characterisation with radioactive sources . . . . . . . . . 3.4 3.4.3.2 Simulation of different material and thickness beam pipe 3.4.4 Case of HER . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

  ).

	Figure 2.2 -Layout of SuperKEKB/Belle II experiment
	2.2 Accelerator layout
	Parameter 2.2.1 Electron source KEKB (LER/HER) SuperKEKB design (LER/HER)
	Energy (GeV) SuperKEKB requires a beam with very high charge (5 nC) and low normalised emittance 3.5/8 4/7
	β * y (mm) (20 mm-mrad), for this reason a new RF gun [11] which generates the electron beam has 5.9/5.9 0.27/0.42
	σ * y (µm) been developed to achieve such requirements. The gun is installed at the start point of 0.94 0.059
	Beam current (mA) the J-linac (fig. 2.2). The RF gun (l.h.s of fig. 2.41.64/1.19	3.6/2.6
	Luminosity (cm -2 s -1 )	2.11 × 10 34	8 × 10 35
	Table 2.1 -Comparison between achieved parameters at KEKB and design parameters
	at SuperKEKB

Table 3 .

 3

1 -Minimum required fractions of Bhabha cross-section in the sensors

  The distribution of the position in time of the maximum of the signal is shown on l.h.s of fig.3.19. The Gaussian distribution is centred at 0.8 ns with an rms of 1.67 ns (> 1 ns). Since we trigger on the scintillator, then the position in time of the maximum of the diamond may strongly depend on the position in time of the maximum of the scintillator. The r.h.s of fig.3.19 represents the position of the maximum of the scintillator. We can clearly see from this figure that the position of the maximum of the scintillator moves a lot with an rms of 1.64 ns, in addition we can see three populations.We subtract the position in time of the maximum of the diamond signal (l.h.s of fig.3.19) from one population of the position in time of the maximum of the scintillator corresponding to times < 4 ns and we get the distribution shown in fig.3.20. The RMS (1.5 ns) is still larger than 1 ns. But since the distribution of the position in time of the

	800							Entries Entries	h h	9998 9998										Entries Entries	h h	9939 9939
								Mean Mean		0.01233 0.01233	1000									Mean 7.259e-09 Mean 7.259e-09
	700							RMS RMS	0.002344 0.002344										RMS 7.382e-10 RMS 7.382e-10
	600										800
	500
											600
	400
	300										400
	200
											200
	100
	0 0	0.005	0.01	0.015	0.02	0.025	0.03	0.035	0.04	4 0		6	8	10	12		14	16	18	20 × 10	-9
								max_of_signal(V)										sigma(s)
	1000							Entries Mean Entries Mean	hn 0.004042 10000 hn 10000 0.004042	1400	×	3 10							Entries Mean Entries Mean	hn hn 1.752e+07 0.000204 1.752e+07 0.000204
								RMS RMS		0.0009093 0.0009093										RMS RMS	0.0006852 0.0006852
	800										1200
											1000
	600
											800
	400										600
											400
	200
											200
	0 0	0.002	0.004	0.006		0.008	0.01	max of signal (V) 0.012 0.014	-0.01 -0.008 -0.006 -0.004 -0.002 0	0	noise (V) 0.002 0.004 0.006 0.008 0.01

Figure 3.17 -Landau reconstruction of 140 µm sCVD on l.h.s and noise reconstruction of the charge amplifier on r.h.s in the case of multistage trigger sCVD is 11.52 mV. The Landau reconstruction from the data is shown on the l.h.s of fig. 3.18, with a MPV at 12.3 mV. The distribution of the fitted RMS of the signal in the sCVD is shown on r.h.s of fig. 3.18. The plot shows an average RMS of ≈ 7 ns.

  .28). The angular distributions of the Bhabha particles are shown in fig.3.29 & 3.30. These distributions are similar to the angular distributions of the beams ( εx βx , εy βy ), electron beam: (2×10 -4 , 6.2×10 -5 ) and positron beam: (2.1×10 -4 , 8.7×10 -6 ).After being generated and to be able to choose the optimal location of our DS in the LER, Bhabha positrons are tracked to different locations downstream of the IP. The updated lattices of both rings were provided by Y. Funakoshi. Tracking took place using SAD (Strategic Accelerator Design)[47]. SAD is developed at KEK in Japan, and is used by scientists at KEK and elsewhere. SAD is a very powerful tool for simulation, design and commissioning studies. All the simulation codes for the SuperKEKB commissioning were prepared within the SAD framework.As shown on l.h.s of fig.3.28, the energy of Bhabha positrons ranges from very low energies to the beam energy (4 GeV). Very low energy Bhabha positrons will be lost after traversing the first optical elements in the lattice downstream of the IP. When a bunch of particles passes through the uniform field of the bending magnet, par-

										Chapter 3. Fast Luminosity Monitoring
	3.4.2.2 Particle Tracking
								h1 h1									h1 h1
	16000					Entries Mean RMS Entries Mean RMS			342501 -1.171e-06 0.0001926 342501 -1.171e-06 0.0001926	30000					Entries Entries Mean RMS Mean RMS		342501 342501 4.989e-07 0.0001276 4.989e-07 0.0001276
	14000					/ ndf Constant 2 χ / ndf 2 χ Constant	1.529e+04 1.529e+04	± ±	5247 / 97 3.433e+01 5247 / 97 3.433e+01						/ ndf / ndf Constant 2 χ 2 χ Constant	1.251e+04 / 97 1.251e+04 / 97 6.980e+01 ± 3.041e+04 6.980e+01 ± 3.041e+04
	12000					Mean Sigma Mean Sigma	3.005e-07 0.000000 ± ± 0.000173 -7.498e-07 3.005e-07 ± -7.498e-07 0.000000 ± 0.000173	25000					Mean Sigma Mean Sigma	2.753e-07 8.528e-05 2.753e-07 8.528e-05	± ± ± ±	1.496e-07 1.262e-07 1.496e-07 1.262e-07
	10000										20000
	8000										15000
	6000
											10000
	4000
	2000										5000
	angle_x(rad) -0.001 -0.0008-0.0006-0.0004-0.0002 0 0.0002 0.0004 0.0006 0.0008 0.001 0	angle_y(rad) -0.001 -0.0008-0.0006-0.0004-0.0002 0 0.0002 0.0004 0.0006 0.0008 0.001 0
						h1 h1									h1 h1
	10000				Entries 116370 Mean 3.198 Entries 116370 Mean 3.198								Entries 116763 Entries 116763 Mean 5.585 Mean 5.585
					RMS RMS	0.9537 0.9537				5000				RMS RMS	1.681 1.681
	8000
											4000
	6000										3000
	4000										2000
	2000										1000
	0 0	0.5	1	1.5	2	2.5	3	3.5		4	0 0	1	2	3	4	5	6	7
								E(GeV)							E(GeV)
	Figure 3.28 -The energy distribution of Bhabha positrons on l.h.s and of Bhabha electrons
	on r.h.s

Table 3 .

 3 4 -Precision on luminosity at different phases of SuperKEKB and different designsdifferent thickness of the beam pipe (1, 2, 3, 4, 5 and 6 mm) as well as different orientations of the diamond sensor. In table3.5, the characteristics of the different materials considered for the vacuum chamber are recalled.

	For each material I studied the dynamics of the interactions of the exiting Bhabha
	Material	Copper (Cu) Aluminium (Al) Beryllium (Be) Titanium (Ti)
	Z	29	13	4	22
	Density (g.cm -3 )	8.96	2.7	1.85	4.506
	Radiation length X 0 (cm)	1.47	8.985	35.28	3.655
	Shower max X (cm)	7.2	35.76	110.2	17
	Molière radius R M (cm)	1.176	3.38	4.86	2.25

Table 3 .

 3 5 -Characteristics of the different materials considered for the design of the vacuum chamber

Table 3 .

 3 3.51, 3.52, 3.53). 6 -The list of the first four bending magnets downstream of the IP in the HER

	Name	Number Length (m) Position from IP (m)	Type
	BLC2RE	First	2.23	8.2	horizontal
	BC1RE	Second	0.344	14.6	vertical
	BC2RE	Third	0.344	17.7	vertical
	BC3RE	Fourth	0.344	23	vertical

  During phase 1, 101 PIN diodes and 32 ion chambers were activated. Each BLM signal can generate an abort signal. Signals from BLMs all over the rings are sent to 4 LCRs (Local Control Rooms). Along with the BLM signals, other signals like the beam intensity, signals from the RF cavities, injection trigger timing signals and others are fed to data loggers which are used to analyse data collected over 600 ms, diagnose the abort signals and give the reason behind each abort. The loss monitor system is shown in fig. 4.4.

  Figure5.12 -The horizontal angular distribution of charged secondary particles exiting the 6 mm Cu cylinder beam pipe on l.h.s and their number per incident particle on r.h.s

	8000								h1 Entries 746142 Mean 72.58 RMS 35.55 h1 Entries 746142 Mean 72.58 RMS 35.55	counts	3000								Entries Mean RMS Entries Mean RMS	h1 h1	54729 13.63 7.623 54729 13.63 7.623
	7000
												2500
	6000
	5000											2000
	4000											1500
	3000
												1000
	2000
												500
	1000
	0 0	20	40	60	80	100	120	140	160	180		0 0	5	10	15	20	25	30	35	40	45
								angle_x(deg)									secondaries per incident
				counts	240 220										Entries Mean RMS Entries Mean RMS	h1 h1	359 2.109 2.553 359 2.109 2.553
					200
					180
					160
					140
					120
					100
					80
					60
					40
					20
					0 0	2	4	6	8	10		12	14	16	18	20
															secondaries

  6.19). For each current, σ y is varied five times (r.h.s of fig.6.[START_REF] Matsumoto | Large-aperture Travelling-wave accelerator structure for positron capture of SuperKEKB injector linac[END_REF]), with data taking lasting for ≈ 2 minutes at each value of σ y .

  Figure 6.20 -Fit of the losses in the sCVD as a function of the inverse of the vertical beam size at 180 mA on l.h.s and at 360 mA on r.h.s

				Touschek scan from 21 June @ I=180 mA				Touschek scan from 21 June @ I=360 mA
	loss in 10ms in sCVD	8 10								2 χ p0 p1 2 χ p0 p1	/ ndf / ndf	153.7 / 180 0.2986 ± 1.189 13.11 ± 48.35 153.7 / 180 0.2986 ± 1.189 13.11 ± 48.35	los in 10 ms in SCVD	30 40 35		2 χ p0 2 χ p0 p1 p1	/ ndf / ndf	193 / 0.9054 ± 193 / 0.9054 ± 60.87 ± 178.8 5.589 5.589 60.87 ± 178.8
														25
		6												20
		4												15
														10
		2
														5
		0 0	0.005	0.01	0.015	0.02	0.025	0.03	0.035	0.04		0 0	0.005	0.01	0.015	0.02	0.025
										1/sigma_y(um-1)				1/sigma_y(um-1)
				Touschek scan from 21 June @ I=540 mA
	loss in 10 ms in sCVD	40 50							2 χ p0 p1 2 χ p0 p1	/ ndf / ndf		175.1 / 124 1.393 ± 14.53 91.05 ± 336.7 175.1 / 124 1.393 ± 14.53 91.05 ± 336.7	loss in 10 ms in sCVD	50 60 70		2 χ p0 2 χ p0 p1 p1	/ ndf / ndf	78.24 / 2.848 ± 27.09 78.24 / 2.848 ± 27.09 ± 988.7 ± 988.7
		30
														40
		20												30
														20
		10
														10
		0 0	0.005	0.01		0.015	0.02			0.025		0 0	0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.022
										1/sigma_y(um-1)				1/sigma_y(um-1)

  Figure 6.22 -Fit of the losses in the sCVD as a function of the inverse of the vertical beam size at 900 mA on l.h.s and at 1000 mA on r.h.sThe information on the Touschek contribution to the total beam losses in the sCVD is contained in the slopes of the fits in figs 6.20, 6.21 and 6.22. To confirm the direct proportionality of Touschek with the inverse of the vertical beam size, the relative losses from data as a function of the relative inverse vertical beam size are shown for the six values of the beam current, resulting in slopes ≈ 1 (see table. 6.4) (fig.6.24, fig.6.25, fig. 6.26). For a global comparison of the data to the simulation, the weighted arithmetic mean (see eq. 6.2) of the slopes for each current was computed, where x i stands for the value of the slope and the σ i is the fit error of the slope, as well as the corresponding error (see eq. 6.3). The result for the data is slope data = 0.97±0.05, which is compatible within its statistical errors with the simulation (fig.6.27): slope simulation =1.095±0.06.

	106	Chapter 6. Measurements of Single Beam Loss at Phase 1
	loss in 10 ms in sCVD To compare the contribution of the losses from Touschek scattering in the sCVD be-80 100 120 / ndf 2 χ 62.72 / 66 p0 4.177 ± 57.98 p1 362.1 ± 1373 / ndf 2 χ 62.72 / 66 p0 4.177 ± 57.98 p1 362.1 ± 1373 Touschek scan from 21 June @ I=900 mA 160 / ndf 2 χ 34.89 / p0 12.08 ± 83.81 p1 ± 1953 / ndf 2 χ 34.89 / p0 12.08 ± 83.81 p1 ± 1953 Touschek scan from 21 June @ I=1000 mA σ 2 x = 1 n i=1 1 σ i (6.3) loss in 10 ms in sCVD 120 140
	40 60		100 Current (mA) Slope ± error 40 80 180 0.95 ± 0.10 60 360 1.1 ± 0.6
	20		540	20 1.02 ± 0.08
	0 0	0.002 0.004 0.006 0.008	1/sigma_y(um-1) 0.01 0.012 0.014 0.016 0.018 720 900	0 1.42 ± 0.55 0.002 0 0.97 ± 0.15	0.004	0.006	0.008	0.01	1/sigma_y(um-1) 0.012
			1000	0.97 ± 0.10
	Table
	Current (mA) Pressure (nTorr) Beam-gas Bremsstrahlung in DS in 10 ms
		180	0.56			1.2 ± 0.3
		360	0.75			5.6 ± 0.9
		540	1			14.5 ± 1.4
		720	1.45			27.1 ± 2.8
		900	2.2			58.0 ± 4.2
		1000	3			83.8 ± 12.0
	Table 6.3 -Beam-gas Bremsstrahlung in 10 ms in the DS for different beam currents and
	vacuum pressure in the LER
	6.2.3.2 Losses from Touschek scattering
			x =	n i=1 ( x i σ 2 i n 1	)				(6.2)
				i=1	σ 2 i

  Figure 6.23 -Losses from beam-gas Bremsstrahlung in the sCVD in the LER as a function of the product of the beam current and the vacuum pressure, from data (red) and simulation (blue) on l.h.s and the fraction of the losses from beam-gas Bremsstrahlung in data and simulation

	loss in 10 ms in the sCVD	60 70 90 80	2 χ p0 p1 2 χ p0 p1	/ ndf / ndf	9.018e-29 / 4 2.913e-15 ± 5.802e-15 1.881e-18 ± 0.009606 9.018e-29 / 4 2.913e-15 ± 5.802e-15 1.881e-18 ± 0.009606	2 χ p0 p1 2 χ p0 p1	/ ndf / ndf	4.056 / 4 0.3392 ± 0.0008347 -1.906 ± 0.02992 4.056 / 4 0.3392 ± -1.906 0.0008347 ± 0.02992		data/sim	3 3.5 4.5 5 4	hist Mean x Entries Mean y hist Entries Mean x Mean y RMS x RMS x RMS y RMS y	6 1156 2.545 6 1156 2.545 1030 1030 0.6526 0.6526
		50														2.5
		40														2
		30														1.5
		20														1
		10														0.5
		0	0	500		1000		1500		2000	2500	3000	0 0	500	1000	1500	2000	2500	3000
												current*pressure(mA.nTorr)		current*pressure(mA.nTorr)
					relative loss of Touschek @ I=180 mA			relative loss of Touschek @ I=360 mA
	loss/loss_0	2.4 2.6					2 χ p0 p1 2 χ p0 p1	/ ndf / ndf	0.06134 / 3 0.1821 ± -0.02733 0.09708 ± 0.9448 0.06134 / 3 0.1821 ± -0.02733 0.09708 ± 0.9448			loss/loss_0	2.4
																2.2	χ χ	2 2	/ ndf / ndf	0.841 / 3 0.841 / 3
		2.2															p0 p0	0.3578 0.3578	± ±	0.9517 0.9517
		2														2	p1 p1	1.081 1.081	± ±	0.6348 0.6348
		1.8														1.8
		1.6														1.6
		1.4														1.4
		1.2														1.2
		1														1
			1	1.2	1.4	1.6	1.8	2	2.2	2.4	2.6	2.8	1	1.2	1.4	1.6	1.8	2
													(1/sigma_y)/(1/sigma_y0)		(1/sigma_y)/(1/sigma_y0)

  Similar to the case of a Copper beam pipe, the precision improves when decreasing the thickness. The best precision is for the smallest thickness (1 mm), with a DS at 90 • (1.6 % in phase 2 (l.h.s of fig.8.15) and 5×10 -3 in phase 3 (r.h.s of fig.8.15)). These precisions are still not satisfactory. The values achieved in the case of the window and radiator, are 4.3 times better for phase 2. Figure 8.15 -Precision in 1 ms on the luminosity as a function of the thickness of the beam pipe for DS at 0 • (blue), DS at 45 • (red) and DS at 90 • (black) compared to the window + radiator (red spot) in phase 2 on l.h.s and in phase 3 on r.h.s

	8.3. Titanium																		131
	8.2.1 Precision of L evaluation 8.3 Titanium
	The dynamics of the secondaries exiting a 6 mm thick Ti beam pipe are represented in
	fig. 8.16.
	Longitudinal profile of secondaries exiting 6 mm of Ti					Secondaries per incident positron exiting 6 mm of Ti
										Entries Mean RMS Entries Mean RMS	h1 h1	212838 1.565 0.8187 212838 1.565 0.8187	400							Entries Entries Mean Mean	h1 h1	9132 9132 34.66 34.66
														350							RMS RMS	12.45 12.45
	Longitudinal profile of secondaries exiting 6 mm of Al precision at phase2 at different Al beampipe thickness and for window at 45 deg				300	secondaries per incident particle exiting 6 mm of Al precision at phase3 at different Al beampipe thickness and for window at 45 deg
	precision 10 -1									Entries Mean RMS Entries Mean RMS	h1 h1	233467 1.651 0.7753 233467 1.651 0.7753	300 200 -2 10 precision 250							Entries Mean RMS Entries Mean RMS	h1 h1	8997 36.74 14.74 8997 36.74 14.74
														250
														150
														200 100
	-2 10													150 50
		0.5		1		1.5		2		2.5	z(m) 3		100 0 0	10	20	30		40	50	60	Secondaries 70 80
	(a) The position in z of the exiting charged sec-	50 (b) The number of charged secondaries per inci-
	1 ondaries -3 10	0.5	2	1	3	1.5	4	2	5	z(m) 3 thickness(mm) 2.5 6 Entries Entries h1 h1	68526 68526	Secondaries 80 Entries 316504 Entries 316504 thickness(mm) 70 6 h1 h1 dent Bhabha positron exiting the beam pipe 0 10 20 30 40 50 60 0 20000 1 2 3 4 5 -3 10 Spectrum of secondaries exiting 6 mm of Ti
										Mean Mean			61.48 61.48								Mean Mean	10.93 10.93
	(a) The position in z of the exiting charged sec-RMS 23.89 RMS 23.89	(b) The number of charged secondaries per inci-RMS 12.79 18000 RMS 12.79
	ondaries		Angle_x of secondaries exiting 6 mm of Al		h1 h1		h1 h1 dent Bhabha positron exiting the beam pipe 14000 Spectrum of secondaries exiting 6 mm of Al 16000
	2500									Entries Mean Entries Mean		74750 66.23 74750 66.23	25000 12000							Entries 330588 Mean 13.27 Entries 330588 Mean 13.27
										RMS RMS			22.51 22.51	10000							RMS RMS	17.55 17.55
	2000													20000 8000
														6000
	1500													15000 4000
														2000
	1000	20	40	60	80		100	120	140	160		180	10000 0 0	10	20	30	40	50	60	70	80
										angle_x(deg)									E(MeV)
	500 (c) The transverse angular profile of the exiting	5000
	0 secondaries 20	40	60	80		100	120	140	160		180	0 0	10	20	30	40	50	60	70	80	90	100
										angle_x(deg)									E(MeV)
	(c) The transverse angular profile of the exiting	(d) The energy spectrum of the exiting secon-
	secondaries												daries
	Figure 8.14 -Dynamics of the secondaries exiting 6 mm thick Aluminium beam pipe

  The dynamics of the secondaries exiting the 6 mm thick Beryllium beam pipe are shown in fig.8.[START_REF] Zang | Design optimization of flux concentrator for SuperKEKB[END_REF]. Beryllium has the longest radiation length (X Be =35.28 cm) and the smallest atomic number (Z=4), which reduces the absorption of the shower, and thus more secondaries exit the beam pipe. Figure 8.18 -Dynamics of the secondaries exiting 6 mm thick Beryllium beam pipe List of Figures2.1 Luminosity at SuperKEKB compared to other colliders . . . . . . . . . . 2.2 Layout of SuperKEKB/Belle II experiment . . . . . . . . . . . . . . . . . 2.3 The final focus Superconducting magnet system at SuperKEKB [10] . . 2.4 The RF gun on l.h.s and the photocathode on r.h.s, at SuperKEKB . . . 2.5 The scheme of positron source on l.h.s and the large aperture S-band accelerating structure at SuperKEKB on r.h.s . . . . . . . . . . . . . . . . 2.6 Electron/positron separator chicane at SuperKEKB . . . . . . . . . . . . 2.7 The layout of the damping ring at SuperKEKB . . . . . . . . . . . . . . 2.8 The layout of the SuperKEKB injector linac . . . . . . . . . . . . . . . . 2.9 The concept of antechamber on l.h.s and the antechambers for wigglers on r.h.s at SuperKEKB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10 The nano beam scheme at SuperKEKB . . . . . . . . . . . . . . . . . . .

	8.4. Beryllium														133
	8.4 Beryllium
	Longitudinal profile of secondaries exiting 6 mm of Be				secondaries per incident particle exiting 6 mm of Al
	1200	Entries Mean Entries Mean	h1 h1	127278 1.815 127278 1.815								300						Entries Mean Entries Mean	h1 h1	8997 36.74 8997 36.74
		RMS RMS			0.7114 0.7114														RMS RMS	14.74 14.74
	1000												250
	800												200
	600												150
	400												100
	200												50
	0 0	0.5			1	1.5		2	2.5	z(m) 3		0 0	10	20	30	40	50	60	70	Secondaries 80
	(a) The position in z of the exiting charged sec-	(b) The number of charged secondaries per inci-
	ondaries	Angle_x of secondaries exiting 6 mm of Al			dent Bhabha positron exiting the beam pipe Spectrum of secondaries exiting 6 mm of Be
	3500									Entries Entries	h1 h1	48389 48389	18000						h1 Entries 127278 h1 Entries 127278
	3000									Mean RMS Mean RMS		69.32 21.81 69.32 21.81	16000						Mean RMS Mean RMS	10.97 16.33 10.97 16.33
													14000
	2500												12000
	2000												10000
	1500												8000
													6000
	1000
													4000
	500												2000
	0 0	20	40		60	80	100	120	140	angle_x(deg) 160 180	0 0	10	20	30	40	50	60	70	E(MeV) 80
	(c) The transverse angular profile of the exiting	(d) The energy spectrum of the exiting secon-
	secondaries											daries

• (red) and DS at 90 • (black) compared to the window + radiator (red spot) in phase 2 on l.h.s and in phase 3 on r.h.s

Acknowledgements

I am honored to have been surrounded by great people during the three years of my PhD thesis work. It is difficult to mention all what they did for me in a short paragraph but I will never forget their direct or indirect contributions to my success.

Appendix A

Copper

Copper has the smallest radiation length (X 0 =1.47 cm) and the largest atomic number (Z=29) among all the other considered materials. Consequently, we believe that Cu will give the worst precision among the materials due to the fact that a great part of the electromagnetic shower will be absorbed in the thickness of the beam pipe.

Chapter 8. Appendix A 8.1. 5

Precision of L evaluation

After analysis of the simulation, we can conclude on the precision of the luminosity measurement in 1 ms for phase 2 (L=10 34 cm -2 s -1 ) and phase 3 (L=10 35 cm -2 s -1 ). The precision improves only moderately when decreasing the thickness of the beam pipe, with the best values for 1 mm (fig. 8.13). The orientation of the DS also affects the precision slightly, with a preference for the cases of 45 ). Even when decreasing the thickness of the beam pipe to 1 mm, we will not be able to improve the precision in the way we can do with the window and radiator. The factor of improvement in the case of the window and the radiator compared to the 1 mm thick beam pipe is 7 for phase 2. The best precision obtained among materials is with a Beryllium beam pipe of 1 mm thickness and a DS oriented at 90 • . This precision (1.3% in phase 2 and 4.1×10 -3 in phase 3) is still far from the aimed precision. The window with radiator shows the best precision among all the designs of the beam pipe (3.7×10 -3 in phase 2 and 1.2×10 -3 in phase 3).

After showing the results of all the simulations done to the vacuum group at KEK, it has been decided to fabricate the 6 mm Cu window at 45 • , to be installed in the drift at 11 m downstream of the IP for the phase 2 of SuperKEKB.