JRA-SRF Work Package 7 Power Couplers

1. JRA-SRFWork Package 7Power Couplers Hassen Jenhani (LAL) CARE’08, CERN, December 2008

2. General context XFEL 800 accelerating superconducting cavities (and couplers) 1.3 GHz / 23.6 MV/m Length (m) 16000 accelerating superconducting cavities (and couplers) 1.3 GHz/ 31.5 MV/m ILC

3. RF conditioning • All couplers need to be cleaned and tested (RF conditioning) before their assembly on the cavities • Coupler RF conditioning: • High RF power • For XFEL: 1 MW for short pulses & 0.5 MW for 1.3 ms pulses • For ILC: 2 MW for short pulses & 1 MW for 1.5 ms pulses • Possibility of arcs • Possibility of strong vacuum bursts • Strong multipacting (resonant electron avalanches) • ….. • RF conditioning should be: • well mastered security for the coupler • optimized short conditioning time Damage the coupler

4. Outline TTF-III: DESY design • Conditioning & multipacting studies on TTF-III couplers (prototypes for XFEL) • Power coupler prototypes: TTF-V & TW60 • Titanium-Nitride (TiN) sputtering technology against multipacting on coupler ceramic windows TW60: LAL design TTF-V (LAL): based on TTF-III design TiN sputtering machine

5. Studies on the TTF-III couplers

6. Without I. B.* before the RF conditioning I. B.* before the RF conditioning + Optimization of the RF conditioning procedure 240 Av. = 157 h 220 I. B.* before the RF conditioning 200 180 160 Av. = 60 h 140 Av. = 21 h 120 100 Conditioning time (h) 80 60 40 20 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Coupler pairs processed RF conditioning activities • Originally :Very long conditioning time in average for the TTF-III couplers • Realizations: • Totally automated conditioning procedure for TTF-III was implemented : systematic studies on a significant number of couplers was possible • Study of the effect of the coupler preparation on the conditioning time • Optimization of the conditioning procedure based on data analysis and calculations • Reference for all the other prototype conditionings at LAL Published in SRF 2005, EPAC’06 (2006) et NIM A (2008) Conditioning time is drastically reduced *I. B. : In-situ baking

7. Experimental test of DC bias 4.5 kV DC bias to eliminate e- current activity for all the power ranges up to 1 MW

8. MP power levels (bellows are neglected) Bellows are considered MP level is not built up within the bellow undulations Multipacting studies Published in LINAC’08 (2008) Comparison between MP simulation and e- current measurements

9. New prototype couplers: TTFV &TW60

10. Need for prototypes • TTF-III is the baseline coupler for the ILC: • Well known coupler • Tested on high performance TESLA cavities (35 MV/m) • Short conditioning time • But: • Complex geometry • Expensive • Two axes for prototypes were investigated at LAL : • Similar coupler with best performances: TTF-V power coupler • New coupler design with simpler geometry: TW60 power coupler • Two coupler pairs of each model were designed, ordered and received at LAL. • Only one pair of each model was already tested using the TTF-III coupler conditioning procedure

11. D TTF-V coupler prototype Pierre Lepercq RF Studies (LAL) TTF-V is very similar to TTF-III, but, have larger cold part diameter in order to shift multipacting (MP) to higher power levels. Multipacting scaling law in coaxial lines: P1-point ~ (f . D)4. Z P2-point ~ (f . D)4. Z2 TTF-V TTF-III Coupler prototype TTF-V

12. TW60 coupler prototype Insulation using dielectric ring and insulating screws Capacitor (kapton) TW60 • TW60 design: • Coaxial planar window • New polarization system • Larger pumping port • Simpler geometry Pierre Lepercq RF Studies (LAL) Coupler prototype TTF-V Coupler prototype TW60

13. TW60 coupler RF conditioning First part of the conditioning: 23 h of conditioning time (20 µs pulses up to 660 kW; 2 Hz) But many e- current and vacuum interlocks Second part of the conditioning: 36 h of conditioning time; many interlocks; but fully conditioned Low level RF measurements (TW60 pair) TW60 coupler pair assembled for the RF tests Second part of the TW60 RF conditioning

14. -30 dB -35 dB 1.3 GHz Frequency (GHz) (dB) TTF-V coupler RF conditioning TTF-V RF conditioning Published in LINAC’08 (2008) TTF-V coupler pair assembled for the RF tests Low level RF measurements (TTF-V pair) Easy conditioning in 24 h only Next step: A TTF-V coupler pair will be conditioned at KEK following their conditioning procedure for ILC couplers (January 2009)

15. Titanium-nitride coating activity for the coupler ceramic windows

16. Objective • Nanometric Titanium-Nitride (TiN) layer is needed to be deposed on the coupler RF windows to avoid harmful multipacting. • Only few laboratories and industries master this technology. • Our aims: • mastery of TiN sputtering technology • optimization of the deposition process • obtaining good surface behavior against MP • and study of the MP process using the produced samples with well known surface characteristics

17. The sputtering machine Titanium target Magnetron Sputtering machine overview Sample of ceramic window Sample pretreatment: RF Etching Sample holder Reactive magnetron sputtering of TiN

18. The sputtering machine: results Published in EPAC’08 (2008) • First optimization of the parameter => stoechiometryis obtained for thick layers • Coating ~ 50 nm • Deposition velocity determined => ~0.3 angstrom/sec • Next steps: • Reduction of the TiN layer thickness • Multipacting tests (thanks to DESY colleagues)

19. Surface Characterization • Diffractometer (financed by IN2P3) • => Characterization of the deposed TiN surface: • Stoechiometric identification • Crystalline phase identification of deposited layer • Layer thickness measurement at nanometric scale • Roughness determination of thin layer deposit (Courtesy of W. Kaabi) Next step: Multipacting resonator stand to validate the TiN deposed layers and study multipacting

20. Summary CARE support had a strong impact on the coupler activities at LAL: • Drastic reduction of the conditioning time of the TTF-III couplers was achieved: very good result for the XFEL and ILC. • Good expertise in RF power couplers was acquired at LAL: LAL in charge of XFEL couplers industrialization. • Two coupler prototypes TTF-V and TW60 (alternatives for the ILC baseline coupler) were designed at LAL. RF conditioning was successful and more tests are planned. • TiN deposition activity is effective at LAL: first results are fully satisfactory & local surface characterization facility is growing with the participation of the IN2P3 (funds for the diffractometer)

21. Acknowledgment Thanks to J. Bastide, S. Cavalier, F. Cordillot, T. Chabaud, L. Grandsire, T. Garvey, W. Kaabi, M. Lacroix, P. Lepercq, B. Mercier, M. Omeich, C. Prévost, Yann Peinaud, A. Thiebault, A. Variola and J. Vieira from LAL. Thanks to our DESY colleagues especially A. Brinkmann, S. Choroba, D. Kostin, W-D. Möller and D. Proch. Thanks to G. Keppel, V. Palmieri and F. Strada from INFN/LNL We acknowledge the support of the European Community-Research Infrastructure Activity under the FP6 ‘‘Structuring the European Research Area’’ programme (CARE, Contract no. RII3-CT-2003-506395).