ilc-bds collimator study adriana bungau and roger barlow the university of manchester cern - october...
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ILC-BDS Collimator Study
Adriana Bungau and Roger Barlow
The University of Manchester
CERN - October 15
Since last time…
Only higher order mode geometric wakefields were implemented in the Merlin code at the last COLSIM meeting
Resistive wakefields were included in the simulations (benchmark with an experiment at SLC)
At PAC - 07: the increase in the bunch size and the decrease in the luminosity due to geometric and resistive wakefields were presented for large offsets
However, large offsets of couple of hundreds of microns are not realistic in a real machine but useful in theory when tried to find the range when the split into modes occurs
Small offsets of several sigmas are more likely to happen Beam jitter in all ILC_BDS collimators Wakefield tests at SLAC in March and July (see Jonny’s talk)
No
Name Type Z (m) Aperture
1 CEBSY1 Ecollimator 37.26 ~
2 CEBSY2 Ecollimator 56.06 ~
3 CEBSY3 Ecollimator 75.86 ~
4 CEBSYE
Rcollimator 431.41 ~
5 SP1 Rcollimator 1066.61 x99y99
6 AB2 Rcollimator 1165.65 x4y4
7 SP2 Rcollimator 1165.66 x1.8y1.0
8 PC1 Ecollimator 1229.52 x6y6
9 AB3 Rcollimator 1264.28 x4y4
10 SP3 Rcollimator 1264.29 x99y99
11 PC2 Ecollimator 1295.61 x6y6
12 PC3 Ecollimator 1351.73 x6y6
13 AB4 Rcollimator 1362.90 x4y4
14 SP4 Rcollimator 1362.91 x1.4y1.0
15 PC4 Ecollimator 1370.64 x6y6
16 PC5 Ecollimator 1407.90 x6y6
17 AB5 Rcollimator 1449.83 x4y4
No Name Type Z (m) Aperture
18 SP5 Rcollimator
1449.84 x99y99
19 PC6 Ecollimator
1491.52 x6y6
20 PDUMP Ecollimator
1530.72 x4y4
21 PC7 Ecollimator
1641.42 x120y10
22 SPEX Rcollimator
1658.54 x2.0y1.6
23 PC8 Ecollimator
1673.22 x6y6
24 PC9 Ecollimator
1724.92 x6y6
25 PC10 Ecollimator
1774.12 x6y6
26 ABE Ecollimator
1823.21 x4y4
27 PC11 Ecollimator
1862.52 x6y6
28 AB10 Rcollimator
2105.21 x14y14
29 AB9 Rcollimator
2125.91 x20y9
30 AB7 Rcollimator
2199.91 x8.8y3.2
31 MSK1 Rcollimator
2599.22 x15.6y8.0
32 MSKCRAB Ecollimator
2633.52 x21y21
33 MSK2 Rcollimator
2637.76 x14.8y9
ILC-BDS colimators
Bunch size - geometric wakefields
- beam parameters at the end of linac:
x = 30.4 10-6 m, y = 0.9 10-6 m
- beam size at the IP in absence of wakefields:
x = 6.51*10-7 m, y = 5.69*10-9 m
- last talk->modes separation at 250 um (on
logarithmic scale!);
- for small offsets, modes separation occurs at
~10 sigmas;
Luminosity - geometric wakefields
- at 10 sigmas when the separation into modes occurs, the luminosity is reduced to 20%
- for a luminosity of L~1038 the offset should be 2-3 sigmas
Resistive wall
pipe wall has infinite thickness; it is smooth; it is not perfectly conducting the beam is rigid and it moves with c; test charge at a relative fixed distance;
bc
cThe fields are excited as the beam interacts with the resistive wall surroundings;
For higher moments, it generates different wakefield patterns; they are fixed and move down the pipe with the phase velocity c;
General form of the resistive wake
Write down Maxwell’s eq in cylindrical coordinates Combined linearly into eq for the Lorentz force components and
the magnetic field Assumption: the boundary is axially symmetric (
are ~ cos mθ and are ~ sin mθ ) Integrate the force through a distance of interest L Apply the Panofsky-Wenzel theorem
sr eBFFF ,|| ,, θ
rFF ,||
sBF ,θ
Lz
c
bzW
mmm 2/1
012
1
)1(
2)(
δπ +−= +
Lz
c
bzW
mmm 2/3
012
' 1
)1(
1)(
δπ += +
The MERLIN code
Previously in Merlin: Two base classes: WakeFieldProcess and
WakePotentials
- transverse wakefields
- longitudinal wakefields
Geometrical wakes:
Some functions made virtual in the base classes Two derived classes:
- SpoilerWakeFieldProcess - does the
summations
- SpoilerWakePotentials - provides
prototypes for W(m,s) functions (virtual) The actual form of W(m,s) for a collimator type is
provided in a class derived from SpoilerWakePotentials
WakeFieldProcess WakePotentials
SpoilerWakeFieldProcess
CalculateCm();CalculateSm();
CalculateWakeT();CalculateWakeL();ApplyWakefield ();
SpoilerWakePotentials
nmodes;virtual Wtrans(s,m);virtual Wlong(s,m);
Implementation of the Resistive wakes
WakeFieldProcess WakePotentials
SpoilerWakeFieldProcess
CalculateCm();CalculateSm();
CalculateWakeT();CalculateWakeL();ApplyWakefield ();
SpoilerWakePotentials
nmodes;virtual Wtrans(s,m);virtual Wlong(s,m);
ResistiveWakePotentials
Modes;Conductivity;pipeRadius;
Wtrans(z,m,AccComp); Wlong(z,m, AccComp);
Resistive wakes
Benchmark against an SLC result
Bunch size - resistive wakefields
For small offsets the mode separation starts at ~10 sigmas
At larger offsets (30-35 sigmas) there are
particles lost in the last collimators
The increase in the bunch size due to resistive wakefields is far greater than in the geometric case
Luminosity - resistive wakes
- at 10 sigmas when the separation into modes occurs, the luminosity is reduced to 10%-for a luminosity of L~1038 the offset should be less than 1 sigma- the resistive effects are dominant!
Bunch Shape Distortion
The bunch shape changes as it passes through the collimator; the gaussian bunch is distorted in the last collimators
But the bunch shape at the end of the linac is not a gaussian so we expect the luminosity to be even lower than predicted
Beam offset in each BDS collimator
No wakefields <y>=4.74e-12; Jitter of 1 nm of maximum tolerable bunch-to-bunch jitter in the train with 300 nm
between bunches; for 1nm: <y>=8.61e-11 Jitter about 100 nm which intratrain ffedback can follow with time constant of ~100
bunches; for 100nm: <y>=5.4e-10 Maximum beam offset is 1 um in collimator AB7 for 1nm beam jitter and 9um for 100 nm
jitter
Beam jitter
Beam jitter of 500 nm of train-to-train offset which intratrain feedback can comfortably capture
The maximum beam offset in a collimator is 40 um (collimator AB7) for a 500nm beam jitter
For 500nm: <y>=2.37e-9
Next plans
Study the wakefields of one collimator for the material damage tests in Japan (Ti coated with Be - emittance dilution and performance with Ti and Be resistivity)
Merlin code development for implementation of ECHO/GDFIDL results
…other suggestions?
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