Sasha Novokhatski “ HOM Effects in the Damping Ring”
Sasha Novokhatski
SLAC, Stanford University
WG2
–
Damping Rings
March 17, 2005
“HOM Effects
in the Damping Ring”
Sasha Novokhatski “ HOM Effects in the Damping Ring”
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
.
0
3
2
11
2
1
23.
10
b
kV
cm cm
c
b
m
cZ eN
E
a
N
E
a
Electric field at the beam pipe wall
If these fields are near a sharp metal corner they may exceed the breakdown
threshold
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Bunch field spectrum
•
Field spectrum goes to higher frequency
with shorter bunches exponentially
Beam spectrum (12 mm bunch)
Bunch
spacing
resonances
1,2,3,...
n
b
n
f n
1,2,3,...
b
RF
m
m
f
Bunch
spacing
2
( ) ~
c
A e
Sasha Novokhatski “ HOM Effects in the Damping Ring”
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
.
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Wake fields and HOMs
Wake fields of a
short bunch
in a
PEP

II cavity
Loss Factor Frequency Integral
,
Main mode
and Higher Order Modes
Sasha Novokhatski “ HOM Effects in the Damping Ring”
HOM power in cavities (2004)
10%RF
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Loss factor and HOM power
2
b
P K I
HOM Power
Bunch Spacing
Loss Factor
Current
2
[ ] [ sec] [ ]
1 4.2 0.026 3
kW n V A
pC
Now small irregularities of the vacuum chamber become very important
Sasha Novokhatski “ HOM Effects in the Damping Ring”
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
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Examples from PEP

II
•
A very small gap in a vacuum chamber is
the source of high intensity wake fields,
which cause electric breakdowns
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Small Gap, Breakdowns and
Temperature Oscillations
Wake fields
due to small
0.2 mm gap
In the flange
connection
Breakdowns
Sasha Novokhatski “ HOM Effects in the Damping Ring”
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
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Wake field Evidence from PEP

II
•
Shielded fingers of some vacuum valves were destroyed by
breakdowns of intensive HOMs excited in the valve cavity
.
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Wake fields outside
•
Wake fields can go outside the vacuum
chamber through heating wires of TSP
pumps.
Sasha Novokhatski “ HOM Effects in the Damping Ring”
HOM leaking from TSP heater connector
HOM spectrum from
Spectrum analyzer
antenna
The power in the wake fields
was high enough to char
beyond use the feed

through
for the titanium sublimation
pump (TSP).
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Wake fields
•
Other possibilities for wakes to go outside
is to escaped from the vacuum pumps
through RF screens
Sasha Novokhatski “ HOM Effects in the Damping Ring”
HOMs go through RF screens
antenna
RF spectrum
RF screens
Sasha Novokhatski “ HOM Effects in the Damping Ring”
A gap ring may be a reason for the
beam instability
Breakdowns traces
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Fast Instability and
vacuum spikes
abort
LER
vacuum
Sasha Novokhatski “ HOM Effects in the Damping Ring”
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
Sasha Novokhatski “ HOM Effects in the Damping Ring”
•
All shielded bellows in LER and HER rings have fans for
air cooling to avoid high temperature rise.
Wake field Evidence from PEP

II
Sasha Novokhatski “ HOM Effects in the Damping Ring”
PEP

II Vertex Bellows
S. Ecklund measured
500 W dissipated in
vertex bellows
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Bunch

spacing resonances in
HER bellows
HER current
Vacuum chamber temperature
3
1
~
~10
bellows
bellows
chamber chamber
bellows
l
f
Q f l
l T
l
Bellows temperature
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Change of temperature raise due to
RF voltage change in bellows
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Localized HOM source
•
Beam collimators are powerful HOM
sources in PEP

II
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Main HOM Source are Collimators
MAC Review
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Detector region
•
Other effect can be the
interaction
of
escaped (from the vacuum chamber)
short wake field pulses with detector
electronics
.
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Wake in IP region of PEP

II
Simulation model
Sasha Novokhatski “ HOM Effects in the Damping Ring”
HOM power is absorbed in ceramic
tiles of Q2

bellows in PEP

II
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Measured HOM power in Q2

bellows
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Loss factor for
PEP

II
IR
Bunch length
dependence
changes
from
2
(14

8
mm)
to

3/2 (6

1 mm)
PEPII Interaction region
Loss factor and approximations
y = 38.575x
1.967
y = 17.379x
1.4934
0
1
2
3
4
5
6
7
8
9
10
0
2
4
6
8
10
12
14
Bunch length [mm]
Loss factor [V/pC]
Sasha Novokhatski “ HOM Effects in the Damping Ring”
IP HOM Power
2 2
( ) 0.5*( )
HOM Pow
er
b
e e
P k I I
Bunch l engt h [ mm] =
12
Loss f act or [ V/pC] =
0.2 3 3
LER current [ A]
3
HER current [ A]
1.7
Bunch spaci ng [ nsec]
4.2
Power l oss ( pul se) [ kW]
10.2 0
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Additional beam power loss comes from the
Cherenkov radiation in Q2 ceramic tiles
PEPII
eps=
30
L [mm] =
59.2
Bunch length [mm] =
12
s=
4.487637
check s/sigma <1 or >1
0.37397
Loss factor [V/pC]=
0.089932
LER current [A]
3
HER current [A]
1.7
Bunch pattenrt by2 T [nsec]
4.2
Power loss [kW] (incoherent)
4.491
No open ceramics for Super B!
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Aborts and vacuum spikes in
interaction region
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Simulation model
spring
0.5mm
gap
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Electric displacement force lines
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Electric field distribution
Tiles
Small Gaps
Sasha Novokhatski “ HOM Effects in the Damping Ring”
In time
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Maximum electric field is near
breakdown limit
Left spring corner
First tiles gap
Metal corner
Tile corner
Sasha Novokhatski “ HOM Effects in the Damping Ring”
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.
Sasha Novokhatski “ HOM Effects in the Damping Ring”
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
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Resistive Wall Wakefield Power
pipe Radius [m]
0.045
0.045
0.045
Material
Cu
Al
SS
resistivity [Ohm m]
1.69E08
2.86E08
7.14E07
S0 [m]
5.67E05
6.75E05
1.97E04
bunch length [m]
0.003
0.003
0.003
loss factor [V/pC]
0.002
0.003
0.015
Bunch spacing [nsec]
2.1
2.1
2.1
beam current [A]
10
10
10
power [kW/m]
1.017
1.320
6.602
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Comparison of 2.5, 1, and 0.5 cm pipes at IP.
pipe Radius [m]
0.025
0.01
0.005
Material
Cu
Cu
Cu
resistivity [Ohm m]
1.69E08
1.69E08
1.69E08
S0 [m]
3.83E05
2.08E05
1.31E05
bunch length [m]
0.003
0.003
0.003
Loss factor
0.004
0.010
0.021
Bunch spacing [nsec]
2.1
2.1
2.1
beam current [A]
23
23
23
power [kW/m]
9.684
24.209
48.418
This is only resistive

wall power!
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Surface roughness wake fields
Tube
R=5mm
Random bumps
<h>=50
m
<g>=50
m
Bunch
=250
m
Sasha Novokhatski “ HOM Effects in the Damping Ring”
What we can do
•
There is only one way :
absorb HOM power
in specially designed water

cooled
RF absorbers
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Effect of absorber
installed in antechamber
Temperature
LER current
Nov. 2002

July 2004
Sasha Novokhatski “ HOM Effects in the Damping Ring”
HOM Power in absorber
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Special absorber device to capture
collimator HOMs
Red line shows absorption in ceramic tiles
S. Weathersby
Sasha Novokhatski “ HOM Effects in the Damping Ring”
Field leakage though
bellows fingers
Will be captured by
ceramic absorbing tiles in
the new vertex bellows
design
Sasha Novokhatski “ HOM Effects in the Damping Ring”
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
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