S. N. HOM Impedance in Vacuum … 1 of 40 Sasha Novokhatski SLAC, Stanford University Machine-Detector Interface Joint Session April 22, 2005 HOM Impedance in Vacuum Designs

S. N. HOM Impedance in Vacuum … 2 of 40 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.

S. N. HOM Impedance in Vacuum … 3 of 40 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.

S. N. HOM Impedance in Vacuum … 4 of 40 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.

S. N. HOM Impedance in Vacuum … 5 of 40 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.

S. N. HOM Impedance in Vacuum … 6 of 40 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.

S. N. HOM Impedance in Vacuum … 7 of 40 Wake field Evidence from PEP-II Shielded fingers of some vacuum valves were destroyed by breakdowns of intensive HOMs excited in a valve cavity.

S. N. HOM Impedance in Vacuum … 8 of 40 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. Wake field Evidence from PEP-II

S. N. HOM Impedance in Vacuum … 9 of 40 HER resonance bellows Resonance at HER current of 1300 mA Temperature difference 9072QUA – 9062QUA Resonance 1-2 degrees F ~dZ~100 micron Q=10^3

S. N. HOM Impedance in Vacuum … 10 of 40 HOM leaking from TSP heater connector

S. N. HOM Impedance in Vacuum … 11 of 40 Effect of absorber installed in antechamber Temperature LER current Nov July 2004

S. N. HOM Impedance in Vacuum … 12 of 40 HOM Power in absorber

S. N. HOM Impedance in Vacuum … 13 of 40 The source of HOM power: Collimators

S. N. HOM Impedance in Vacuum … 14 of 40 Beams passing by collimators generate dipole and quadruple modes. These modes can easily penetrate though shielded fingers S. Weathersby

S. N. HOM Impedance in Vacuum … 15 of 40 HOM Power from collimators goes downstream

S. N. HOM Impedance in Vacuum … 16 of 40 Hottest Bellows 2012 takes HOM power from four Y and X Collimators Y and X collimators

S. N. HOM Impedance in Vacuum … 17 of 40 Collimator Loss Factor

S. N. HOM Impedance in Vacuum … 18 of 40 Bunch length dependence straight section collimator

S. N. HOM Impedance in Vacuum … 19 of 40 Collimator HOM Power Low HOM type collimators are needed for super B

S. N. HOM Impedance in Vacuum … 20 of 40 Special absorber device to capture collimator HOMs Red line shows absorption in ceramic tiles S. Weathersby

S. N. HOM Impedance in Vacuum … 21 of 40 Wake in IP region of PEP-II Simulation model

S. N. HOM Impedance in Vacuum … 22 of 40 PEP-II Vertex Bellows Bellows Cavity bunch field Mode Converter S. Ecklund measured 500 W dissipated in vertex bellows

S. N. HOM Impedance in Vacuum … 23 of 40 Field leakage though bellows fingers Will be captured by ceramic absorbing tiles in the new vertex bellows design

S. N. HOM Impedance in Vacuum … 24 of kW HOM power absorbed in ceramic tiles of Q2-bellows in PEP-II Stan Ecklund measurements

S. N. HOM Impedance in Vacuum … 25 of 40 Loss factor for PEP-II IR Bunch length dependence changes from (14-8 mm) to -3/2 (6-1 mm)

S. N. HOM Impedance in Vacuum … 26 of 40 IP HOM Power

S. N. HOM Impedance in Vacuum … 27 of 40 Additional beam energy loss due to Cherenkov radiation in open ceramic pipes. Loss factor HEACC92 page 537

S. N. HOM Impedance in Vacuum … 28 of 40 Additional beam power loss in Q2-bellows Using this formula for Q2 No open ceramics for Super B!

S. N. HOM Impedance in Vacuum … 29 of 40 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.

S. N. HOM Impedance in Vacuum … 30 of 40 RF antenna in a pump HV connector

S. N. HOM Impedance in Vacuum … 31 of 40 Antenna in other pump

S. N. HOM Impedance in Vacuum … 32 of 40 Q-value estimation

S. N. HOM Impedance in Vacuum … 33 of 40 Moveable collimator changes HOM spectrum in near pump

S. N. HOM Impedance in Vacuum … 34 of 40 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.

S. N. HOM Impedance in Vacuum … 35 of 40 Resistive Wall Wakefield Losses Loss factor asymptotic (M. Sands, K. Bane)

S. N. HOM Impedance in Vacuum … 36 of 40 Resistive Wall Wakefield Power

S. N. HOM Impedance in Vacuum … 37 of 40 Comparison of 2.5, 1, and 0.5 cm pipes. This is only resistive-wall power!

S. N. HOM Impedance in Vacuum … 38 of 40 Total HOM Loss Estimation

S. N. HOM Impedance in Vacuum … 39 of 40 DESY news F. Willeke

S. N. HOM Impedance in Vacuum … 40 of 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