Download - CLIC crab cavity design
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CLIC crab cavity design
Praveen Ambattu
24/08/2011
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Monopole x dipole mode @ 12 GHz
abs E, V/m abs H, A/m abs S, W/m2
TM010
TM110
5 mm
5 mm
Crab cavity has a E, H and S distributions different from the main linac
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CLIC crab cavity numbers
• Bunch rotation:10 mrad
• Transverse space: ~100 cm
• Mode: 2/3, 11.9942 GHz
• Voltage: 2.55 MV per cavity
• Available peak power: 15 MW
• RF tolerance for 98 % luminosity:
dVrf/Vrf=2 %, drf=18 mdeg
• Peak surface field: <250 MV/m
• Peak pulsed heating: <40 K
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Cell shape
• Initial optimisation was for a cylindrical cell
• 5 mm radius beampipe was chosen as a compromise among surface fields and short range wakefields
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Vertical modes for 10 cell cavity
SOM band
• Vertical wakes are dominated by the SOM band which is the 1st dipole mode itself but in 90 deg plane
• For r0=35 m, SOMs needs Q<100 to meet the luminosity requirements
Kick factor x frequency SOM Q x HOM Q
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• By shifting the SOM frequency with highest kick to 6.5th bunch harmonic (13 GHz), the last bunch in the train will see zero sum wakefield
• This allows relaxing the SOM damping requirement
•This can be implemented by using an asymmetric cell shape
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Asymmetric cell shapes
• Achieving 1 GHz shift in dipole frequency with less structure complexity would be easy with racetrack shape.
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E-field (1 J stored energy)
Single racetrack cell
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H-field (1 J)
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Power flow and Sc (1 J)
Sc=max(ReS+ImS/6)
ReS=sqrt(ReSx^2+ReSy^2+ReSz^2)
ImS=sqrt(ImSx^2+ImSy^2+ImSz^2)
Sc/Et2=3.87 mA/V
Transverse gradient, Et=Vt/Lcell
Vt=jVz(r)*(c/r)
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Property Value
QCu 6395
Rt/Q, Ohm 54.65
Kick, MV 2.03
vgr, % -2.92
Attenuation, Nep/m 0.0056
Es/Et 3.425
Hs/Et, T(K) 0.0114, 24
Sc/Et2, mA/V 3.87
Single cell properties (1 J)
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Standard coupler Waveguide coupler
Structure size mainly depends on
• power coupler type :- standard, waveguide, mode launcher etc
• feed geometry :- single or dual
• length of damping waveguides
Power coupler
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Standard coupler
More space needed to include damper, also to help cell tuning
Waveguide coupler
Splitter size
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Coupler comparison
Property Waveguide Standard
Longitudinal size More Less
Transverse size Less More
Wakefield More Less
End cell tuning complexity
Low High
Mechanical complexity
Low High
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Dual-feed x single-feed coupler
• DFC perfectly centres the dipole mode in the end cells due to symmetric feeding and do not excite other modes
• This needs the waveguide arms of the splitter be temperature stabilised
• Assembly tolerance is also critical
• Inclusion of dampers and tuning the end cells will be difficult unless longer waveguide arms are used
• More trapped modes, hence more wakefield
• Single-feed coupler avoids all above• But the mode is not centred causing beamloading• This can be minimised by flipping the two couplers 180 deg with each other to reduce the effect of beam loading • Not needed in the prototype 1 as there is no beam
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Single-feed coupler
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slot_a
slot_a
slot_h
slot_h
Microwave Studio copper model
• Used a dummy waveguide, cut-off to 12 GHz
• Could be used as damping waveguide in the final cavity
• Could be avoided in the 1st prototype
• Waveguides can be on the same plane
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Power flow (1W)
z
y
x
All waveguides are terminated by ports
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S-parameters
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Complex field (Hx) in the band
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Field amplitudes at 11.9942 GHz
Hx(0)
Ey(0)
Ez(r)
on-axis
on-axis
0.5 mm off-axis
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Internal reflection and phase advance at 11.9942 GHz from Hx(0)
beam pipe
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Beamloading in the end cells
Ez(0) x y
Ez(0) x z
wg2 wg1
wg1
wg2
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Hsurf=360 kA/m T=28 K
Coupler cell x Regular cell
Hsurf=269 kA/m T=16 K
R0.5 mm
For 13.5 MW peak power and 242 ns pulse
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RF properties
Parameter Value
Total length, mm 149.979
Active length, mm 99.979
Transverse size, mm 106.424
Kick, MV 2.55
Peak power, MW 13.5
Esurf, MV/m 110
Hsurf, kA/m360 (cell) 269 (coupler)
T, K28 (cell) 16 (coupler)
Sc, W/m2 3.85*
* TD26_vg1p8_R05, Sc~5 W/m2
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Cavity tuning
pin in 0 MHz/mm
pin out -0.6 MHz/mmpin in 18.2 MHz/mm
pin out -11.4 MHz/mm
pin in 27.2 MHz/mm
pin out -16.8 MHz/mm
For 1 MHz tuning, ~1.26 m in radius~50 m in pit
Simulated 0.5 mm deep pit
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Bead pull• Tuning could be done by ‘non-resonant perturbation’ technique,
combined with bead-pull, identical to what has been done for the accelerating cavity
• Simulated beadpull result using a metallic disk (1.5 mm dia x 1mm thick) shown below seems to give well defined perturbation
• More accurate field measurement needs a fine cylinder made of thin surgical needle
complex S11
![Page 28: CLIC crab cavity design](https://reader030.vdocuments.net/reader030/viewer/2022033018/568143ee550346895db073bc/html5/thumbnails/28.jpg)
HFSS x MWS fields @ 1 W, 11.9942 GHz
Hx on axis
HFSS MWS
0
200
400
600
800
1000
1200
1400
1600
0 20 40 60 80 100 120 140 160
Ez off axis
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RF properties MWS x HFSS
Parameter MWS HFSS
Frequency, GHz 11.9942 11.9942
|S11|, dB -50 -32
Kick, MV 2.56 2.56
Peak power, MW 13.5 9.75
Esurf, MV/m 110 110
Hsurf, MA/m 360 354
T, K 28 27
Sc, W/m2 3.85* ??
• Fields amplitudes in HFSS are higher than in MWS by a factor of 1.15
• Needs more investigation but seems OK !!
For coarse mesh inside the cavity
![Page 30: CLIC crab cavity design](https://reader030.vdocuments.net/reader030/viewer/2022033018/568143ee550346895db073bc/html5/thumbnails/30.jpg)
Discussion
• Single feed without dummy wg ?– Yes it is, as the priority is to RF test the undamped cavity
• Cooling pipes on iris or equator ?– equator
• Tuning pins 0 deg or 45 deg ?– 45 deg
• Timescales ?– Finish drawing by Dec 2011, start procurement of copper
by Jan 2012, so EuCARD money could be spent before March 2012
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CLIC crab cavity final design (using CST Microwave Studio 2010)
Praveen Ambattu
26/08/2011
The design changed from what shown at the RF group meeting
• Removed extra waveguide on coupler cells
• Increased coupler slot rounding to 1 mm from 0.5 mm
• Increased waveguide corner rounding to 4 mm from 2 mm
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Single cell with periodic boundary of 2/3
Property Value
Energy stored, J 1
QCu 6395
Rt/Q, Ohm 54.65
vgr, % -2.92
Esurf/Et 3.43
Hsurf/Et 0.0114
Esurf
Hsurf
Ssurf
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Comparison with HFSSv13(Vasim F. Khan, CERN fellow)
Property MWS (PEC) HFSS (Cu)
Freq, GHz 11.9941 11.9959
QCu 6395 6106
Rt/Q, Ohm 54.65 53.78
Esurf/Et 3.43 3.28
Hsurf/Et 0.0114 0.0106
• MWS supports only PEC material in eigenmode simulation
• MWS used Perfect Boundary Apprx, 134,912 hexahedra per quarter (lines/lamda=40, lower mesh limit=40, mesh line ratio limit=40)
• HFSS used 8,223 tetrahedra per quarter (surface apprx= 5m, aspect ratio=5)
Mesh view
MWS HFSS
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12 cell structure frequency domain simulation
• Full structure with one symmetry plane
• 1.75 m tetrahedral elements
• Calculation at 11.9942 GHz and 1 W ‘peak’ input power
Mesh view
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S-parameters
![Page 36: CLIC crab cavity design](https://reader030.vdocuments.net/reader030/viewer/2022033018/568143ee550346895db073bc/html5/thumbnails/36.jpg)
Complex field (Hx) in the band
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Magnetic field profile-Hx on axis at 11.9942 GHz for 1 W
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Electric field profile -Ey on axis at 11.9942 GHz for 1 W
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Electric field profile -Ez(y) at 11.9942 GHz for 1 W
Red: at y=0.5 mm Green: at y=0
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Power flow (1 W)
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Internal reflection at 11.9942 GHz
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Phase advance at 11.9942 GHz
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Properties of full cavity
RF properties
Mode (rad, GHz) 2/3, 11.9942
Kick (MV) 2.56
Group vel, % -2.9
Fill time, ns 11.5
Peak power (MW) 13.35
Esurf (MV/m) 103
Hsurf (kA/m) 348 (reg cell) 207 (coup slot)
T (K)26 (reg cell) 10 (coup slot)
Sc (W/m2) 3.32*
Size
Total length
(mm)149.984
Active length (mm)
99.984
Vertical size
(mm)59.354