Choke Mode Damped X-Band Structure for CLIC Main Linac: Collaboration between CERN and Tsinghua University

1. Choke-mode Damped X-band Structure for CLIC Main Linac Hao ZHA, Jiaru SHI CERN Sep 27, 2011 Jiaru Shi, LCWS11 Workshop, Granada 1

2. Jiaru Shi, LCWS11 Workshop, Granada Acknowledgement Collabration between CERN and Tsinghua University in China Ph.D Student visiting CERN for 6 months, ZHA Hao Manufacturing capability at Tsinghua Outline The Idea of choke-mode-damping Parameters Comparison with waveguide damping Wakefield damping study Future plans 2

3. The choke mode cavity Progress at KEK and SLAC Applications at Japanese XFEL 3

4. Radial Choke [1] Jiaru Shi, LCWS11 Workshop, Granada [1] A. Grudiev 4

5. Parameters [1] Jiaru Shi, LCWS11 Workshop, Granada Cell NDS WDS CDS1mm CDS1.5mm CDS2mm Q Cu 6825 5635(0.83) 4720(0.7) 5129(0.75) 5617(0.82) Vg / c (%) 1.32 1.2(0.9) 1.12(0.85) 1.04(0.79) 0.97(0.73) R/Q (kOhm/m) 17.8 16.1(0.91) 15.2(0.85) 14.1(0.79) 13.2(0.74) Rs (Mohm/m) 121.5 90.7 (0.75) 71.7 (0.59) 72.3 (0.59) 74.1 (0.60) E surfmax /E acc 1.93 1.93 1.93 1.93 1.93 H surfmax /E acc [mA/V] 2.5 4.0(1.6) 2.5 2.5 2.5 Pulse d T rise (K) 13 32 13 13 13 Plused T rise in first cell. @ 118MV/m 20 47 20 [1] Based on CLIC-G middle cell [2] 250 ns, 100MV/m 5

6. Advantages Lower pulsed surface heating, no magnetic field enhancement Bonding joint at low magnetic field, no problem on the chamfer Easy manufacturing, turning only Considerations Knowledge from SLAC [1] , small gap (1mm) severe breakdown / 4mm gap OK different gap size Prototype structures for high power test 6 Jiaru Shi, LCWS11 Workshop, Granada

7. Jiaru Shi, LCWS11 Workshop, Granada Damping simulation with Gdfidl/HFSS (Model in HFSS) Gap 1mm 2mm Good damping for first dipole Mode reflected by the choke, to be studied E field, fundamental mode E field of a dipole mode that is reflected by the choke Impedance and wakefield simulated in Gdfidl 7

8. EM field Jiaru Shi, LCWS11 Workshop, Granada High H, Low E Zero H, High E Fundamental mode at 12GHz High order dipole at 30GHz 8

9. EM field after detuning with bubble Jiaru Shi, LCWS11 Workshop, Granada H H E E Fundamental mode tuned back to 12GHz By changing choke-length 9

10. Smith Chart 10 A B C A B C B C Bubble,+j Bubble,+j No effect when adding to infinity Off from open point Red: fundamental mode Blue; 3h-HOM

11. Model in HFSS for optimization 11 Port 1 Port 2 Absorber in radial line absorb in the radial line: sqrt( 1-S11^2-S21^2 )

12. Radial line absorption 12 Choke reflection

13. Results of bubble-choke s 0.15m origin 20~30 Bubble- choke 20~25 WDS 6~8 13 31GHz 34GHz (in V/pC/m/mm) Push the second peak, more and more

14. Smith chart double 14 A B C A B C B double 2*z double 2*z C Red: fundamental mode Blue; 3h-HOM

15. double choke, bottle-choke 15 40GHz! 50GHz!

16. First dipole 16 s 0.15m origin 20~30 CDS 50~60 WDS 6~8 (in V/pC/m/mm)

17. Thin-neck bottle Blue: 1:1:3 bottle-choke Red: 3:1:3 thin-neck bottle-choke 17 12GHz 52GHz Narrow passband!

18. Result of thin-neck bottle 18 s 0.15 0.30 CDS- origin 20~25 10~15 CDS 2~4 0.5~0.8 WDS 6~8 1~1.5 (in V/pC/m/mm) Note: applied to Tapered structure

19. Problem of the thin neck 220MV/m electric field in the choke (@100MV/m gradient) 19

20. Impedance matching Reduced field in choke: 1:1 to gradient; as max surface field @ iris. Optimizing the depth, length, position of the step, try to match the 16G,18G mode 20

21. Absorption after impedance matching Blue: 1:1:3 bottle-choke Red: 1.5:1:3 bottle-choke with impedance matching 21

22. Result: 22 s 0.15 0.30 CDS- origin 20~25 10~15 CDS 6~8 1~2.2 WDS 6~8 1~1.5 (in V/pC/m/mm)

23. Possible design 23 Pin: 61.3MW Loaded gradient: 100MV/m CLIC-G TD24-Choke same irises Pin: 61.3MW Loaded gradient: 96MV/m

24. Summary Jiaru Shi, LCWS11 Workshop, Granada Study choke as transmission line, with smith chart Simple model in HFSS for optimization, also possible for a prototype to do RF measurement Choke mode damping with comparable result to waveguide damping Future plans Finalize the alternative RF design with choke-mode damping that could possibly replace the baseline design Propose wakefield measurement at SLAC Prototype for high power test 24

25. Thank you! Jiaru Shi, LCWS11 Workshop, Granada 25