HOM effects in the damping ring - LNF

clappergappawpawUrban and Civil

Nov 16, 2013 (3 years and 8 months ago)

82 views

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)

PEP-II 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

PEP-II
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]
1.69E-08
2.86E-08
7.14E-07
S0 [m]
5.67E-05
6.75E-05
1.97E-04
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]
1.69E-08
1.69E-08
1.69E-08
S0 [m]
3.83E-05
2.08E-05
1.31E-05
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