“HOM Effects in the Damping Ring”

1. “HOM Effects in the Damping Ring” Sasha Novokhatski SLAC, Stanford University WG2 –Damping Rings March 17, 2005

2. Luminosity and electromagnetic fields • We need high current beams of very short bunches to achieve super high luminosity • These beams carry high intensity electromagnetic fields. Electric field at the beam pipe wall If these fields are near a sharp metal corner they may exceed the breakdown threshold

3. Bunch field spectrum • Field spectrum goes to higher frequency with shorter bunches exponentially Beam spectrum (12 mm bunch) Bunch spacing resonances Bunch spacing

4. Luminosity and wake fields • Any geometric disturbance, finite electric conductivity or even surface roughness of a beam pipe may lead to diffraction of these fields. • The diffracted fields are separated from the beam and propagate free in the beam pipe. • We call these field as wake fields.

5. Wake fields and HOMs Loss Factor Frequency Integral, Main mode and Higher Order Modes Wake fields of a short bunch in a PEP-II cavity

6. HOM power in cavities (2004) 10%RF

7. Loss factor and HOM power HOM Power Bunch Spacing Loss Factor Current Now small irregularities of the vacuum chamber become very important

8. Main HOM Effects • Heating of vacuum elements • Temperature and vacuum rise • Deformations and vacuum leaks • Decreasing pumping speed • Breakdowns and multipacting • Vacuum leaks • Melting thin shielded fingers • Longitudinal instabilities • Electromagnetic waves outside vacuum chamber • Interaction with high sensitive electronics

9. Examples from PEP-II • A very small gap in a vacuum chamber is the source of high intensity wake fields, which cause electric breakdowns

10. Small Gap, Breakdowns and Temperature Oscillations Wake fields due to small 0.2 mm gap In the flange connection Breakdowns

11. HOMs with transverse components • Wake fields, which have transverse components may penetrate through small slits of shielded fingers to vacuum valves volumes and excite high voltage resonance fields, which may destroy the fingers

12. Wake field Evidence from PEP-II • Shielded fingers of some vacuum valves were destroyed by breakdowns of intensive HOMs excited in the valve cavity.

13. Wake fields outside • Wake fields can go outside the vacuum chamber through heating wires of TSP pumps.

14. HOM leaking from TSP heater connector The power in the wake fields was high enough to char beyond use the feed-through for the titanium sublimation pump (TSP). antenna HOM spectrum from Spectrum analyzer

15. Wake fields • Other possibilities for wakes to go outside is to escaped from the vacuum pumps through RF screens

16. HOMs go through RF screens RF spectrum antenna RF screens

17. A gap ring may be a reason for the beam instability Breakdowns traces

18. Fast Instability and vacuum spikes LER vacuum abort

19. Temperature raise • Propagating in the vacuum chamber wake fields may transfer energy to resonance High Order Modes (HOMs) excited in the closed volumes of shielded bellows. • Main effect is the temperature rise

20. Wake field Evidence from PEP-II • All shielded bellows in LER and HER rings have fans for air cooling to avoid high temperature rise.

21. PEP-II Vertex Bellows Bellows Cavity S. Ecklund measured 500 W dissipated in vertex bellows bunch field ‘‘Mode Converter”

22. Bunch-spacing resonances in HER bellows HER current Bellows temperature Vacuum chamber temperature

23. Change of temperature raise due to RF voltage change in bellows

24. Localized HOM source • Beam collimators are powerful HOM sources in PEP-II

25. Main HOM Source are Collimators MAC Review

26. Detector region • Other effect can be the interaction of escaped (from the vacuum chamber) short wake field pulses with detector electronics.

27. Wake in IP region of PEP-II Simulation model

28. HOM power is absorbed in ceramic tiles of Q2-bellows in PEP-II

29. Measured HOM power in Q2-bellows

30. Loss factor for PEP-II IR Bunch length dependence changes from s-2 (14-8mm) to s-3/2 (6-1 mm)

31. IP HOM Power

32. Additional beam power loss comes from the Cherenkov radiation in Q2 ceramic tiles No open ceramics for Super B!

33. Aborts and vacuum spikes in interaction region

34. Simulation model 0.5mm gap spring

35. Electric displacement force lines

36. Electric field distribution Small Gaps Tiles

37. In time

38. Maximum electric field is near breakdown limit Left spring corner First tiles gap Tile corner Metal corner

39. Resistive-wall wake fields • Other type of wake fields is excited due to finite conductivity of vacuum chamber walls. • Resistive-wall wake fields give temperature rise everywhere in the ring.

40. Change of temperature raise due to RF voltage change in chambers RF Voltage was changed from 4.5 MV to 5.4 MV Temperature of the vacuum chamber changed by 4F around the ring

41. Resistive Wall Wakefield Power

42. Comparison of 2.5, 1, and 0.5 cm pipes at IP. This is only resistive-wall power!

43. Surface roughness wake fields TubeR=5mm Random bumps <h>=50 m <g>=50 m Bunch s =250 m

44. What we can do • There is only one way : absorb HOM power in specially designed water-cooled RF absorbers

45. Effect of absorberinstalled in antechamber Temperature LER current Nov. 2002-July 2004

46. HOM Power in absorber

47. Special absorber device to capture collimator HOMs Red line shows absorption in ceramic tiles S. Weathersby

48. Field leakage though bellows fingers Will be captured by ceramic absorbing tiles in the new vertex bellows design

49. Summary • Vacuum chamber must be very smooth. • HOM absorbers must be installed in every region that has unavoidable discontinuity of vacuum chamber • Increase the bunch length in damping rings