Sasha Novokhatski

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
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HOM power in cavities (2004)
10RF
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Loss factor and HOM power
HOM Power
Bunch Spacing
Loss Factor
Current
Now small irregularities of the vacuum chamber
become very important
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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

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Examples from PEP-II

• A very small gap in a vacuum chamber is the
  source of high intensity wake fields, which cause
  electric breakdowns

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Small Gap, Breakdowns and Temperature Oscillations
Wake fields due to small 0.2 mm gap In the flange
connection
Breakdowns
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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
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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
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A gap ring may be a reason for the beam
instability
Breakdowns traces
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Fast Instability and vacuum spikes
LER
vacuum
abort
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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

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Wake field Evidence from PEP-II

• All shielded bellows in LER and HER rings have
  fans for air cooling to avoid high temperature
  rise.

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PEP-II Vertex Bellows
Bellows Cavity
S. Ecklund measured 500 W dissipated in vertex
bellows
bunch field Mode Converter
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Bunch-spacing resonances in HER bellows
HER current
Bellows temperature
Vacuum chamber temperature
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Change of temperature raise due to RF voltage
change in bellows
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Localized HOM source

• Beam collimators are powerful HOM sources in
  PEP-II

25
Main HOM Source are Collimators
MAC Review
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Detector region

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

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Wake in IP region of PEP-II
Simulation model
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HOM power is absorbed in ceramic tiles of
Q2-bellows in PEP-II
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Measured HOM power in Q2-bellows
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Loss factor for PEP-II IR
Bunch length dependence changes from s-2
(14-8 mm) to s-3/2 (6-1 mm)
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IP HOM Power
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Additional beam power loss comes from the
Cherenkov radiation in Q2 ceramic tiles
No open ceramics for Super B!
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Aborts and vacuum spikes in interaction region
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Simulation model
0.5mm gap
spring
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Electric displacement force lines
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Electric field distribution
Small Gaps
Tiles
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In time
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Maximum electric field is near breakdown limit
Left spring corner
First tiles gap
Tile corner
Metal corner
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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.

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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
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Resistive Wall Wakefield Power
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Comparison of 2.5, 1, and 0.5 cm pipes at IP.
This is only resistive-wall power!
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Surface roughness wake fields
Tube R5mm Random bumps lthgt50 m ltggt50 m Bunch s
250 m
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What we can do

• There is only one way
• absorb HOM power
• in specially designed water-cooled RF
  absorbers

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Effect of absorberinstalled in antechamber
Temperature
LER current
Nov. 2002-July 2004
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HOM Power in absorber
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Special absorber device to capture collimator
HOMs
Red line shows absorption in ceramic tiles
S. Weathersby
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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