Nail-Boucherie

D Aunis

PhD JE A Depaulis
email: antoine.depaulis@ujf-grenoble.fr

Evidence for a role of the parafascicular nucleus of the thalamus in the control of epileptic seizures by the superior colliculus

Keywords: epilepsy, animal model, control, substantia nigra, superior colliculus, parafascicular nucleus, microdialysis, microinjections, thalamus, absence epilepsy Running title : Parafascicular nucleus and control of seizures

Purpose

The aim of this study was to investigate whether the nucleus parafascicularis (Pf) of the thalamus could be a relay of the control of epileptic seizures by the superior colliculus (SC). The Pf is one of the main ascending projection of the SC, the disinhibition of which has been shown to suppress seizures in different animal models and has been proposed as the main relay of the nigral control of epilepsy.

Methods

Rats with genetic absence seizures (Generalized Absence Epilepsy Rat from Strasbourg or GAERS) were used in this study. The effect of bilateral microinjection of picrotoxin, a GABA antagonist, in the SC on the glutamate and GABA extracellular concentration within the Pf was first investigated using microdialysis. In a second experiment, the effect of direct activation of Pf neurons on the occurrence of absence seizures was examined by microinjection of low doses of kainate, a glutamate agonist.

Results

Bilateral injection of picrotoxin (33 pmol/side) in the SC suppressed spike and wave discharges for 20 min. This treatment resulted in an increase of glutamate but not GABA levels in the Pf during the same time course. Bilateral injection of kainate (35 pmol/side) into the Pf significantly suppressed spike and wave discharges for 20 min, whereas such injection were without effects when at least one the site was located outside the Pf.

Conclusions

These data suggest that glutamatergic projections to the Pf could be involved in the control of seizures by the SC. Disinhibition of these neurons could lead to seizure suppression and may be involved in the nigral control of epilepsy.

Introduction

Several lines of evidence have suggested that the basal ganglia and especially the substantia nigra pars reticulata (SNR), their main output station, are involved in the control of epileptic seizures [START_REF] Gale | Mechanisms of seizure control mediated by gamma aminobutyric acid: role of the substantia nigra[END_REF][START_REF] Deransart | The role of basal ganglia in the control of generalized absence seizures[END_REF]. In particular, it has been shown that any pharmacological or electrical manipulation of the basal ganglia that results in inhibition of the SNR, suppresses the occurrence of most epileptic seizures (for review see 2). Yet, the mechanisms and circuits involved in this "nigral control of epilepsy" remain to be characterized.

Recently, we have shown that the neuronal populations of the intermediate and superficial layers of the caudal superior colliculus (SC) may constitute an important relay for the control of absence seizures by the basal ganglia, via the SNR (3). One of the main output of the SC is the ascending projection to the centromedial and the parafascicular nuclei (Pf) of the thalamus, two main components of the intralaminar nuclei (4, 5). Both experimental and clinical studies show that the intralaminar nuclei may play a role in the modulation and/or maintenance of seizures and neuronal oscillations [START_REF] Banerjee | Thalamic NMDA receptors in the gamma-hydroxybutyrate model of absence seizures: a cerebral microinjection study in rats[END_REF][START_REF] Velasco | Centromedian-thalamic and hippocampal electrical stimulation for the control of intractable epileptic seizures[END_REF][START_REF] Seidenbecher | Contribution of intralaminar thalamic nuclei to spike-and-wavedischarges during spontaneous seizures in a genetic rat model of absence epilepsy[END_REF].

The aim of this study was thus to investigate whether the Pf, the main intralaminar nucleus, could be a relay of the nigral control of epileptic seizures. To this aim, the genetic model of absence epilepsy in the rat (Genetic Absence Epilepsy Rat from Strasbourg or GAERS) was used and involvement of the collicular projection to the Pf during suppression of absence seizures was investigated by measuring variations in the extracellular levels of GABA and glutamate in the Pf in GAERS. In a second experiment, the consequences of Pf activation on the occurrence of absence seizures was studied by measuring the effects of local injection of a low, non toxic, dose of kainate on the occurrence of spike and wave discharges (SWD) in GAERS.

Methods

Adult male GAERS rats (250-350 g), a strain with spontaneous generalized non-convulsive seizures, bred at U398 Inserm in Strasbourg were kept in individual cages under a 12/12 h normal light/dark cycle, with food and water ad libitum. They were implanted under general anesthesia (diazepam 4 mg/kg + ketamine 1,000 mg/kg, i.p.) with four single-contact recording electrodes as previously described [START_REF] Nail-Boucherie | Suppression of absence seizures by electrical and pharmacological activation of the caudal superior colliculus in a genetic model of absence epilepsy in the rat[END_REF] After at least a week of recovery and habituation to handling, the electroencephalograms (EEGs) were recorded between 9.00 and 12.00 a.m (left frontal-parietal; right frontal-parietal) in awake freely moving animals using a digital acquisition system (Coherence 3NT, Deltamed, France). The animals were injected bilaterally in a volume of 200 nl/side (i) in the SC with saline or picrotoxin at a dose of 33 pmoles/side (Sigma, France) dissolved in NaCl 0.9 %, as in our previous study [START_REF] Nail-Boucherie | Suppression of absence seizures by electrical and pharmacological activation of the caudal superior colliculus in a genetic model of absence epilepsy in the rat[END_REF] or (ii) in the parafascicular nucleus with saline or a low non toxic dose of kainate (35 pmoles/side) dissolved in NaCl 0.9 %. This range of kainate dose is known to activate cell bodies for about 20-30 min, without lesioning effects, upon intracerebral microinjections [START_REF] Bandler | Elicitation of intraspecific defence reactions in the rat from midbrain periaqueductal grey by microinjection of kainic acid, without neurotoxic effects[END_REF]. For each recording session, intracerebral injections were performed after a 20-min reference EEG recording. The EEG recording was then resumed for up to 60 min and the behaviour of the animal was observed.

Dialysis in the Pf was carried out 60 min before and 100 min after injection of picrotoxin or saline in the SC. The microdialysis probe (polycarbonate-polyether, 20-kDa molecular mass cutoff, 2 mm long and 0.52-mm diameter, CMA 12; CMA Microdialysis) was first inserted into the guide and was connected to a microinjection pump (CMA 100; CMA Microdialysis).

The perfusion medium contained 147 mM NaCl, 4.0 mM KCl and 2.4 mM CaCl2, pH 6.5, filtered through 0.22 µM Millex Millipore filters before each experiment and was perfused at 1.0 µl/min (20-min collection). The GABA and glutamate contents of the dialysates were determined by fluorimetric detection (Waters fluorimeter 470; excitation, 345 nm; emission, 455 nm) of the o-phthaldehyde derivatives after their separation by HPLC, using a modification of the method of [START_REF] Allison | O-Phtalaldehyde derivatives of amines for high-speed liquid chromatography/electrochemistry[END_REF] [START_REF] Allison | O-Phtalaldehyde derivatives of amines for high-speed liquid chromatography/electrochemistry[END_REF]. All chromatographies were performed with a Nucleosil C 18 column (5 m, 25 x 0.4 cm), a Waters pump (model 626), a controller 600 S and a Millenium 32 integrator. . Upon completion of the experiments, the animals were killed with an overdose of nembutal and the brains were removed, cut in 20-µm coronal sections, stained with cresyl violet and each injection site and placement of the dialysis probe were localized with reference to the atlas of Paxinos and Watson (11) in the caudal aspect of the SC or near the Pf, respectively.

When an injection or a microdialysis site were respectively located outside the SC or the parafascicular nucleus, the data were not included in the analysis.

The EEG data were expressed as mean ± SEM cumulated absence seizure duration (s) of spike-and-wave discharges (SWD) per 20-min period. Paired comparison versus control conditions (saline) were made using an ANOVA and were confirmed with a non parametric test (Wilcoxon). The microdialysis data were expressed as mean ± SEM of percentage of basal release of the 4 first baseline samples (i.e., before injection) and were compared to control conditions (saline) using a one-way ANOVA followed by a Student-Newman-Keuls as multiple comparisons test.

Results

Experiment 1 : Effects of bilateral injection of picrotoxin in the caudal superior colliculus on amino acid release in the parafascicular nucleus

Bilateral injection of picrotoxin (33 pmoles/side; n=4) in the caudal part of the SC significantly decreased cumulated duration of SWD during the first 20 min post injection period as compared to saline injection, as previously reported [START_REF] Nail-Boucherie | Suppression of absence seizures by electrical and pharmacological activation of the caudal superior colliculus in a genetic model of absence epilepsy in the rat[END_REF][START_REF] Depaulis | Suppression of spontaneous generalized non-convulsive seizures in the rat by microinjection of GABA antagonists into the superior colliculus[END_REF] (results not shown).

When Pf microdialysis was performed in these animals, the mean baseline extracellular concentration was 37.8 ± 3.9 pmoles/20 min for glutamate and 3.91 ± 1.75 pmoles/20 min for GABA. Picrotoxin injection in the SC induced a significant increase of extracellular levels of glutamate in the Pf, 40 min post injection (+ 28.8 % of the basal level, p <0.05, Newman-Keuls ; figure 1) as compared to saline injection, whereas no change was observed in the extracellular GABA level (figure 1). Since the dead volume of the catheter (20 µl) corresponded to a 20-min collection, the extracellular increase of glutamate observed at 40 min post injection of picrotoxin corresponded to the antiepileptic effect observed 20 min post injection. When one of the injection sites was located outside the Pf (n=3), injection of KA did not result in significant suppression of SWD (Table 1).

  Stainless-steel guide-cannulae (o.d. = 0.4 mm; i.d. = 0.3 mm) were aimed bilaterally at (i) the deep layers of the caudal part of the SC (A/P: -7.3 mm; M/L: 2.0 mm; D/V: -4.0 mm from bregma) or (ii) the Pf (A/P:-4.80 mm; M/ L: 1.5 mm; D/V: 4.4 mm from bregma). A microdialysis guide was also aimed just above the parafascicular nucleus (A/P:-4.80 mm; M/L: 1.5 mm; D/V: 4.4 mm from bregma) in animals implanted in the SC. All experiments were carried out in accordance with the European Community Council Directive of 24 November 1986 (86/609/EEC).

Figure 1 about here Experiment 2 :

 2 Figure 1 about here

Figure 2

 2 Figure 2 about here

Table 1 about here Table 1 :

 1here1 Effects of bilateral injections of kainate (35 pmol/side) outside the parafascicular nucleus on absence seizures in the GAERS. The results are expressed as percent of cumulated duration of spike-and-wave discharges versus control conditions for each 20 min period postinjection. Pf : parafascicular nucleus; VPM : ventral posteromedial nucleus of the thalamus

	Location	sites	0-20 min	21-40 min	41-60 min
	(right/left)
	Pf/VPM		51	151	125
	VPM/Pf		59	135	77
	No/Pf		79	94	41

Discussion

The main results of the present study are that (i) disinhibition of the caudal part of the SC which results in suppression of absence seizures is associated with a significant increase of glutamate in the Pf whereas no change of GABA concentration was observed and (ii) direct pharmacological bilateral activation of the Pf suppresses absence seizures. Although preliminary, these data suggest that the glutamatergic projection from the caudal aspect of the SC to the Pf is a critical relay in the output circuit involved in the control of seizures by the basal ganglia via the SNR.

The data of the first experiment of this study confirmed previous results obtained in our laboratory showing that disinhibition of the intermediate layers of the caudal SC suppresses absence seizures without behavioural side effects [START_REF] Nail-Boucherie | Suppression of absence seizures by electrical and pharmacological activation of the caudal superior colliculus in a genetic model of absence epilepsy in the rat[END_REF][START_REF] Depaulis | Suppression of spontaneous generalized non-convulsive seizures in the rat by microinjection of GABA antagonists into the superior colliculus[END_REF]. It is also in agreement with data from the literature showing an antiepileptic effect in different animal models of seizures following activation or disinhibition of the SC [START_REF] Redgrave | Anticonvulsant role of nigrotectal projection in the maximal electroshock model of epilepsy -I. Mapping of dorsal midbrain with bicuculline[END_REF]. The microdialysis data of this experiment suggest that glutamatergic fibers are critically involved in mediating the antiepileptic effects of SC disinhibition. The involvement of these glutamatergic projections is further supported by the time course of the glutamate increase which is similar to the anti-epileptic effects.

Little is known about the neurotransmitter nature of the SC-Pf projections [START_REF] Yamasaki | Superior collicular projection to intralaminar thalamus in rat[END_REF][START_REF] Krout | Superior colliculus projections to midline and intralaminar thalamic nuclei of the rat[END_REF] and our data suggest that glutamatergic fibers are an important component of this pathway. It is not possible from our study to determine the origin of the GABA measured in the Pf, but our data suggest that GABAergic neurons may not be an important part of the SC-Pf projection.

Further experiments are required to confirm the involvement of the glutamatergic neurons in the control of seizures by the disinhibition of the SC.

The second experiment of the present study further confirmed that activation of glutamate receptors in the Pf leads to absence-seizure suppression and is likely to mediate, at least in part, the suppressing effects of SNR inhibition and/or SC disinhibition. In the present study, microinjection of low doses of kainate was used to activate cell bodies, since intracerebral microinjection of this compound have been shown to result in behavioural or physiological effects lasting up to 30 min [START_REF] Bandler | Elicitation of intraspecific defence reactions in the rat from midbrain periaqueductal grey by microinjection of kainic acid, without neurotoxic effects[END_REF]. The time course of suppressive effect on SWD is thus in agreement with data from the literature. In addition, at the dose used, no cell death is observed, especially in brain structures with moderate concentration of kainate receptors, as it is the case for the Pf [START_REF] Petralia | Histological and ultrastructural localization of the kainate receptor subunits, KA2 and GlurR6/7, in the rat nervous system using selective antipeptide antibodies[END_REF]. In our study, no overt loss of cells was observed in the area of the injection. These data remain to be confirmed with other pharmacogical or electrical means of activation. However, they are in agreement with clinical reports showing that electrical stimulation of the centromedian nucleus of the thalamus (which shares similar connections with Pf) results in potent anti-epileptic effects in human patients [START_REF] Velasco | Centromedian-thalamic and hippocampal electrical stimulation for the control of intractable epileptic seizures[END_REF]. Whether similar mechanisms are involved in seizure suppression in animal models and human patients and whether this control depends on the type of seizures remain to be determined. However, our data are also in agreement with experimental reports indicating that an increase of activity of intralaminar neurons is associated with the cortical desynchronized activity that characterizes wakefulness [START_REF] Glenn | Discharge rate and excitability of cortically projecting intralaminar thalamic neurons during waking and sleep states[END_REF]. Altogether, our results suggest that direct or indirect activation of Pf neurons could reduce the capacity of the cortical neurons to synchronize, thus limiting the occurrence of epileptic discharges.

Neurons of intralaminar thalamic nuclei such as the centrolateral and paracentral nuclei are recruited rythmically during SWDs through mechanisms underlying glutamatergic excitation modulated by GABAergic influences [START_REF] Seidenbecher | Contribution of intralaminar thalamic nuclei to spike-and-wavedischarges during spontaneous seizures in a genetic rat model of absence epilepsy[END_REF]. These neurons may be involved in the maintenance of cortico-thalamic oscillations and a change in their excitability, as induced by activation of glutamatergic input or by direct activation of kainate receptor as in the present study, could thus interrupt SWD. Whether this suppressive effect results from a blockage of neuronal synchronization or oscillations in the frequency range of SWD remains to be determined. In addition, although injection of kainate at sites located outside the Pf did not result in significant effects in the present study, we cannot exclude the possibility that SWD suppression results from a change of excitability of adjacent thalamic nuclei. In particular, diffusion of kainate to neurons of the ventro-basal nuclei, known to be critical in the generation of SWD [START_REF] Crunelli | Childhood absence epilepsy: genes, channels, neurons and networks[END_REF], could suppress their oscillating activity and result in seizure suppression. Further studies conducted in our laboratory are currently aimed to determine the specificity of the glutamatergic SC-Pf projection in the control of seizures by the SC and to better understand the mechanisms through which activation of these fibers suppress SWD.

Aknowledgments

The authors want to thank Viviane Hechler for her help in the HPLC analysis. KNB was the recipient of research fellowship from Université Louis Pasteur -CNRS -Eli Lilly.


P.-E Bernaudin

M Aburas

M Di

A Ghribi

P Robillard

L Valentin

THREE YEARS OF OPERATION OF THE SPIRAL2 LINAC: CRYOGENICS AND SUPERCONDUCTING RF FEEDBACK

Keywords: Accel. Conf. LINAC2022, Liverpool, UK JACoW

The superconducting LINAC of SPIRAL2 at the GANIL facility is in operation since October 2019. Its 26 superconducting quarter wave resonating cavities (88 MHz) are operated at a nominal gradient of 6.5 MV/m, but most of the cavities can be operated up to 8 MV/m. They are integrated into 19 cryomodules and cooled down at 4 K by a dedicated refrigeration cryogenic system.

In this paper, we will present a feedback after five years of operation of the cryogenic system, focusing on the main problems that have been faced, and on the diverse evolutions performed in order to improve the cryogenic system and to increase its reliability. We will also provide a feedback of the superconducting cavities performances status after three years of operation.

THE CRYOGENIC SYSTEM

Cryogenic System Overview

The SPIRAL2 cryogenic system is based on a HELIAL LT Air Liquide cold box. Fed by two Kaeser screw compressors, this cold box supplies 4 K liquid helium (1.3 kW) and 60 to 70 K helium gas to cool down cavities and thermal screens respectively.

The cryogenic distribution is composed of a 9 m long vertical line, linking the ground floor located cryoplant to the LINAC tunnel, and by 19 cryogenic valves boxes, connected one to the next to form the horizontal cryolines. Five valves per valves box supplies the cryogenic fluids to each cryomodule (2 valves for the thermal screen circuit, 2 valves for the liquid helium one, and 1 valve for the cryomodule cool down). Various non insulated, auxiliary lines are available for conditioning, warm gas collection, etc. For more details about the SPIRAL2 cryogenics system please refer to Ref. [START_REF] Ghribi | Status of the SPIRAL 2 LINAC cryogenic system[END_REF].

LINAC Cool Down

From 2017 up to today, the LINAC has been cooled down and warmed up once a year. Each cool down lasts three full weeks, plus a fourth week dedicated to thermalization of cryomodules and various controls and tuning operation. Hence the accelerator's cryogenics is in nominal configuration four weeks after the beginning of cool down.

As the niobium cavities have not been baked against the 100 K effect, the LINAC cool down is performed in a way that minimizes the time spent by each cavity in the 50-150 K temperature window. The temperatures of all the cavitiies are slowly decreased to 150 K, to allow homogeneous cool down of the cold mass, and once the 150 K threshold is reached, the process is accelerated as fast as possible. The acceleration is applied to only one cryomodule at a time, due to the volumetric flow rate capacity of the compression station and takes one full day per cryomodule. The volumetric flow rate capacity of the compression station limits the number of simultaneous fast cool down to one cryomodule at a time, so these phases shall be properly programmed to avoid any overlap.

Thermo-acoustic Oscillations (TAO)

The cryogenic operation has been hampered by two TAO (Taconis) issues since 2017 [START_REF] Ghribi | Cryogenic thermoacoustics in the SPIRAL2 LINAC[END_REF].

The cryoplant's main dewar (5 000 L) suffered from TAO once connected to the LINAC. This problem was detected early and easily solved using a dedicated buffer volume of a few litres.

More critical was the TAO that appeared on the cryogenic valves boxes. Positioned on the 5 K helium return line, it was detected only thanks to the cavity RF instabilities. A solution was hastily developed: the helium vessels inside the cryomodules were linked to the faulty connection point on the valves boxes using a warm line and a needle valve. This solution allowed RF operation, but it has some drawbacks; thus an alternative, more suitable solution based on a pre-tuned hydraulic RLC circuit, was developed in parallel with operation. After preliminary tests in 2021, it was installed early this year and successfully tested in July 2022.

Other Cryogenic Issues

The main cause of downtime over the last five years of cryogenic operation were linked to utilities. Several air utility failures were suffered, stopping completely the cryoplant. These failures were overcome thanks to upgrades performed on the GANIL air utility system in 2020; the cryogenic team is also working on a backup air supply system, to provide 2-hours autonomy to the cryoplant.

Main electrical power failures cause one cryogenic emergency stop per year. In particular, the cycle compressors are sensitive to voltage drops. Water refrigeration stops also caused several compression station trips.

In such cases, 3-5 hours are necessary to recover the LINAC's operational status from a cryogenic point of view.

During the five years of cryogenic operation, only two failures were caused by the cryogenic system, both in 2021. During the cool down, the cryoplant stopped. This stop was caused by mishandling of cryomodules cool down sequence by the cryogenics operators. During cold box restart, another operator mistake caused one turbine break. Thanks to the availability of spare turbines, the installation was quickly restarted with no impact on beam schedule. Later in 2021, the main cycle compressor stopped three times within 36 hours. The cause was the same each time: the disconnection of a wire to an internal temperature sensor. The problem was definitively fixed during the 2022 winter maintenance.

From a general point of view, cryogenics is a very minor source of beam downtime, as is evident from Fig. 1. To be fair, the dozen of downtime hours caused by the 2021 compressor station failure does not appear on these statistics because the beam was already down due to other reasons. Most cryogenic connexions, both in the cryomodules and between valves boxes, are CF flanges. Hence helium cold leak rates are a significant concern; they are monitored twice a year for each cryomodule and cryogenic lines vacuum section. The result of this monitoring is shown in Fig. 2. Over time, leaks tend to get more homogeneous (initially, leak tightness was much better inside high beta cryomodules, which is not the case anymore). Only one section (valves boxes section A) shows significant cold leaks. They remain manageable thanks to the dynamic pumping system, and only further diagnostic (localization) is intended in a near future.

Advanced Control and Monitoring

The control of the diphasic helium bath pressure is the critical point of cryogenic stability. Available RF power margins require a ±5 mbar pressure stability. PID standard control proved unable to satisfy this requirement and thus advanced feedback, based on a linearized model and a LQ controller, was developed. Full details can be found in Ref. [START_REF] Vassal | Dynamic modeling and control of the SPI-RAL2 cryomodules[END_REF].

This modelling effort led to the development of a heat load observer. This virtual sensor computes in real time the helium mass flow in and out of each cryomodule, based on the various sensors and actuators. For more details, please refer to Ref. [START_REF] Ghribi | Advanced cryogenic process control of the SPIRAL2 superconducting LINAC[END_REF].

THE SUPERCONDUCTING CAVITIES

Performances and Evolution

Performances of the superconducting (SC) cavities are monitored on a yearly basis. Except cavity B13, all cavities can be operated at 8 MV/m since their installation on the LINAC. Cavity B13 limitation at 6.2 MV/m (quench gradient) predates its installation on the accelerator.

Figure 3 show the evolution of the cavities performances since their test stand acceptance. Table 1 show the total losses (RF and static evolution) over time. The decrease of heat load from the test stands to the accelerator are the consequence of three factors: first, on the accelerator, static load was reduced thanks to the use of 65 to 70 K helium, instead of nitrogen, for the thermal screens of the cryomodules. Second, it appears clearly on the dynamic load graph that the low beta cavities heat load was significantly reduced, hence the result of RF conditioning. This conditioning effect is also observed between 2019 and 2020 for the high beta cavities. Lastly, calibration of the accelerating gradient is a significant source of error. All gradients have been calibrated using beam energy during the 2019 run, showing significant differences with the calibration values used since the acceptance tests.

Only cavity A2 shows a loss of performances since the beginning of operation; significant field emission is observed, with no consequence as it is only used at low gradient; all other (including A11) cavities performances remain stable.

A11 Cavity Issue

One cavity does not behave as the other ones: #11 low beta cavity's heat load at nominal gradient is extremely high. While the cavity was compatible to SPIRAL2 specifications when qualified on the tests stands, the dynamic load was measured around 65 W on the accelerator. Extended tests were performed in order to understand the origin of the problem. These tests allowed to locate the heat source on the lower section of the cavity. As the magnetic field zone is located on top of the cavity, this definitively proves that the issue is caused by a normal conducting area on the cavity. This cavity is slightly different from the other low beta ones: the sealing between the cavity itself and the removable bottom is different. It is therefore foreseen that the issue is linked to this sealing method. It is assumed that only a complete re-assembly in clean room might solve the issue. As the cavity runs steadily despite its high heat load, this operation is not foreseen in a near future. Only pressure control valve's flow coefficient (Kv) has been modified successfully to get more operational margin.

In Situ Helium Processing of B2 Cavity X-ray field emission might be fatal to beam operation, even if it has no consequence on the cavity operational status. The accelerator Beam Loss Monitors (BLM) are used to tune the beam acceleration, but they can be blinded if the X-rays emitted by the cavities are intense enough. This phenomenon happened in 2021 on high beta cavity #2. It was therefore necessary to perform the first in-situ helium processing of a SPIRAL2 SC cavity.

Helium was injected inside the cavity using the FPC pumping port. RF parameters were progressively raised to:  Cavity pressure: 1.2 10 -5 mbar  Peak field : 9.2 MV/m (peak)  Pulses length : 3.5 ms  Duty cycle: 35%

In less than 3 hours of such treatment, the BLM hits decreased from 2500 to 50 counts/sec at nominal gradient (6.5 MV/m).

Cryomodule Maintenance Perspectives

One site maintenance capacity is enhanced in parallel to regular operation. Early in 2022, the ISO5 GANIL clean room, its water treatment system and the operator's proficiency have been qualified. To do so, a low beta spare cavity has been prepared at GANIL (and tested at CEA Saclay, as the GANIL SC cavities test stand is not yet available). The performance of the cavity is the same as after the initial acceptance test, as shown in Fig. 4; hence GANIL has proved its capacity to treat SC cavities on-site. The same activity will be performed on a high beta cavity, in order to qualify the specific tooling and procedures. GANIL also participates to a national effort in order to develop a plasma processing apparatus, compatible with the SPIRAL2 LINAC. This work involves several accelerator (CEA/DACM, CNRS/IJC Lab) and plasma (IP Paris/LPP, CNRS/INSP) laboratories and the goal is to deliver an operational system late in 2024.

CONCLUSION

SPIRAL2 cryogenics system is running since 2017 and experienced no significant problem since. Work is going on in order to improve its stability and reliability, especially against potential TAO resurgences.

In the meantime, the 26 SC cavities are running with stable performances, apart from some field emission which remains low enough not to be a hindrance to beam operation, or weak enough to be helium processed in a few hours. GANIL is working to improve its maintenance and test capacity in case of significant cavity failure.

Figure 1 :

 1 Figure 1: SPIRAL2 beam downtime statistics (in percent), classified by technical cause (in % of beam downtime).

Figure 2 :

 2 Figure 2: SPIRAL2 cryomodules and cryogenic lines cold leaks evolution. For each family (low beta cryomodules, high beta cryomodules, cryogenic lines), both the total leak rates and the mean leak rate values of the various volumes are shown (in mbar.L/s).

Figure 3 :

 3 Figure 3: SPIRAL2 cryomodules deposited heat load at 4 K during acceptance tests, and on the LINAC in years 2019 and 2020. Top: dynamic loads for each cavity; bottom: total load for each cryomodule. Cavities at nominal gradient in both cases (6.5 MV/m).

Figure 4 :

 4 Figure 4: Spare low beta cavity performances upon reception and after HP rinsing in the GANIL clean room, 2022.

Table 1 :

 1 Total Cryomodules Load Evolution

	Item	Test Stands	2019	2020
	CMA (minus A11)	141.0	97.1	106.5
	A11	20.8	>65	66.6
	CMB	202.5	172.7	149.8
	Total	364.3	338	322.9

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Technology

Cryomodules and cryogenics