M El Khaldi

L Garolfi

RF DESIGN OF A HIGH GRADIENT S-BAND TRAVELLING WAVE ACCELERATING STRUCTURE FOR THOMX LINAC

There is growing demand from the industrial and research communities for high gradient, compact RF accelerating structures. The Thomx high gradient structure (HGS) is travelling wave (TW), quasi constant gradient section and will operate at 2998.55 MHz (30°C in vacuum) in the 2/3 mode. The optimization of the cell shape (Electromagnetic design) has been carried out with the codes HFSS and CST MWS, in order to improve the main RF characteristics of the cavity such as shunt impedance, accelerating gradient, group velocity, modified Poynting vector, surface fields, etc. Prototypes with a reduced number of cells have been designed. For an input power of about 20 MW, EM simulation results show that an average accelerating gradient of 28MV/m is achieved which corresponds to a peak accelerating gradient of 35 MV/m, a peak surface gradient of 44 MV/m and peak modified Poynting vector Scmax of 0.24 MW/mm 2 .

INTRODUCTION

ThomX is a Compton source project in the range of the hard X rays (45/90 keV). The machine is composed of a 50-70 MeV injector Linac and a storage ring where an electron bunch collides with a laser pulse accumulated in a Fabry-Perot resonator. The final goal is to provide an X-rays average flux of 10 11 /10 13 ph/s. The emitted flux will be characterized by a dedicated X-ray line. Different users are partners in the ThomX project [START_REF] Variola | The ThomX project status[END_REF], especially in the area of medical science and cultural heritage. Their main goal will be the transfer of all the experimental techniques developed on big synchrotron rings to these more compact and flexible machines. The ThomX project has recently been funded and will be located on the Orsay University campus.

The beam will be generated by a 2.5 cells standing wave RF gun and accelerated with a conventional LIL-type structure with an operating peak accelerating gradient around 20 MV/m during the first commissioning phase. The Thomx injector linac energy will be upgraded from 50 MeV to more than 70 MeV by replacing a low gradient S-band TW section with a short high gradient S-band one.

A LAL-PMB collaboration on high gradient S-band structure research has been established in order to push the performance envelope of RF structures towards higher accelerating gradients. The program foresees the design, prototyping and high power testing of a high gradient compact S-band accelerating structure. The Thomx high gradient structure (HGS) is travelling wave, quasi constant gradient section and will operate at 2998.55 MHz (30°C in vacuum) in the 2/3 mode. The section consists of five constant impedance (CI) landings with 16 identical cells plus four transition cells between the landings, in which the iris diameter decreases by 0.175 mm from one cell to another.

Before the construction of the final high gradient structure, prototypes with a reduced number of cells will be realized. The goals of these prototypes are to improve the machining of the cells and the brazing process.

The electromagnetic design of the prototypes has been performed with the codes CST MWS and HFSS. The choice of a single cell shape derives from an optimization aiming to maximize RF efficiency and minimize surface fields and modified Poynting vector at very high accelerating gradients. Such gradients can be achieved utilizing shape optimized elliptical irises, surface finish, appropriate materials and specialized fabrication procedures developed for high gradient structures.

DESIGN OPTIMIZATION AND ELECTROMAGNETIC SIMULATIONS

Traditionally, the surface electric field was long considered to be the main quantity which limits accelerating gradient because of its direct role in field emission [START_REF] Kilpatrick | Criterion for vacuum sparking designated to include both RF and DC[END_REF]. There is clear evidence however, in data from both CLIC and NLC [START_REF] Wuensch | The scaling of the travelling-wave RF breakdown limit[END_REF][START_REF] Adolphsen | Advances in normal conducting accelerator technology from the Xband linear collider program[END_REF] which covers structures with a wide range of RF parameters, that a simple constant surface field limit is insufficient to predict the performances of the different structures. Recently, new quantities such as the averaged power flow through the cell by the iris aperture circumference P/C [START_REF] Wuensch | Observations about RF breakdown from the CLIC high gradient testing program[END_REF] and the peak modified Poynting vector Sc = Re{S} + Im{S}/6 [START_REF] Grudiev | A new local field quantity describing the high gradient limit of accelerating structures[END_REF] have been considered to be responsible for high gradient limits.

During the design phase, the optimization of the main RF properties for the accelerating cavities (shunt impedance, accelerating gradient, group velocity, modified Poynting vector, surface fields, etc.) was carried out, by using the 3D simulation codes HFSS and CST MWS. A scan over several parameters has been performed in order to find the best combination of geometrical parameters in term of high gradient operations performance and reduced power consumption. In order to have a quantitative approach, it has been decided to find the cell geometry which minimizes the quantity η:

≡ < > . < > = . < > ⁄ ⁄ (1)
This corresponds to having simultaneously the minimum power consumption and the minimum risk of breakdown (based on the Sc model) for a given accelerating field. Where ω is the angular frequency, r is the effective shunt impedance per unit length and Q is the quality factor of the cell. The plots of figures 1 and 2 summarize the results obtained for the 2/3 phase advance cell as a function of the iris aperture radius (a) and cell to cell iris thickness (t). From these plots, the main trend is that the accelerating gradient decreases when the iris aperture is increased. However, a structure consisting of purely small-aperture cells would not be practical, as the group velocity in these cells is very small. The thickness of the iris has been carefully studied as well. The modified Poynting vector Sc and the surface electric field increase when the iris thickness is decreased for a given iris aperture. The cell-to cell iris thickness t has been optimized in order to get an acceptable compromise between the desired effective shunt impedance, the acceptable filling time and the mechanically rigidity, and is fixed at 3 mm along the structure.

These irises have an elliptical cross-section with an aspect ratio 1.7:1. An elliptical rounding profile at the iris reduces the peak surface field by 10-15%, which is desirable in high gradient applications. The rounding of the cell edge (ρ=10 mm) noticeably improves the quality factor by more than 10% reduces the wall power consumption. An average iris radius of 9.5 mm is chosen for RF efficiency and short-range Wakefield considerations. The optimized cell shape is the result of a trade-off between RF efficiency, accelerating gradient, optimal filling time, Wakefield considerations, and breakdown limitations. Figure 3 shows the distribution of electric field and modified Poynting vector Sc in the cell volume. Both peak surface electric field and peak Poynting vector are located on the iris insert. The S-band prototypes are TW constant impedance (CI) sections with reduced number of cells. Single coupling slots introduce a distortion in the field distribution and multi-pole components can appear and affect the beam dynamics. The simple way to compensate the dipole field is to introduce a symmetric compensating slot opposite to the RF input one. The opposite slot is not feed by RF power and can be used, for example, by pumping the structure. The goals of these prototypes are to test all design, manufacturing procedures and to verify the validity of all technical choices. Table 1 reports the main parameters for the 7-cell HGS prototype. The average and peak accelerating gradients are related to input power (peak value) by the following expressions:

< > [ / ] = . √ [ ] (2) [ / ] = . √ [ ] (3)

HIGH GRADIENT LIMITATIONS

The major obstacle to higher gradient is RF breakdown. RF Breakdown (BD) is a phenomenon that abruptly and significantly changes transmission and reflection RF power directed toward the structure under test. BD is accompanied by a burst of x-rays and by a bright flash of visible light. A local field quantity which predicts the high gradient performance of an accelerating structure is the modified Poynting vector Sc. The dependence of the modified Poynting vector on RF pulse length tp at a fixed breakdown rate (BDR) [START_REF] Sjobak | New criterion for shape optimization of normal-conducting accelerator cells for high-gradient applications[END_REF] has well established scaling law observed in many experiments.

The BDR is defined as the probability of having a breakdown and it is typically measured in "breakdown per pulse" (bpp) for 1 m long structure. As design guideline for a new RF structures, Sc should not exceed 4 MW/mm 2 if the structure is supposed to operate at a breakdown rate smaller than 10 -6 bpp/m and a pulse length of 200 ns [START_REF] Grudiev | A new local field quantity describing the high gradient limit of accelerating structures[END_REF][START_REF] Sjobak | New criterion for shape optimization of normal-conducting accelerator cells for high-gradient applications[END_REF].

By re-scaling these data to the pulse length of Thomx linac, i.e. 3 µs flat top, the maximum threshold of Sc, <Ea>, and peak surface electric field Es peak are presented in figure 5.

In our case, Sc should not exceed 1.6 MW/mm 2 which corresponds to a maximum surface electric field of 117MV/m, and maximum average accelerating gradient of 72 MV/m if the structure is supposed to operate at BDR smaller than 10 -6 bpp/m (the acceptable breakdown rate for a medical accelerator), and a RF flat top pulse length of 3 µs. For an input power of 20 MW the simulated peak modified Poynting vector Scmax is 0.24 MW/mm 2 . The expected BDR is less than 10 -18 bpp/m at given Scmax=0.24 MW/mm 2 , <Ea>max=28 MV/m and for a RF pulse length tp=3µs.

These limits are derived for structures which were constructed based on the high gradient assembly procedure which includes keeping everything rather clean and having high temperature bonding or brazing cycle in Hydrogen or in vacuum Figure 5: Dependence of the expected BDR versus average accelerating gradient, surface electric field and the modified Poynting vector for the prototype. Cyclic thermal stresses produced by RF pulsed heating due to intense magnetic fields that is usually at the coupling slot area can be the limiting factor on the attainable reliable gradients for linear accelerators. Electromagnetic Simulations show that a surface magnetic field reaches a peak value of 1.7 *10 5 [A/m] for an input power of 20 MW as illustrated in figure 6. This field value causes a temperature rise of about 11 °C for a RF flat top pulse length of 3 µs, which is far below the safe limit of 100 °C in the case of copper. As general experimental rule, if this pulsed heating exceeds ~ 110 °C serious damage to the coupler region has a high probability of occurrence.

CONCLUSION

Prototypes with a reduced number of cells have been designed and will be fabricated and high power tested in order to verify the validity of all technical choices. With the mode parameters of these sample prototypes and with fitting procedures, a final quasi-constant high gradient TW accelerating structure can be synthetized and assembled.

Figure 1 :

 1 Figure 1: Normalized average accelerating gradient as function of the iris radius and thickness.

Figure 2 :

 2 Figure 2: The optimization quantity η in the iris region.

Figure 3 :

 3 Figure 3: Distribution of (a) electric field and (b) modified Poynting vector in the regular cell.The S-band prototypes are TW constant impedance (CI) sections with reduced number of cells. Single coupling slots introduce a distortion in the field distribution and multi-pole components can appear and affect the beam dynamics. The simple way to compensate the dipole field is to introduce a symmetric compensating slot opposite to the RF input one. The opposite slot is not feed by RF power and can be used, for example, by pumping the structure. The goals of these prototypes are to test all design, manufacturing procedures and to verify the validity of all technical choices. Table1reports the main parameters for the 7-cell HGS prototype.Table1: Prototype RF Parameters

Figure 4

 4 Figure4shows the amplitude and phase distribution of the longitudinal electric field along the structure's axis with a reflection coefficient less than -40 dB at the input port. The

Figure 4 :

 4 Figure 4: Amplitude and phase of the accelerating gradient along the prototype's axis. The average and peak accelerating gradients are related to input power (peak value) by the following expressions:

Figure 6 :

 6 Figure 6: Surface magnetic field distribution for an input power of 20 MW (peak value).

Table 1 :

 1 Prototype RF Parameters

	Parameter	HFSS	CST MWS
	Freq.	3 [GHz]	3 [GHz]
	Phase advance per cell	2π/3
	Number of cells	7
	Iris radius	9.5 [mm]
	Group velocity (vg/c)	0.9%
	Field attenuation (α)	0.236 [m -1 ]	0.232 [m -1 ]
	Series impedance	38.6 [MΩ.m -2 ]	38 [MΩ.m -2 ]
	Shunt impedance	80 [MΩ.m -1 ]	81[MΩ.m -1 ]
	Unloaded quality factor Q	15000	15100
	Pin @ <Ea> = 28 MV/m	20 [MW]	20 [MW]
	Es peak/<Ea>	1.61	1.62
	Es peak/ Ea peak	1.28	1.29
	Scmax/< Ea > 2	3.10 -4 [A/V]	3.1.10 -4 [A/V]
	Hs peak @ 20MW (cells)	65 [kA/m]	65 [kA/m]
	Pulsed heating @ 20 MW	< 2°C	< 2°C
	BDR @ < Ea >=28 MV/m and RF pulse tp=3 µs	≤ 10 -18 [bpp/m]	≤ 10 -18 [bpp/m]

Proceedings of IPAC2015, Richmond, VA, USA ISBN 978-3-95450-168-7


M El Khaldi

W Kaabi

EXPERIMENTAL STUDY OF MULTIPACTOR SUPPRESSION IN DIELECTRIC MATERIALS

Keywords:

A novel coaxial resonator to investigate two-surface multipactor discharges on metal and dielectric surfaces in the gap region under vacuum conditions (~10 -8 mbar) has been developed and tested. The resonator is ~ 100 mm in length with an outer diameter of ~ 60 mm (internal dimensions). A pulsed RF source delivers up to 30 W average power over a wide frequency range 650-900 MHz to the RF resonator. The incident and reflected RF signals are monitored by calibrated RF diodes. An electron probe provides temporal measurements of the multipacting electron current with respect to the RF pulses. In this paper we compare and contrast the results from the RF power tests of the alumina (97.6% Al 2 O 3 ) and quartz samples without a coating, "the non-coated samples" and the Alumina and quartz samples with a TiN coating in order to evaluate a home made sputtered titanium nitride (TiN) thin layers as a Multipactor suppressor. The effectiveness of this method is presented and discussed in the paper.

INTRODUCTION

A multipactor discharge is a phenomenon in which electrons impact one or more material surfaces in resonance with an alternating electric field [START_REF] Kishek | Multipactor discharge on metals and dielectrics: Historical review and recent theories[END_REF]. Two main conditions must be met in order to develop a multipactor discharge between two material surfaces. First, electrons traversing the gap must impact the electrode near the time the E-field reverses the direction. Second, electrons must impact the surface with enough energy to create electron multiplication by secondary emission, i.e. δ (E) ≥1. The discharge is sustained by the population of electrons that remain in phase with the RF electric field. Because the discharge is sustained by secondary electrons, multipactor discharges typically occur under vacuum conditions.

Multipactor discharges are considered detrimental to RF systems in most applications. The discharges can cause vacuum window failure, limit the delivery of RF power, detune resonant cavities, damage high power RF sources, and cause a local pressure rise due to the desorption of surface gases [START_REF] El Khaldi | RF conditioning of high power input couplers for XFEL superconducting cavities[END_REF]. As mentioned, multipactor (MP) is often an undesirable phenomenon, and avoidance can be critical in the operation of certain systems, it can be avoided in several ways. A general cure against multipacting is to avoid the resonant conditions by either a proper choice of the geometry of the device [START_REF] Lyneis | Elimination of electron multipacting in superconducting structures for electron accelerators[END_REF][START_REF] Klein | Multipacting in superconducting RF structures[END_REF] or by coating the critical areas with a material with a lower secondary yield [START_REF] Nayaiesh | Properties of thin anti-multipactor coatings for klystron windows[END_REF][START_REF] Proch | Measurement of multipacting currents of metal surfaces in RF fields[END_REF]. This way the effect of multipacting can be significantly reduced, but very often the MP phenomenon cannot be removed. Titanium nitride (TiN) coating is used as a proven anti-multipacting coating for dielectric-loaded accelerating (DLA) structures, and RF vacuum windows. Several experimental investigations have been performed at DESY and LAL-Orsay, aimed at reducing secondary electron emission and multipactor effects by TiN layers generation on dielectric or metal surfaces [START_REF] Lorkiewicz | Surface TiN coating of Tesla couplers at DESY as an anti multipactor remedy[END_REF][START_REF] Kaabi | Titanium nitride (TiN) coating as multipactor suppressor on RF coupler ceramic windows[END_REF].

We propose here an experiment facility for investigating two-surface multipactor discharges on dielectric and metal surfaces. The compact apparatus consists of a RF coaxial resonator in a high vacuum system (~ 10 -8 mbar). The RF resonator is ~ 100 mm in length with an outer diameter of ~ 60 mm (internal dimensions), powered with a pulsed RF source delivering up to 30W average power. These experiments were successful in identifying multipacting and allowed us the evaluation of a homemade sputtered titanium nitride (TiN) thin layers as a Multipactor suppressor.

TiN COATING TECHNOLOGY

The presence of a dielectric window on a high power RF line has in fact a strong influence on the multipactor phenomenon, a resonant electron discharge that is strongly limiting for the RF components performances. The most important method to reduce the multipactor is to decrease the secondary emission yield of the dielectric window and the dielectric-loaded accelerating structures (DLA). Due to its low secondary electron emission coefficient, TiN thin film is used as a multipactor suppressor coating on RF ceramic coupler windows. For this purpose, a reactive DC magnetron sputtering bench has been developed at LAL-Orsay with the collaboration of the consortium Ferrara Ricerche-Italie in the frame of CARE program (Figure 1).

Within the sputtering process gas ions out of a plasma are accelerated towards a target consisting of the material to be deposited. Material is detached ('sputtered') from the target and afterwards deposited on a substrate in the vicinity. The process is realized in a closed recipient, which is pumped down to a vacuum base pressure ~ 10 -8 mbar before deposition starts (Figure 1).

In the both sides of vacuum chamber, the machine is equipped with a 10 inch titanium disc target of high quality (grade2, minimum 99.7 % Ti). Two rotary magnet packs are placed just behind the targets to increase plasma density at their surface, and thus improve sputtering yield. Two power supplies are used to fix both target bias. A special rotating sample holder was designed to allow uniform deposition on the samples made of different materials and had different sizes and shapes. The machine allows also the RF etching of the substrate, a pretreatment step in order to remove particle contamination, as it is not possible to clean a ceramic (alumina) with solvent due to the high porosity of the material and the possibility of solvent trapping.

Optimal parameters for depositing TiN thin films by DC reactive magnetron sputtering were determined. X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Diffraction (XRD) analysis were performed to control the compositions and the stoechiometries of the obtained films [START_REF] Kaabi | Titanium nitride (TiN) coating as multipactor suppressor on RF coupler ceramic windows[END_REF][START_REF] Variola | Titanium Nitride Coating of RF Ceramic Windows by Reactive DC Magnetron Sputtering[END_REF].

COAXIAL RESONATOR MULTIPACTOR EXPERIMENT

The RF coaxial resonator is ~ 100 mm in length with an outer diameter of ~ 60 mm (internal dimensions) [START_REF] Khaldi | RF Coaxial Resonator for Investigating Multipactor Discharges on Metal and Dielectric Surfaces[END_REF]. The resonator's dimensions were chosen because of the available laboratory equipment. It is made out of stainless steel whose inner walls plated with copper and equipped with 4 ports: on the top, a small coaxial line penetrates the upper center electrode to provide a direct measure of the multipacting current, the bottom port for the pumping system, and two horizontal ports for RF coupling. Both RF antennas have the same coupling in order to maintain the symmetry of the central electric field. The RF antennas and diagnostics are accessed with vacuum feedthroughs. The flanges are also fabricated from stainless steel. Several pairs of center electrodes have been made from Cu, Al, and Stainless steel (S.S). One electrode sample holder made out of Cu was developed for investigating multipactor discharges on different material surfaces (Alumina ceramic (97.6% Al 2 O 3 ), TiN coated Alumina, quartz and TiN coated quartz). The dielectric samples have the square shape with a 10 mm edge length and 1 mm thickness. The vacuum system allows low pressures of ~ 10 -8 mbar.

A pulsed RF source delivers up to 30 W average power over a wide frequency range 650-900 MHz to the RF resonator. The pulse width and repetition rate used were 50 ms and 4Hz, respectively (20% duty cycle). The power supply is fed into a circulator which is then attached to a dual directional coupler. The forward and reflected RF powers are monitored by calibrated RF diodes (RF detectors). The electron probe is connected to an electronic circuit. A four 15 V batteries in series allow the bias of the electron probe and a current to voltage converter (1mA/V) provides temporal measurements of the multipacting electron current with respect to the RF power. Data acquisition is controlled by Visual Basic for Applications (VBA). All measured data are recorded and stored in tab delimited format which the signals can be plotted in an Excel © spread sheet. The test bench is straightforward and illustrated in Figure 2.

EXPERIMENTAL RESULTS

For each measurement the electrodes are prepared for vacuum by abrasively cleaning to a moderately polished finish. They are then vacuum cleaned with alcohol before entering the vacuum vessel.

Test Procedure

After assembly of a pair of electrodes the upper conflat flange is closed and the resonator is pumped to a base pressure around 4.10 -9 mbar through its bottom port. In order to initiate the multipactor discharge, The RF resonator is first tuned to the desired frequency. The RF source is then locked to the high-Q RF resonator. At the multipactor onset, the multipacting current, incident and reflected power and pressure variation were measured.

The S parameters' measurement were carried out with a Vector Network Analyzer (VNA). A good RF matching of the RF resonator was obtained. Main RF resonator parameters are summarized in Table 1.

TiN Coating as Multipactor Suppressor

In this section we compare and contrast the results from the RF power tests of the alumina and quartz without a coating "the uncoated samples", and the TiN (20 nm) coated Alumina and TiN (50 nm) coated quartz samples in order to evaluate a home made sputtered titanium nitride (TiN) thin layers as a Multipactor suppressor. The Alumina was chosen in this experiment because it has a high secondary emission coefficient and is commonly used in the RF vacuum windows. The quartz is used in the dielectric loaded accelerating structures. Figure 4 depicts the view of the resonator with a dielectric sample holder. The alumina and quartz samples (1 cm square sample, of height 1mm) are placed in a notch or a nick (square shaped cut) on top of one of the Cu electrodes, and the gap distance is d=6 mm.

CONCLUSIONS

The RF coaxial resonator multipacting experiment provides a test bed for producing and studying twosurface multipactor discharges under various conditions. The apparatus is also used to explore ways to prevent, or extinguish, multipactor by techniques such as coating the dielectrics or the metals. TiN coatings with different thicknesses will be explored.

Figure 1 :

 1 Figure 1: View of the DC Magnetron sputtering bench.

Figure 2 :

 2 Figure 2: (a) Scheme and (b) View of the Coaxial resonator multipacting test bench.

Figure 3

 3 Figure 3 shows typical scope traces for the quartz sample of the TTL signal, incident and reflected RF signals detected by the diodes, and the multipactor current provided by an electron probe.

Figure 3 :

 3 Figure 3: Typical scope traces of diode detectors and electron probe.

Figure 4 :

 4 Figure 4: View of the RF coaxial resonator with a dielectric sample holder.The electron inside the d empty gap is accelerated due to the RF electric field. The RF magnetic field in the center gap of this resonator is small as compared to the

Figure 5 :

 5 Figure 5: Multipacting current vs. time for a constant transmitted RF power (a) Quartz & TiN (50 nm) coated Quartz; (b) Alumina & TiN (20 nm) coated Alumina.

Figure 5 .

 5 Figure 5. Shows the efficiency of the TiN coatings, a significant reduction of multipactor current and time during RF tests were reached. The RF power is set to around 4.5 W, above the MP threshold value. The multipactor times for TiN coated quartz were suppressed to 10 min whereas for uncoated quartz surface it was close to 150 min.

Table 1 :

 1 RF Resonator Parameters

	Resonant frequency	742 MHz
	Insertion loss (I.L)	-1.6 dB
	Return Loss	-21 dB
	Loaded quality factor Q L	500
	Coupling coefficient β c	0.85

Proceedings of IPAC2015, Richmond, VA, USA ISBN 978-3-95450-168-7

Proceedings of IPAC2015, Richmond, VA, USA WEPMA007 7: Accelerator Technology T06 -Room Temperature RF ISBN 978-3-95450-168-7

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