CLIC crab cavity design

1. CLIC crab cavity design Praveen Ambattu 24/08/2011

2. abs E, V/m abs H, A/m abs S, W/m2 TM010 5 mm TM110 5 mm Monopole x dipole mode @ 12 GHz Crab cavity has a E, H and S distributions different from the main linac

3. CLIC crab cavity numbers • Bunch rotation:10 mrad • Transverse space: ~100 cm • Mode: 2p/3, 11.9942 GHz • Voltage: 2.55 MV per cavity • Available peak power: 15 MW • RF tolerance for 98 % luminosity: • dVrf/Vrf=2 %, dfrf=18 mdeg • Peak surface field: <250 MV/m • Peak pulsed heating: <40 K

4. Cell shape • Initial optimisation was for a cylindrical cell • 5 mm radius beampipe was chosen as a compromise among surface fields and short range wakefields

5. Vertical modes for 10 cell cavity Kick factor x frequency SOM Q x HOM Q SOM band • Vertical wakes are dominated by the SOM band which is the 1st dipole mode itself but in 90 deg plane • For r0=35 mm, SOMs needs Q<100 to meet the luminosity requirements

6. By shifting the SOM frequency with highest kick to 6.5th bunch harmonic (13 GHz), the last bunch in the train will see zero sum wakefield • This allows relaxing the SOM damping requirement • This can be implemented by using an asymmetric cell shape

7. Asymmetric cell shapes • Achieving 1 GHz shift in dipole frequency with less structure complexity would be easy with racetrack shape.

8. Single racetrack cell E-field (1 J stored energy)

9. H-field (1 J)

10. Power flow and Sc (1 J) Sc=max(ReS+ImS/6) ReS=sqrt(ReSx^2+ReSy^2+ReSz^2) ImS=sqrt(ImSx^2+ImSy^2+ImSz^2) Sc/Et2=3.87 mA/V Transverse gradient, Et=Vt/Lcell Vt=jVz(r)*(c/wr)

11. Single cell properties (1 J)

12. Power coupler • Structure size mainly depends on • power coupler type :- standard, waveguide, mode launcher etc • feed geometry :- single or dual • length of damping waveguides Waveguide coupler Standard coupler

13. Waveguide coupler Standard coupler More space needed to include damper, also to help cell tuning Splitter size

14. Coupler comparison

15. Dual-feed x single-feed coupler • DFC perfectly centres the dipole mode in the end cells due to symmetric feeding and do not excite other modes • This needs the waveguide arms of the splitter be temperature stabilised • Assembly tolerance is also critical • Inclusion of dampers and tuning the end cells will be difficult unless longer waveguide arms are used • More trapped modes, hence more wakefield • Single-feed coupler avoids all above • But the mode is not centred causing beamloading • This can be minimised by flipping the two couplers 180 deg with each other to reduce the effect of beam loading • Not needed in the prototype 1 as there is no beam

16. Single-feed coupler

17. slot_a slot_h slot_a slot_h Microwave Studio copper model • Used a dummy waveguide, cut-off to 12 GHz • Could be used as damping waveguide in the final cavity • Could be avoided in the 1st prototype • Waveguides can be on the same plane

18. Power flow (1W) y z x All waveguides are terminated by ports

19. S-parameters

20. Complex field (Hx) in the band

21. Hx(0) on-axis Ey(0) on-axis Ez(r) 0.5 mm off-axis Field amplitudes at 11.9942 GHz

22. beam pipe Internal reflection and phase advance at 11.9942 GHz from Hx(0)

23. wg2 wg1 wg1 Ez(0) x y wg2 Ez(0) x z Beamloading in the end cells

24. R0.5 mm Coupler cell x Regular cell Hsurf=360 kA/m  DT=28 K Hsurf=269 kA/m  DT=16 K For 13.5 MW peak power and 242 ns pulse

25. RF properties * TD26_vg1p8_R05, Sc~5 W/mm2

26. Cavity tuning Simulated 0.5 mm deep pit pin in 0 MHz/mm pin out -0.6 MHz/mm pin in 27.2 MHz/mm pin out -16.8 MHz/mm pin in 18.2 MHz/mm pin out -11.4 MHz/mm For 1 MHz tuning, ~1.26 mm in radius~50 mm in pit

27. Bead pull • Tuning could be done by ‘non-resonant perturbation’ technique, combined with bead-pull, identical to what has been done for the accelerating cavity • Simulated beadpull result using a metallic disk (1.5 mm dia x 1mm thick) shown below seems to give well defined perturbation • More accurate field measurement needs a fine cylinder made of thin surgical needle complex DS11

28. HFSS x MWS fields @ 1 W, 11.9942 GHz MWS HFSS Hx on axis Ez off axis

29. RF properties MWS x HFSS For coarse mesh inside the cavity • Fields amplitudes in HFSS are higher than in MWS by a factor of 1.15 • Needs more investigation but seems OK !!

30. Discussion • Single feed without dummy wg ? • Yes it is, as the priority is to RF test the undamped cavity • Cooling pipes on iris or equator ? • equator • Tuning pins 0 deg or 45 deg ? • 45 deg • Timescales ? • Finish drawing by Dec 2011, start procurement of copper by Jan 2012, so EuCARD money could be spent before March 2012

31. CLIC crab cavity final design (using CST Microwave Studio 2010) Praveen Ambattu 26/08/2011 • The design changed from what shown at the RF group meeting • Removed extra waveguide on coupler cells • Increased coupler slot rounding to 1 mm from 0.5 mm • Increased waveguide corner rounding to 4 mm from 2 mm

32. Single cell with periodic boundary of 2p/3 Esurf Ssurf Hsurf

33. Comparison with HFSSv13 (Vasim F. Khan, CERN fellow) Mesh view MWS HFSS • MWS supports only PEC material in eigenmode simulation • MWS used Perfect Boundary Apprx, 134,912 hexahedra per quarter (lines/lamda=40, lower mesh limit=40, mesh line ratio limit=40) • HFSS used 8,223 tetrahedra per quarter (surface apprx= 5mm, aspect ratio=5)

34. 12 cell structure frequency domain simulation Mesh view • Full structure with one symmetry plane • 1.75 m tetrahedral elements • Calculation at 11.9942 GHz and 1 W ‘peak’ input power

35. S-parameters

36. Complex field (Hx) in the band

37. Magnetic field profile-Hx on axis at 11.9942 GHz for 1 W

38. Electric field profile -Ey on axis at 11.9942 GHz for 1 W

39. Red: at y=0.5 mm Green: at y=0 Electric field profile -Ez(y) at 11.9942 GHz for 1 W

40. Power flow (1 W)

41. Internal reflection at 11.9942 GHz

42. Phase advance at 11.9942 GHz

43. Properties of full cavity