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“HOM Impedance in Vacuum Designs”

“HOM Impedance in Vacuum Designs”. Sasha Novokhatski SLAC, Stanford University Machine-Detector Interface Joint Session April 22, 2005. Luminosity and wake fields. We need high current beams of very short bunches to achieve super high luminosity

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“HOM Impedance in Vacuum Designs”

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  1. “HOM Impedance in Vacuum Designs” Sasha Novokhatski SLAC, Stanford University Machine-Detector Interface Joint Session April 22, 2005

  2. Luminosity and wake fields • We need high current beams of very short bunches to achieve super high luminosity • These beams carry high intensity electromagnetic fields. • Any geometric disturbance or even surface roughness of a beam pipe leads to diffraction of these fields. • The diffracted fields can propagate free in the beam pipe. We call these field wake fields.

  3. Wake fields • High frequency part of wake fields can penetrate through small holes of shielded fingers to bellows or through RF screens to vacuum pumps. • These fields can also go outside vacuum chamber through heating wires of NEG pumps or through pump high voltage or BPM connectors.

  4. Wake fields • In a time wake fields are absorbed in conducting chamber walls. • Main effect from wake fields is temperature rise of different vacuum chamber elements, like shielded bellows, vacuum valves and pumps. In this case wake fields transfer energy to resonance High Order Modes (HOMs) excited in closed volumes.

  5. Wake fields • The amplitude of the HOM electric field can rich the breakdown limit and bring damage to the metal surface • Other effect can be the interaction of escaped (from the vacuum chamber) short wake field pulses with detector electronics.

  6. 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 mainly to chamber walls. • In all cases beams lose energy for wake field production. This energy has to be restored in RF cavities.

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

  8. Wake field Evidence from PEP-II • All shielded bellows in LER and HER rings have fans for air cooling to avoid high temperature rise. • All chambers have water cooling against resistive-wall wake fields.

  9. HER “resonance” bellows Resonance at HER current of 1300 mA Resonance 1-2 degrees F ~dZ~100 micron Q=10^3 Temperature difference 9072QUA – 9062QUA

  10. HOM leaking from TSP heater connector

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

  12. HOM Power in absorber

  13. The source of HOM power: Collimators

  14. Beams passing by collimators generate dipole and quadruple modes. These modes can easily penetrate though shielded fingers S. Weathersby

  15. HOM Power from collimators goes downstream

  16. Hottest Bellows 2012 takes HOM power from four Y and X Collimators Y and X collimators

  17. Collimator Loss Factor

  18. Bunch length dependencestraight section collimator

  19. Collimator HOM Power Low HOM type collimators are needed for super B

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

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

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

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

  24. 10 kW HOM power absorbed in ceramic tiles of Q2-bellows in PEP-II Stan Ecklund measurements

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

  26. IP HOM Power

  27. Additional beam energy loss due to “Cherenkov” radiation in open ceramic pipes. HEACC’92 page 537 Loss factor

  28. Additional beam power loss in Q2-bellows Using this formula for Q2 No open ceramics for Super B!

  29. RF screens.NEG chamber and a vacuum pump flange Temperature rise in NEG chambers due to HOM heating changed the vacuum. M. Sullivan attached a lot of thermocouples to NEG chambers to understand the problem.

  30. RF antenna in a pump HV connector

  31. Antenna in other pump

  32. Q-value estimation

  33. Moveable collimator changes HOM spectrum in near pump

  34. RF screens and coupling • Screen impedance scales with frequency (inversely to bunch length) • Holes must be 5 times smaller than for PEP-II, or two times thicker • Decreasing Q-value of a bellow or NEG cavity by placing absorber • Low Q additionally decreases field coupling.

  35. Resistive Wall Wakefield Losses Loss factor asymptotic (M. Sands, K. Bane)

  36. Resistive Wall Wakefield Power

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

  38. Total HOM Loss Estimation

  39. DESY news F. Willeke

  40. Summary • Electron and positron bunches generate electromagnetic fields at any discontinuity of vacuum chamber • These fields can travel long distance and penetrate inside bellows, pumps and vacuum valves. • Vacuum chamber must be optimized for minimum wake loss

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