simulations and rf measurements of sps beam position monitors (bpv and bph)
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1
Simulations and RF Measurements
of SPS Beam Position Monitors(BPV and BPH)
G. Arduini, C. Boccard, R. Calaga, F. Caspers, A. Grudiev, E. Metral, F. Roncarolo, G. Rumolo, B. Salvant, B. Spataro, C. Zannini
Acknowledgments: J. Albertone, M. Barnes, A. d’Elia, S. Federmann, F. Grespan, E. Jensen
R. Jones, G. de Michele, Radiation Protection, AB-BT workshop
GSI/CERN collaboration meeting – Darmstadt, Feb 19th 2009
2
Agenda
• Context• Simulations
– Creating the model– Time domain (Particle Studio)– Frequency Domain (Microwave Studio)– Consistency between Frequency and time domain
• RF Measurements– Setup and strategy– Without wire– With wire
• Open questions• Perspectives
3
Context
• High intensity in the CERN SPS for nominal LHC operation, and foreseen LHC upgrade
• Need for a good knowledge of the machine beam impedance and its main contributors
• To obtain the total machine impedance, one can:– Measure the quadrupolar oscillation frequency shift (longitudinal) or the tune shift
(transverse) with the SPS beam
– obtain the impedance of each equipment separately and sum their contributions:• Analytical calculation (Burov/Lebedev, Zotter/Metral or Tsutsui formulae) for simple
geometries
• Simulations for more complicated geometries
• RF Measurements on the equipment
available impedance and wake data compiled in the impedance database ZBASE
In this talk, we focus on the simulations and RF measurements of the SPS BPMs
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Objective
• Obtain the wake field and impedance of the SPS BPH and BPV
Notes:• Impedance of these SPS BPMs is expected to be small, but ~200 BPMs
are installed in the machine. Summed effect?
• 2 mm gaps seen by the beam are small would affect only high frequencies? Is that really
correct?
5
Broader objectives for the “impedance team”:
1) Which code should we trust to obtain the wake?2) Assess the reliability of bench measurements with wire
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Agenda
• Context• Simulations
– Creating the model– Time domain (Particle Studio)– Frequency Domain (Microwave Studio)– Consistency between Frequency and time domain
• RF Measurements– Setup and strategy– Without wire– With wire
• Open questions• Perspectives
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Creating the Model
SPS BPVSPS BPH
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Creating the BPH model
Input for simulations:- Technical drawings- Available prototype
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Structure of the BPHvacuumPerfect conductor (PEC)
BeamElectrodes
Cut along x=0
Cut along y=0
Casing
Output coax
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Agenda
• Context• Simulations
– Creating the model– Time domain (Particle Studio)– Frequency Domain (Microwave Studio)– Consistency between Frequency and time domain
• RF Measurements– Setup and strategy– Without wire– With wire
• Open questions• Perspectives
11
SPS BPH – time domain simulations
• Wakefield solver
• Boundary conditions: perfect conductor except for beam pipe aperture (open)
• Indirect testbeam wake calculation
• 106 mesh cells
• Simulated wake length=15 mFrequency resolution ~ 0.02 GHz
Material modelled as perfect conductor
1 cm rms Bunch length (=1)
FFT calculated by particle studio
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BPH - Time domain simulations
y
x
y
x
y
x
Wake is calculated at the location of the beam “Total” impedance (dipolar + quadrupolar+…)
1.08GHz
1.68GHz
1.90GHz
2.58GHz Same resonance frequencies as longitudinal
0.55GHz
0.97GHz
1.29GHz
1.69GHz2.14GHz
Longitudinal Horizontal Vertical
Negative imaginary part of the vertical impedance.
1.92GHz
s (mm) s (mm) s (mm)
A few remarks…
Remark 1/4 : Wake length and particle studio fft
20 meters wake
3 meters wake
Need for long wakes to obtain a sufficient frequency resolution Particle Studio FFT seems to introduce more ripple
Remark 2/4: How about “low” frequencies?Im
ag
ina
ry p
art
of
the
lon
gitu
din
al I
mp
ed
an
ce (
in O
hm
)
Z/n=Z/(f/f0)
= 20 cm
Low frequency imaginary longitudinal impedance is Z/n ~ 1 mΩ
Remark 3/4 : Comparing the full BPH with the simple structure with slits
At f=1.06 GHzSimple Structure
longitudinal electric field Ez
on plane x=0 at f=1.06 GHz
Simple Structure with slits
Full BPH structure
The gaps are small, but the electrode are so thin that the cavities behind the electrodes perturb the beam down to low frequencies (~1GHz)
Effect of matching the impedance at electrodes coaxial ports in Particle Studio simulations (BPH)
Electrodecoaxial port
Modes are damped by the “perfect matching layer”
at the coaxial port
In particle studio, ports can be defined and terminated
Effect of matching the impedance at electrodes coaxial ports in Particle Studio simulations (BPV)
Modes are damped by the “perfect matching layer”
at the coaxial port
And for the real long SPS bunch ?(BPH)
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SPS BPV – time domain simulations
• Wakefield solver
• Boundary conditions: perfect conductor except for beam pipe aperture (open)
• Indirect testbeam wake calculation
• 106 mesh cells
• Simulated wake length=15 mFrequency resolution ~ 0.02 GHz
Material modelled as perfect conductor
1 cm rms Bunch length (=1)
FFT calculated by particle studio
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y
x
y
x
y
x
BPV - Time domain
1.13GHz
2.22GHz
1.97GHz
0.73GHz
1.58GHz
1.14GHz~ same resonance frequencies longitudinal
1.97GHz
2.22GHz
Longitudinal Horizontal Vertical
Negative imaginary part of the vertical impedance, again.
Wake is calculated at the location of the beam “Total” impedance (dipolar + quadrupolar+…)
s (mm) s (mm) s (mm)
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Agenda
• Context• Simulations
– Creating the model– Time domain (Particle Studio)– Frequency Domain (Microwave Studio)– Consistency between Frequency and time domain
• RF Measurements– Setup and strategy– Without wire– With wire
• Open questions• Perspectives
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SPS BPH – Frequency domain simulations
• Eigenmode AKS solver
• 2 106 mesh cells
• Material modelled as perfect conductor
• Shunt impedance, frequencies and quality factor obtained from MWS Template postprocessing
•Longitudinal shunt impedance: Rs=Vz
2/W along z at (x,y)=(0,0)
•Transverse shunt impedance: Rs=Vz
2/W along z at (x,y)=(x,0) or (0,y)
•Boundary conditions : perfect conductor.
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BPH simulation : 30 first modes obtained with the eigenmode solver
Mode frequency (GHz) Shunt impedance (center) Shunt impedance (x=5mm) Shunt impedance (y=5mm) Quality factor
1 0.282 7.05E-02 6.95E-02 7.20E-02 13992 0.287 1.93E-25 1.00E-05 2.09E-25 14173 0.549 9.78E-28 1.33E-07 1.52E-26 19854 0.553 3.25E-06 3.10E-06 3.58E+00 20985 0.932 1.67E+00 1.64E+00 1.69E+00 21016 0.938 1.51E-28 2.86E-02 3.56E-28 21927 1.030 3.07E-23 4.62E-09 1.28E-27 78428 1.082 9.43E+04 9.48E+04 9.26E+04 32749 1.093 6.71E-28 1.27E+03 4.31E-28 3304
10 1.196 5.03E-06 4.65E-06 6.48E+03 799811 1.301 3.24E-05 3.19E-05 1.30E+02 267812 1.318 3.56E-25 1.84E-06 2.35E-27 291013 1.402 1.59E-23 1.78E-06 3.30E-27 635914 1.640 1.54E-03 1.50E-03 9.58E+03 405815 1.663 1.38E-29 3.12E-06 5.15E-28 501816 1.676 2.09E-27 2.87E-03 5.34E-27 269117 1.692 2.55E+02 2.51E+02 2.50E+02 281718 1.800 2.42E-06 2.06E-06 1.28E+03 1377719 1.875 5.79E+05 5.68E+05 5.67E+05 363220 1.899 2.66E-26 4.63E+01 4.96E-26 352121 1.923 5.12E-02 5.03E-02 2.30E+02 658222 2.019 1.07E-26 1.66E-05 6.27E-29 516923 2.075 8.11E-22 8.84E-06 3.58E-26 581324 2.127 9.27E-25 8.75E-06 5.02E-25 869825 2.132 3.73E-03 3.71E-03 2.16E+00 475826 2.188 5.01E-03 5.00E-03 1.33E+04 694927 2.248 6.32E+02 6.18E+02 6.10E+02 478028 2.255 2.06E-25 7.59E+01 1.18E-25 487629 2.363 1.37E-03 1.38E-03 1.40E+04 605630 2.370 2.51E-12 5.56E-05 9.67E-14 4938
longitudinal mode horizontal mode vertical mode
Transverse modes should show a strong transverse gradient of the longitudinal shunt impedance
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SPS BPV – Frequency domain
Eigenmode AKS solver
2 106 mesh cells
Material modelled as perfect conductor
Shunt impedance, frequencies and quality factor obtained from MWS template postprocessing
Boundary conditions perfect conductor.
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Mode frequency (GHz) Shunt impedance (center) Shunt impedance (x=5mm) Shunt impedance (y=5mm) Quality factor
1 0.305 2.37E-03 2.52E-03 2.16E-03 1207
2 0.310 7.39E-17 9.02E-17 7.88E-05 1218
3 0.719 6.93E-18 2.34E-16 2.51E-06 2013
4 0.730 3.31E-04 8.38E-01 3.28E-04 2135
5 1.104 1.40E+05 1.39E+05 1.39E+05 2561
6 1.131 7.98E-29 1.98E-28 1.91E+03 2648
7 1.246 5.55E+01 5.52E+01 5.47E+01 2228
8 1.278 2.56E-18 2.34E-18 1.25E-01 2330
9 1.573 1.36E-17 6.61E-11 5.46E-08 2843
10 1.582 2.75E-04 3.30E+03 2.68E-04 2917
11 1.645 4.58E-16 1.85E-11 2.23E-05 2937
12 1.686 2.78E-04 5.29E+01 2.92E-04 2531
13 1.880 7.79E+02 8.30E+02 7.01E+02 13136
14 1.925 1.13E-11 7.48E-11 1.32E-04 8921
15 1.972 8.50E-14 2.28E-12 7.73E+02 7961
16 2.037 1.69E-03 1.13E+02 1.55E-03 15783
17 2.110 1.24E+05 1.25E+05 1.16E+05 4660
18 2.169 2.08E-11 1.29E-11 3.32E+02 3811
19 2.204 3.93E-12 3.27E-12 3.56E+02 3265
20 2.231 4.07E-07 4.91E-07 1.28E-04 19292
21 2.255 1.62E+05 1.61E+05 1.53E+05 4192
22 2.261 1.07E-10 1.64E-10 3.87E+00 4293
23 2.284 7.46E+04 7.37E+04 7.13E+04 3683
24 2.301 1.25E+05 1.23E+05 1.20E+05 5548
25 2.347 1.39E+02 4.32E+03 2.31E+02 4601
26 2.438 7.75E-04 1.86E+02 7.31E-04 12187
27 2.482 3.17E-10 9.63E-10 3.80E+02 17597
28 2.572 3.07E+07 2.96E+07 2.96E+07 13116
29 2.589 9.67E-08 4.94E-04 2.30E-05 3794
30 2.599 6.63E-02 1.52E+02 6.39E-02 4710
BPV simulation : 30 first modes obtained with the eigenmode solverlongitudinal mode horizontal mode vertical mode
Transverse modes should show a strong transverse gradient of the longitudinal shunt impedance
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Agenda
• Context• Simulations
– Creating the model– Time domain (Particle Studio)– Frequency Domain (Microwave Studio)– Consistency between Frequency and time domain
• RF Measurements– Setup and strategy– Without wire– With wire
• Open questions• Perspectives
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Modefrequency
(GHz)Shunt
impedance9 1.09 1.27E+03
20 1.90 4.63E+0128 2.25 7.59E+01
Are frequency simulations and time domain simulations consistent?BPH case
1.08GHz
1.68GHz
1.90GHz
Same resonance frequencies as longitudinal
0.55GHz
0.97GHz
1.29GHz
1.69GHz2.14GHz
Modefrequency
(GHz)Shunt
impedance5 0.93 1.67E+008 1.08 9.43E+04
17 1.69 2.55E+0219 1.88 5.79E+0527 2.25 6.32E+02
Modefrequency
(GHz)Shunt
impedance4 0.55 3.58E+00
10 1.19 6.48E+0311 1.30 1.30E+0214 1.64 9.58E+0318 1.80 1.28E+0321 1.92 2.30E+0226 2.18 1.33E+0429 2.36 1.40E+04
Longitudinal Horizontal Vertical
1.92GHz
Most of the modes are observed in both time and frequency domain.Reasonable agreement
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Are frequency and time domain simulations consistent?BPV case
Longitudinal Horizontal Vertical
More mixing between time and frequency domain modes than for the BPH. Coupling?
Modefrequency
(GHz)
Shunt impedance
(center)5 1.104 1.40E+057 1.246 5.55E+01
13 1.880 7.79E+0217 2.110 1.24E+0521 2.255 1.62E+0523 2.284 7.46E+0424 2.301 1.25E+0525 2.347 1.39E+0228 2.572 3.07E+07
Modefrequency
(GHz)
Shunt impedance
(center)4 0.730 8.38E-01
10 1.582 3.30E+0312 1.686 5.29E+0116 2.037 1.13E+0225 2.347 4.32E+0326 2.438 1.86E+0230 2.599 1.52E+02
Modefrequency
(GHz)
Shunt impedance
(center)6 1.131 1.91E+03
15 1.972 7.73E+0218 2.169 3.32E+0219 2.204 3.56E+0222 2.261 3.87E+0027 2.482 3.80E+02
1.13GHz
2.22GHz
1.97GHz
0.73GHz
1.58GHz
1.14GHz~ same resonance frequencies longitudinal
1.97GHz
2.22GHz
Comparison with Bruno (BPH longitudinal)
1.08GHz
1.68GHz
1.90GHz
f [GHz] R [Ω] R/Q [Ω] Q
0.932 1.67 7.95E-04 2101
1.082 9.43E+04 2.88E+01 3274
1.692 255 9.05E-02 2817
1.875 5.79E+05 1.59E+02 3632
2.248 632 1.32E-01 4780
Comparison with Bruno (BPH vertical)
0.55GHz
0.97GHz
1.29GHz
1.69GHz2.14GHz
Modefrequency
(GHz)Shunt
impedance4 0.55 3.58E+00
10 1.19 6.48E+0311 1.30 1.30E+0214 1.64 9.58E+0318 1.80 1.28E+0321 1.92 2.30E+0226 2.18 1.33E+0429 2.36 1.40E+04
1.92GHz
Comparison with Bruno
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Agenda
• Context• Simulations
– Creating the model– Time domain (Particle Studio)– Frequency Domain (Microwave Studio)– Consistency between Frequency and time domain
• RF Measurements– Setup and strategy– Without wire– With wire
• Open questions• Perspectives
35
Setup for the measurement
SPS BPV SPS BPH
36
Measurement strategy• Not ideal to measure the impedance with a wire (small
signal expected, radioactive device, tampering with the device would mean reconditioning before being able to put it back in the machine).
• Idea: first, try to measure S-parameters from the available N-ports at the BPM electrodes, to benchmark the simulations and the measurements
N connectorsLinked to BPMElectrodes with a coax
37
Measurement setup• VNA parameters
– Number of point: 20001 (max)– IF bandwidth: 1 kHz– Linear frequency sweep between 1 MHz and 3 GHz– 2-port calibration (short, open load for each port + transmission)– Port 1 is next to the beam pipe– Port 2 is next to the flange
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Agenda
• Context• Simulations
– Creating the model– Time domain (Particle Studio)– Frequency Domain (Microwave Studio)– Consistency between Frequency and time domain
• RF Measurements– Setup and strategy– Without wire– With wire
• Open questions• Perspectives
39
S parameters measurements for the BPH
Not much difference between S11 and S22
S11
S22
40
Simulations and measurements BPH
Measurement and simulations are shifted in frequency Frequency shift seems to increase with frequency
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Measurements, HFSS and Particle Studio simulations(BPH)
HFSS simulation: courtesy of F. Roncarolo
42
This benchmark with measurements without wire indicate that the model is not completely wrong.
But do they give information on impedance peaks, by any chance?
Apparently yes!!!
Observed S21 peaks are the longitudinal impedance frequency peaks
Useful for more than just the benchmark!
Let’s compare with the BPH time domain simulation!
43
Comparison between measurements and simulations BPV
Similar conclusions as for the BPH
44
And if we compare with time domain?
Again, agreement between time domainand frequency domain is not so good as with the BPH
To be understood
45
Agenda
• Context• Simulations
– Creating the model– Time domain (Particle Studio)– Frequency Domain (Microwave Studio)– Consistency between Frequency and time domain
• RF Measurements– Setup and strategy– Without wire– With wire
• Open questions• Perspectives
46
Measurements with wire (only BPV)
Available BPV prototype equipped with a wire. However, nobody has looked inside for a long while
CST model
47
BPV S21 Measurements and simulations with and without wire
Measurement with wire behaves like the measurement without wire Measurement without wire behaves
like both simulations
Port 1Port 2
48
Powering the wire: Transmission should yield the longitudinal impedance
Port 3
Port 4
Still a frequency-dependant frequency shift between measurements and simulations
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Agenda
• Context• Simulations
– Creating the model– Time domain (Particle Studio)– Frequency Domain (Microwave Studio)– Consistency between Frequency and time domain
• RF Measurements– Setup and strategy– Without wire– With wire
• Outlook• Open questions
50
Outlook and future plans• Reasonable agreement between time domain, frequency domain, eigenmode, and
bench RF measurements.
• The agreement seems better for the BPH than for the BPV
• Powering the electrode without the wire gives information on the impedance related resonances.
• From simulations, putting a wire in the BPV affects moderately the impedance spectrum.
• Not discussed here: Time Domain Simulations of both BPH and BPV indicate that the ouput signals (corrected by the time delay) at both electrodes are not equal when the bunch is centered. This could explain difficulties to calibrate these specific BPMs.
• Future plans:– Check dipolar, quadrupolar, coupled and higher order terms of the wake, and ways to obtain
these terms in frequency domain.
– Use the same approach to simulate the SPS kickers (much larger impedance contribution is expected)
– Explore more in detail the effect of finite resistivity.
– Effect of these wakes on the SPS beam
51
Some open questions…
• Negative impedance for both BPV and BPH. Convention?• Linux version of CST?• How to decouple dipolar and quadrupolar terms in
frequency domain for structures with no symmetry?• Are there limitations to calculating the transverse wake
from the longitudinal?• Open boundary condition for low energy beams?
(important for the PS Booster and the PS)• FFT in particle studio?• Which windowing should we use?
Adding the ceramic spacers
Ceramic insulator spacers designed to mechanically stabilize the thin electrodes(homemade at CERN, cf BPH/BPV technical specs, 1973)
BPV
BPV BPV
BPH
Taking into account the losses (BPV)
Taking into account the losses does not fundamentally change the S21.
The casing is not PEC.
Stainless Steel 304L conductivity: 3 106 S/m
Thank you for your attention!
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