update on design of standing- wave accelerator structure
TRANSCRIPT
Update on Design of Standing-Wave Accelerator Structure
Jeff Neilson, Sami Tantawi, and Valery Dolgashev SLAC National Accelerator Laboratory
5th Collaboration Meeting on X-band Accelerator Structure Design and Test Program 16-18 May, 2011
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Outline
• Motivation • Conceptual Approach • Feed System Design • Cavity Design • Fabrication • Conclusions
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
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Approach*
• Individually fed π mode cavities
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� � � �
RF source
Directional Coupler Sc = (1 – i + N)-1/2
Accelerator Cavity
Nth Accelerator Cavity
Load
*S. Tantawi,” RF distribution system for a set of standing-wave accelerator structures”, Phys. Rev., ST Accel. Beams,vol. 9, issue 11
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
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Cross-guide Coupler
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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)
Biplanar Directional Coupler*
• Can be designed for coupling over desired range • Compact, minimal field enhancement • Planar shape – easy to machine
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*MIT Radiation Laboratory Series, Vol. 8, “Principles of Microwave Circuits”
Electric field for 3dB Coupler
Coupling Sensitivity to Parameter Variation
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• 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
Coupling Histogram for 12.5 dB Design Tolerance = +/- .0025 cm
v u
d
Coupling Histogram for 3 dB Design Tolerance = +/- .0025 cm
Freq
uenc
y of
Occ
urre
nce
Freq
uenc
y of
Occ
urre
nce
Difference from Design Value (%) Difference from Design Value (%)
12.5 dB Coupler Measurement
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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
• Three 12.5 dB couplers built with +/- .0025 cm tolerance • Measured coupling values off by 18%
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
X WR-90
Modal Amplitude a vs w
Biplanar Coupler
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P 15 MW Emax 17 MV/m Hmax 50 kA/m
Rc 10.6mm
Coupling Histogram for 12.5 dB Design Tolerance = +/- .0025 cm Variation u, v, d, and rc
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Freq
uenc
y of
Occ
urre
nce
Difference from Design Value (%)
Improved 12.5 dB Coupler
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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Width and length of coupler arm
Iris radius of curvature
Cavity radius of curvature
Cavity radius Beam tunnel radius
and thickness
Circumference radiusing (Rc)
Cavity Design Parameters
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Cavity parameters optimized to maximize shunt impedance with minimum enhancement of magnetic surface field relative to closed cavity
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Magnetic Field
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] 0.153
Emax/Accel gradient 2.53
Hmax Zo/Accel gradient 1.29
Design Cavity Results for 100 MV/m
45 Degree Wedge of Cavity
F 11.424 GHz Ql 5340 β 0.7 P 6.5 MW for 100MV/m acceleration gradient (center cavity)
On Axis Field
Initial Test Configuration
1
2 3
4
Frequency Response (dB)
Cavity Driven Through RF Feed (F = 11.424GHz)
On Axis Field Phase Error (degrees relative to π shift)
-5
0
+5
-20
0
-40
Phase Arm Error on Last Cavity Feed (30 Deg)
Frequency Response (dB)
--- no phase error
On Axis Field Phase Error (degrees relative to π shift)
-5
0
+5
-20
0
-40
15 MW Input Power Emax 23MV/m Hmax 73kA/m
Return Loss
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Planar Geometry 180 Degree Elbow
Frequency (GHz)
Ret
urn
Loss
Electric Field
Summary & Plans
• Conceptual design for parallel fed SW structure completed, mechanical design underway
• Primary issues for achieving a structure with superior performance to existing TW designs are: – pulse heating from waveguide coupling to cavities – achieving sufficient HOM suppression
• Evidence of strong coupling through beam holes suggests alternate configurations (3 cells per feed?) may be preferable
• Construction and test of 5 cell structure late summer
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