higher-order modes and beam-loading compensation in clic main linac oleksiy kononenko be/rf, cern...
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Higher-Order Modes and Beam-Loading Compensation in
CLIC Main Linac
Oleksiy KononenkoBE/RF, CERN
CLIC RF Structure Development Meeting, March 14, 2012
2
Outline
• Motivation• Beam-loading compensation scheme• Frequency Domain: HFSS/ACE3P benchmark• Time Domain: HFSS/ACE3P/gdfidl benchmark• Effect of the higher-order modes to the
compensation scheme• Conclusion
3
Motivation: CLIC Performance Issue
*CLIC-Note-764, private conversations with Daniel Schulte (CERN)
In order to have luminosity loss less than 1%, the RMS bunch-to-bunch relative energy
spread must be below 0.03%
CLIC Drive Beam Generation Complex
*CLIC-Note-764
5
Energy Spread Minimization SchemeUnloaded Voltage in AS
- fix phase switch times in buncher- generate corresponding drive beam profile- take into account PETS (+PETS on/off) bunch response- calculate unloaded voltage
Loaded Voltage in AS - calculate AS bunch response - calculate total beam loading voltage
- add to unloaded voltage
Energy Spread Minimizationvarying buncher delays
6
Beam-Loading Compensation
Main results are published: O. Kononenko, A. Grudiev, Transient beam-loading model and compensation in Compact Linear Collider main linac, Physical Review, Special Topics on Accelerators and Beams, 2011, Vol. 14, Issue 11, 10 pages, http://prst-ab.aps.org/abstract/PRSTAB/v14/i11/e111001
7
HFSS Simulation Setup Model:
- 90 deg of the structure- copper outer walls
H-plane
Port 2
Port 1
H-plane
Simulation profile: - second order basis functions
- curvilinear elements enabled- 0.001 s-parameters accuracy
leads to ~300K tet10 mesh
8
HFSS Port and Plane Wave Excitations
Thanks to Valery Dolgashev from SLAC for the idea to use the plain wave source
9
s3p Simulation Setup Model:
- 90 deg of the structure- copper outer walls
H-plane
Port 2
Port 1
H-plane
Simulation profile: - second order basis functions
- curvilinear elements enabled- 2M tet10 mesh
10
Reflection Coefficient s11
11.99 11.991 11.992 11.993 11.994 11.995 11.996 11.997 11.998 11.999 12-65
-60
-55
-50
-45
-40
-35
-30
-25
-20
Frequency, GHz
Re
fle
cti
on
, d
B
s3pHFSS
11
Complex Magnitude Ez, f=11.994GHz
0 50 100 150 200 250 3000
5
10
15
20
25
30
35
40
45
z, mm
|Ez|
, k
V/m
s3pHFSS|s3p-HFSS|
12
Ez(z) in a Complex Plane, f=11.994GHz
-50 -40 -30 -20 -10 0 10 20 30-40
-30
-20
-10
0
10
20
30
40
Re(Ez), kV/m
Im(E
z), k
V/m
HFSSs3p
13
s3p/HFSS Benchmark SummaryTD26 RF Design
Remarks s3p HFSSf, GHz 11.994
Filling time, ns 67.3393 66.98
Q-factor, Cu 5682.5388 5657
S12, dB -3.8784 -3.8750
S11, dB -60.7318 -58.2715
Voltage, V (Pin=4W) 7022.376 7040
There is a very good agreement between the HFSS and s3p results
14
t3p Simulation Setup Model:
- 90 deg of the structure- bunch sigma = 1mm- ABC/WG condition: couplers, beam-pipe, damping waveguides- PEC/copper outer walls
Simulation profile: - second order basis functions
- curvilinear elements enabled- 2, 3, 6, 12M tet10 meshes
H-plane
H-plane
Beam
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Bunch Passage through TD26
DC trail which is caused by the numerical errors can be observed, 2M mesh has been used
16
ACE3P Wake Convergence Study
0 10 20 30 40 50 60 70 800
20
40
60
80
100
120
140
Time, ns
Lo
ng
itu
din
al
Wa
ke
, V
/pc
2M_p1_1ps12M_p2_1ps_IMP2M_p2_1ps6M_p1_1ps12M_p1_1ps
Different wake length is simulated because of the limited computer resources. Maximum 6hours x 2400 CPU per one run
17
gdfidl Simulation Setup Model:
- 90 deg of the structure- bunch sigma = 1mm- PEC outer walls
Simulation profile: - mesh planes fixed to the irises, thanks to Vasim - 100, 50 um uniform cubic meshes, 50x50x25um mesh
Model:- 90 deg of the structure- bunch sigma = 1mm- PML condition: couplers, beam-pipe, damping waveguides- PEC outer walls
H-plane
H-plane
Beam
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gdfidl Convergence upon the Mesh Size
0 10 20 30 40 50 60 70 80 90 100-20
0
20
40
60
80
100
120
140
160
Time, ns
Wa
ke
, V
/pc
25um50um100um
Convergence is observed while wake rises at the tail for some reason
19
12 14 16 18 20 22 24 2610
-6
10-4
10-2
100
102
Frequency, GHz
Imp
ed
an
ce
, V
/A
HFSSACE3Pgdfidl
Beam Coupling Impedance HFSS/ACE3P/gdfidl
Strange ACE3P Resonances
Monopole band
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Lowest Monopole Band ImpedanceHFSS/ACE3P/gdfidl
24 24.5 25 25.5-0.04
-0.02
0
0.02
0.04
0.06
0.08
Frequency, GHz
Lo
ng
itu
din
al
Imp
ed
an
ce
, V
/A
HFSSACE3Pgdfidl
21
Fundamental Mode ImpedanceHFSS/ACE3P/gdfidl
11.5 11.6 11.7 11.8 11.9 12 12.1 12.2 12.3 12.4 12.50
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Frequency, GHz
Lo
ng
itu
din
al
Imp
ed
an
ce
, V
/A
HFSSACE3Pgdfidl
22
HFSS/ACE3P/gdfidl Wakes
0 10 20 30 40 50 60 70 80-50
0
50
100
150
200
Time, ns
Lo
ng
itu
din
al
Wa
ke
, V
/pc
HFSS, copper wallsACE3P, PEC wallsgdfidl, PEC walls
HFSS and gdfidl are ok at the beginning, while ACE3P/gdfidl are ok after that because of the PEC boundary condition (copper in HFSS), also no ACE3P/HFSS wake rise is observed in the tail
23
-10 0 10 20 30 40 50 60 70 80 90 1000
20
40
60
80
100
120
140
160
Time [ns]
Wa
ke
Po
ten
tia
l [V
/pC
]
30 GHz0.6 GHz1 GHz
HFSS wake shape vs BW
This wake (wake function) for the delta function bunch is used for the compensation scheme, since bunch length in CLIC is only 44um.
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 60
50
100
150
Time [ns]
Wa
ke
Po
ten
tia
l [V
/pC
]
30 GHz0.6 GHz1 GHz
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Energy Spread vs BW
0 50 100 150 200 250 300 350-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
Bunch Number
10
0*
E /
<E
> [
%]
0.6 GHz1 GHz30 GHz
Fixed optimized buncher delays and injection time for 1 GHz BW
BW, GHz ΔE/E,%
0.6 0.0257
1.0 0.0253
30 0.028
Bunch Number
25
Conclusions
• Good agreement between ACE3P and HFSS in frequency domain
• Some difference has been observed between HFSS/ACE3P/gdfidl in time domain
• HOM’s taken into account don’t affect the developed beam-loading compensation scheme on the level of 0.03%
• Beam-loading compensation scheme should work
26
Acknoledgement
I would like to thank my supervisor Alexej Grudiev, all of the members of the CERN CLIC RF team, SLAC ACD group.
Thank you!
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