1. Design of Standing-Wave Accelerator Structure
Jeff Neilson, Sami Tantawi, and Valery Dolgashev
SLAC National Accelerator Laboratory
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US High Gradient Research Collaboration Workshop February 9-10, 2011

2. Outline Motivation Conceptual Approach Feed System Design
Cavity Design
Fabrication
Conclusions
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3. Motivation
Provide robust high-gradient (>100 MV/m) accelerator structure
Potential advantages of parallel fed, π mode standing-wave (SW) structures over travelling-wave structures
minimizes energy available during breakdown
maximizes power distribution efficiency
enhanced vacuum pumping conductance
empirical evidence π mode have lower breakdown rate at given gradient vs. travelling wave structures
What is typical fraction of RF drive energy dumped at end of structure
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4. Approach* Individually fed π mode cavities  RF source
Directional Coupler
Sc = (1 – i + N)-1/2
Accelerator
Cavity
Nth Accelerator
Load
What is initial response (large reflection?)
Individually fed π mode cavities
*S. Tantawi,” RF distribution system for a set of standing-wave accelerator structures”, Phys. Rev., ST Accel. Beams,vol. 9, issue 11
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5. Approach - Cont Four RF feed ports per cavity Module of 18 cells
eliminate RF driven dipole modes
damp long range wakefields
maximizes pump conductance
Module of 18 cells
60 MW power (100MV/m)
15 MW each arm
directional coupling factors would range from to -3dB
-loaded , unloaded power
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6. Coupler Design
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7. RF Arm Feed to Cavity Coupling
source
Load
Accelerator
Cavity
Short cavity spacing (1.3 cm) precludes use of inline coupler along axis of accelerator structure
Optimal configuration has coupler in same plane as cavity
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8. Cross-guide Coupler
3.0 dB coupling
12.5 dB coupling
Provides required range of coupling required but not ideal solution
large field enhancement on slot edges
high construction complexity
space limitation would require half-height waveguide (increased loss)
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9. RF Feed Using Cross-Guide Couplers
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10. Biplanar Directional Coupler*
Electric field for 3dB Coupler
Can be designed for coupling over desired range
Compact, minimal field enhancement
Planar shape – easy to machine
*MIT Radiation Laboratory Series, Vol. 8, “Principles of Microwave Circuits”
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11. Coupling Sensitivity to Parameter Variationd
Variation in coupling will reduce average gradient over structure from optimal value
Monte Carlo calculation performed varying u, v, d by +/ cm
12.5 dB design has significantly more sensitivity than 3dB design
Coupling Histogram for 3 dB Design
Tolerance = +/ cm
Coupling Histogram for 12.5 dB Design
Tolerance = +/ cm
Frequency of Occurrence
Frequency of Occurrence
Difference from Design Value (%)
Difference from Design Value (%)
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12. 12.5 dB Coupler Measurement
Three 12.5 dB couplers built with +/ cm tolerance
Measured coupling values off by 18%
Design coupling factor (-12.5 dB)
Measured (3 couplers) ( /- 0.1dB)
Calculated with (-14.1 dB)
measured offsets of u, v, d
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13. Biplanar Coupler
Modal Amplitude a vs w a w X
WR-90
Natural coupling value for WR-90 (w=2.3cm) waveguide is very close to 3dB
Potential coupling of 0.24 (-12.5 dB) for width ~3.1cm
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14. Directivity
Rc 10mm 2d d
Coupling
Page 14 d

15. Improved 12.5 dB Coupler Rc 10.6mm P 15 MW Emax 17 MV/m Hmax 50 kA/m
Coupling Histogram for 12.5 dB Design
Tolerance = +/ cm
Variation u, v, d, and rc
Frequency of Occurrence
Rc 10.6mm
P 15 MW
Emax 17 MV/m
Hmax 50 kA/m
Difference from Design Value (%)
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16. Cavity Design
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17. Cavity Design Goals Proof of concept
Achieved results will determine relevant applications of SW approach
Nominal goal is CLIC G
acceleration gradient 100 MV/m
iris a/λ 0.11 (average CLIC G)
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18. Cavity Design Four port coupling designed to provide With
rf drive to beam
long range wakefield damping
high pump conductance
With
Minimal pulse heating and electric field enhancement
maintain high shunt impedance
minimizing construction complexity
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19. Cavity Shape Many design options explored
rf choke coupling
optimized iris shaping
multiple slots (>4)
complex cavity shape
All designs had excessive surface heating or minimal improvement over simple cavity shape
Shaped iris
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20. Simple Cavity Configuration
Width and length of coupler arm
Iris radius of curvature
Cavity radius of curvature
Cavity radius
Beam tunnel radius and thickness
Circumference radiusing (Rc)
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21. Design Cavity Results for 100 MV/m
Parameter
Beam Tunnel radius (mm)
2.75
Iris thickness (mm) 2
Stored Energy [J]
0.153
Q-value
8580
Shunt Impedance [MOhm/m]
103.5
Max. Mag. Field [KA/m]
342
Max. Electric Field [MV/m]
253
Normalized Max. Mag. Field [290 KA/m]
Emax/Accel gradient
2.53
Hmax Zo/Accel gradient
1.29
Magnetic Field
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22. Fabrication
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23. RF Feed Using Biplanar Coupler
~ 7 cm
~ 3 cm
~ 24 cm
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24. Planar Geometry 180 Degree Elbow
Electric Field
Return Loss
Return Loss
15 MW Input Power
Emax 23MV/m
Hmax 73kA/m
Frequency (GHz)
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26. Page 26

27. Summary & Plans
Conceptual design for parallel fed SW structure completed
Primary issues for achieving a structure with superior performance to existing TW designs are:
uniformity of rf feed system power coupling
pulse heating from waveguide coupling to cavities
achieving sufficient HOM suppression
Construction of 18 cell structure by October 2011
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