DUAL FEED RF GUN DESIGN FOR LCLS

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DUAL FEED RF GUN DESIGN FOR LCLS

• Liling XIAO, Zenghai LI
• Advanced Computations Department
• Stanford Linear Accelerator Center

Nov.3 2004, SLAC-LCLS Injector RF System Internal
Review
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Outline

• Introduction
• Design via Simulation
• Pulsed Heating
• Quadruple mode
• Dual Feed Gun Parameters
• 6. Summary

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1. Introduction

• Requirements and Parameters
• - 2.856GHz operating
  frequency
• ? mode field imbalance within 10
• Coupling factor around 2
• ?T lt 50oC due to pulsed heating
• for Ecathode120MeV, t-pulse 3?s with
  120Hz pulse
• repetition rate

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D.T.Palmer, et al., Microwave Measurements of
the BNL/SLAC/UCLA 1.6 cell Photocathode RF Gun
SLAC-PUB-95-6799

• Previous Study

J.Hodgson
R.Boyce, et al., Design Considerations for the
LCLS RF Gun, LCLS TN 04-4, April 2004
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• New Design Features
• Dual RF feeds
• - to eliminate the dipole modes
• Larger rounding of coupling iris
• - to reduce the pulsed heating
• Racetrack cell shape
• - to minimize the quadruple modes

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Original Design
2. Design via Simulation
A
A
RF Coupling Aperture
B
B
Laser Port
Results f2.8625GHz, ß2.28 (E0E110.908),
?T150oC for the single feed gun dimensions.
(comparing the data on Slide 15)
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hc
2b0
2b1
lc
r2
New Optimized Design- Adjust labeled dimensions
by modeling with ACDs parallel eigensolver
Omega3P and parallel S-parameter solver S3P
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• Modeling with S3P

? mode
S12
GHz
Typical transmission curves calculated with S3P
(solid curve) and fitted using Lorentzian
profiles (dotted curve)
Model is generated and meshed using Cubit.
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Iteration to optimized dimensions (Pulsed heating
and quadruple correction not considered)
b0, b1, lc, hc (inch) 1.6335,1.6435, 0.868, 0.056 (starting point) 1.6370, 1.6470, 0.868, 0.056 1.6370, 1.6482, 0.82, 0.056 1.6370, 1.6475, 0.868, 0.09
F0 (GHz) 2.86247 2.85605 2.85622 2.85617
Q0 11384 13214 13337 13307
beta 2.28 2.45 1.93 1.93
E0/E1 10.908 10.908 10.985 10.96
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3. Pulsed Heating
Previous Results from ANSYS on Temperature
Distribution in the LCLS RF Gun (LCLS TN 04-4)
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Breakdowns due to the pulsed heating had been
observed in the Fermilab RF Gun G4
J.P.Carneiro, et al., TESLA Note 2003-13
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• The temperature rise at the end of RF pulse is

Hsmax(t)
D.T.Pritzkau, RF Pulsed Heating,
SLAC-Report-577
Maximum surface magnetic field located on the
coupling aperture edge.
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Temperature Rise vs. Rounding Radius of RF
Aperture
Start point
Final point
Assuming Ecathode 120MV/m, f 2.856GHz, ß
2,Q0 13300, tp 3?s
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Starting point hc0.056, r20.022 - ?Tmax150oC
Final point hc0.2165, r20.185- ?Tmax42oC
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4. Quadruple Mode in Coupler Cell
Coupling Factor vs. Length of Aperture
Start point
Final point
hc0.2165 inch
hc0.2165 inch
hc0.056 inch
Field imbalance for all the points are within 3.
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Quadruple Moment in Cylindrical Cavity
?ßr/mm
4th order finite element basis functions in
Omega3P have to be used to compute the quadruple
fields accurately in the models. And the fields
from Omega3P are being used in PARMELA emittance
calculations.
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Quadruple Moment in Racetrack Cavity
?ßr/mm
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Effect of Laser Ports
1 max ?T36oC
?ßr/mm
2 Quadruple moment due to the laser ports is
about 5 of that in the cylindrical cavity.
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The racetrack corrects only the quadruple in the
coupler cell. The skew quadruple effect in the
half cell due to laser ports needs to be
evaluated racetrack shape can be added to the
half cell if necessary.
?ßr/mm
?ßr/mm
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5. Dual Feed Corrected RF Gun
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RF-GUN Final Dimensions
Parameter Value
Race track arc radius b1 1.5999 inch (original1.6435)
Race track arc separation d 0.134 inch (original 0)
Race track cell length l1 1.28 inch
Half cell radius b0 1.6361 inch (original1.6335, not consider the laser ports)
Half cell length l0 0.896 inch
RF coupling hole size (slot length) lc 0.95 inch (original0.868)
RF coupling hole radius of curvature rc 0.1875 inch
RF coupling hole size (slot width) 2rc 0.375 inch
RF coupling hole thickness hc 0.2165 inch (original 0.056)
RF coupling hole rounding radius r1 on up side 0.022 inch
RF coupling hole rounding radius r2 on down side 0.185 inch (original 0.022)
Cell iris radius a 0.492 inch
Disk thickness t 0.868 inch
Disk rounding radius r 0.375 inch
Laser port hole size (slot length) ll 0.433 inch
Laser port hole radius of curvature rl 0.125 inch
Laser port hole size (slot width) 2rl 0.250 inch
Laser port hole rounding radius r3 0.030 inch
Laser port offset the cathode plate 0.531 inch
Laser port angle 18 degree
Waveguide 2.840 inch1.340inch
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Corrected RF GUN Parameters
RF Parameters
F0 (GHz) 2.856022
Q0 13325
ß 2.07
Mode Sep. ?f (MHz) 3.4
?T max (oC) 44
E0E1 10.962
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On-axis Accelerating Field Profile in RF Gun
Cavity
E0E110.962
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Frequency and Field Sensitivity to Major Gun
Dimensions
Dimension Freq. error per 0.0001 Field error per 0.0001
Full cell radius b1 113KHz 5.1
Half cell radius b0 70KHz 5.2
Racetrack arc separation d 45KHz 2
Cell iris radius a 40KHz 0.3
Disk rounding radius r 24KHz 0.2
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6. Summary

• The original RF gun design has been optimized
  to
• - eliminate the dipole modes
• - reduce pulsed heating
• - minimize the quadruple moment
• A corrected design has been generated using
  SLACs parallel codes Omega3P
  S3P with 4th order elements
• The steady-state thermal properties needs to be
  studied again for the new dual feed gun design.