Design of Standing-Wave
Accelerator Structure
Jeff Neilson, Sami Tantawi, and Valery Dolgashev
SLAC National Accelerator Laboratory
US High Gradient Research Collaboration Workshop
February 9-10, 2011
Outline
•
•
•
•
•
•
Motivation
Conceptual Approach
Feed System Design
Cavity Design
Fabrication
Conclusions
Page 2
Motivation
• Provide robust high-gradient (>100 MV/m) accelerator
structure
• Potential advantages of parallel fed, mode standingwave (SW) structures over travelling-wave structures
– minimizes energy available during breakdown
– maximizes power distribution efficiency
– enhanced vacuum pumping conductance
– empirical evidence mode have lower breakdown rate at
given gradient vs. travelling wave structures
Page 3
Approach*
RF
source
Directional Coupler
Sc = (1 – i + N)-1/2
Load
   
Accelerator
Cavity
Nth Accelerator
Cavity
• Individually fed  mode cavities
*S. Tantawi,” RF distribution system for a set of standing-wave accelerator
structures”, Phys. Rev., ST Accel. Beams,vol. 9, issue 11
Page 4
Approach - Cont
• Four RF feed ports per cavity
– 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 12.5 to -3dB
Page 5
Coupler Design
Page 6
RF Arm Feed to Cavity Coupling
RF
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
Page 7
Cross-guide Coupler
12.5 dB coupling
3.0 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)
Page 8
RF Feed Using Cross-Guide Couplers
Page 9
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”
Page 10
Coupling Sensitivity to Parameter Variation
•
v
d
u
Coupling Histogram for 12.5 dB Design
Tolerance = +/- .0025 cm
Coupling Histogram for 3 dB Design
Tolerance = +/- .0025 cm
Frequency of Occurrence
•
Variation in coupling will reduce average gradient
over structure from optimal value
Monte Carlo calculation performed varying u, v, d
by +/- .0025 cm
12.5 dB design has significantly more sensitivity
than 3dB design
Frequency of Occurrence
•
Difference from Design Value (%)
Page 11
Difference from Design Value (%)
12.5 dB Coupler Measurement
• Three 12.5 dB couplers built with +/- .0025 cm tolerance
• Measured coupling values off by 18%
Design coupling factor
0.236 (-12.5 dB)
Measured (3 couplers)
0.20 (-14.0 +/- 0.1dB)
Calculated with
0.198 (-14.1 dB)
measured offsets of u, v, d
Page 12
Biplanar Coupler
Modal Amplitude a vs w
X
WR-90
a
a
w
• 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
Page 13
Directivity
Rc 10mm
d
2d
Coupling
Page 14
d
Improved 12.5 dB Coupler
Frequency of Occurrence
Coupling Histogram for 12.5 dB Design
Tolerance = +/- .0025 cm
Variation u, v, d, and rc
Rc 10.6mm
P 15 MW
Emax 17 MV/m
Hmax 50 kA/m
Difference from Design Value (%)
Page 15
Cavity Design
Page 16
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)
Page 17
Cavity Design
• Four port coupling designed to provide
– 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
Page 18
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
Page 19
Shaped iris
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)
Page 20
Design Cavity Results for 100 MV/m
Parameter
Beam Tunnel radius (mm)
Iris thickness (mm)
2.75
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]
0.153
Emax/Accel gradient
2.53
Hmax Zo/Accel gradient
1.29
Magnetic Field
Page 21
Fabrication
Page 22
RF Feed Using Biplanar Coupler
~ 7 cm
~ 3 cm
~ 24 cm
Page 23
Planar Geometry 180 Degree Elbow
Electric Field
Return Loss
Return Loss
15 MW Input Power
Emax 23MV/m
Hmax 73kA/m
Frequency (GHz)
Page 24
Page 25
Page 26
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
Page 27