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Design of Standing-Wave Accelerator Structure
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  1. 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

  2. Outline • Motivation • Conceptual Approach • Feed System Design • Cavity Design • Fabrication • Conclusions Page 2

  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 Page 3

  4. Approach* RF source Directional Coupler Sc = (1 – i + N)-1/2 Load     Accelerator Cavity Nth Accelerator Cavity *S. Tantawi,” RF distribution system for a set of standing-wave accelerator structures”, Phys. Rev., ST Accel. Beams,vol. 9, issue 11 Individually fed πmode cavities Page 4

  5. 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

  6. Coupler Design Page 6

  7. 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

  8. Cross-guide Coupler 3.0 dB coupling 12.5 dB coupling Page 8 • 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)

  9. RF Feed Using Cross-Guide Couplers Page 9

  10. Biplanar Directional Coupler* Electric field for 3dB Coupler *MIT Radiation Laboratory Series, Vol. 8, “Principles of Microwave Circuits” Can be designed for coupling over desired range Compact, minimal field enhancement Planar shape – easy to machine Page 10

  11. Coupling Sensitivity to Parameter Variation Coupling Histogram for 12.5 dB Design Tolerance = +/- .0025 cm Coupling Histogram for 3 dB Design Tolerance = +/- .0025 cm Frequency of Occurrence Frequency of Occurrence d v u Difference from Design Value (%) Difference from Design Value (%) Page 11 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

  12. 12.5 dB Coupler Measurement 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 Three 12.5 dB couplers built with +/- .0025 cm tolerance Measured coupling values off by 18%

  13. Biplanar Coupler Modal Amplitude a vs w a a X WR-90 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

  14. Directivity Rc 10mm d 2d Coupling Page 14 d

  15. Improved 12.5 dB Coupler Coupling Histogram for 12.5 dB Design Tolerance = +/- .0025 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 (%) Page 15

  16. Cavity Design Page 16

  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) Page 17

  18. 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

  19. Cavity Shape Shaped iris • 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

  20. Simple Cavity Configuration Iris radius of curvature Width and length of coupler arm Cavity radius of curvature Cavity radius Beam tunnel radius and thickness Circumference radiusing (Rc) Page 20

  21. Design Cavity Results for 100 MV/m Magnetic Field Page 21

  22. Fabrication Page 22

  23. RF Feed Using Biplanar Coupler ~ 7 cm ~ 3 cm ~ 24 cm Page 23

  24. 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

  25. Page 25

  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 rffeed system power coupling • pulse heating from waveguide coupling to cavities • achieving sufficient HOM suppression • Construction of 18 cell structure by October 2011 Page 27