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J. Haimson and B. Mecklenburg Haimson Research Corporation

AAC 08. 1. DESIGN FEATURES AND INITIAL RF PERFORMANCE OF A GRADIENT HARDENED 17 GHz LINAC. J. Haimson and B. Mecklenburg Haimson Research Corporation. Work performed under the auspices of the U.S. Department of Energy SBIR Grant No. DE- FG02-05ER84362. 2. AAC 08.

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J. Haimson and B. Mecklenburg Haimson Research Corporation

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  1. AAC 08 1 DESIGN FEATURES AND INITIAL RF PERFORMANCE OF A GRADIENT HARDENED 17 GHz LINAC J. Haimson and B. Mecklenburg Haimson Research Corporation Work performed under the auspices of the U.S. Department of Energy SBIR Grant No. DE-FG02-05ER84362

  2. 2 AAC 08 To Prevent Surface Erosion During High Gradient Operation and to Assist inStudying RF Breakdown, the Gradient Hardened 17 GHz Linac Structurewas Designed to have Brazed and Machined Stainless Steel Surfaces in the High Field Regions of the Circuit. HAIMSONRESEARCHCORPORATION

  3. AAC 08 3 Topics to be Presented 1. DESIGN PARAMETERS AND SPECIAL FEATURES OF THE GRADIENT HARDENED LINAC AND THE HIGH POWER DUAL RESONANT RING SYSTEM USED FOR RF TESTING. 2.THE STATUS OF THE SST/CU LINAC STRUCTURE PRESENTLY BEING RF PROCESSED AT MIT. 3. RF WAVEFORM DYNAMICS RECORDED WHILE RF PROCESSING AT HIGH SURFACE GRADIENTS AND LOW REPETITION RATES. HAIMSONRESEARCHCORPORATION

  4. 4 To Prevent Surface Erosion During High Gradient Operation and to Assist inStudying RF Breakdown, the Gradient Hardened 17 GHz Linac Structurewas Designed to have Brazed and Machined Stainless Steel Surfaces in the High Field Regions of the Circuit. AAC 08 PAC 2007 HAIMSONRESEARCHCORPORATION

  5. 5 AAC 08 The Filling Time, Insertion Loss and Physical Dimensions of the Gradient Hardened (SST/Cu) Structure were Specifically Chosen to Match the Requirements of an existing Dual Resonant Ring High Power Amplifying System (having an RF Bridge Ratio of 3) that had been Used Previously with a Similar Geometry 17 GHz All-Copper Structure. HAIMSONRESEARCHCORPORATION

  6. 6 DC – High Directivity Directional Coupler FC – Faraday Cup M – Motor Drive PSh – Phase Shifter W – Ceramic RF Window R F DC W AAC 08 17 GHz TWRK FC TRIPLE HYBRID SPECTROMETER FC F R R F R F PSh (b) DC LOAD DC HYBRID (A) DC 22 CAVITY TEST LINAC M M INJECTOR 94 CAVITY TW LINAC DC (B) F R LOAD F R HYBRID LOAD DC HAIMSONRESEARCHCORPORATION

  7. 7 7 AAC 08 DESIGN AND OPERATING PARAMETERS OF THE 17 GHz SST/CU LINAC STRUCTURE AND RESONANT RING Frequency . . . . . . . . . . . . . . . . 17.140 GHz Number of Cavities . . . . . . . . . . . 22 Phase Advance . . . . . . . . . . . . . 120 Linac Structure Beam Aperture (a/λ). . . 0.1715 Linac Structure Disc Thickness (t/λ). . . 0.0835 Group Velocity . . . . . . . . . . . . . . 0.044c Cavity Phase Velocity . . . . . . . . . . 4 at c, 2 at 1.005c, 2 at 1.010c, and 14 at1.015c Structure Attenuation Parameter . . . . 0.178 Np Resonant Ring Loop Loss . . . . . . . . 1.65 dB RF Power Buildup [= C/(1-TcTf)] . . . . . . 3.1 HAIMSONRESEARCHCORPORATION

  8. 8 AAC 08 HAIMSONRESEARCHCORPORATION

  9. 9 AAC 08 HAIMSONRESEARCHCORPORATION

  10. 10 AAC 08 HAIMSONRESEARCHCORPORATION

  11. 11 AAC08 High Power RF Waveforms of the Bridge Protected High Gradient 17 GHz Linac Frequency = 17137 MHz. Time Base = 50 ns/div. WITH ARC IN LINAC “A Linear Accelerator Power Amplification System for High Gradient Structure Research,” in Advanced Accelerator Concepts, AIPConf. Proc., No. 472, pp. 1003–1013, 1998. RF Source Power Bridge Load Power Linac Input Power Linac Reflected Power NORMAL LINAC OPERATION Showing that when an Arc Occurs in the Linac, the Linac Input Power (blue) is Rapidly Truncated and, for the Remaining Portion of the Klystron RF Pulse the Bridge Input Power (red) is Automatically Directed into the Bridge Load (green). Thus, the Linac Power Amplifying Bridge Assists in Automatically Protecting both the RF Source and the High Gradient Linac Structure. HAIMSONRESEARCHCORPORATION

  12. AAC 08 BECAUSE THE RING LIMITS THE ENERGY DEPOSITION IN THE STRUCTURE DURING THE BREAKDOWN, AND BECAUSE THE KLYSTRON POWER IS SIMULTANEOUSLY SWITCHED INTO THE LOAD ARM AND DISSIPATED OUTSIDE THE HIGH VACUUM ENVELOPE, BREAKDOWNS CAN BE TOLERATED WITHOUT TRIPPING OR SWITCHING OFF THE SYSTEM, AND BY REDUCING THE REP RATE TO ½ or 1Hz, THE SYSTEM CAN BE RUN CONTINUOUSLY WHILE OBSERVING THE PULSE BY PULSE PROGRESSION OF THE RF CONDITIONING PROCESS. HAIMSONRESEARCHCORPORATION

  13. 13 AAC 08 AFTER THE PULSE WIDTH IS INCREASED BY ONLY 10 ns, IT TAKES ONLY 1 (0R 2) PULSES AT THE NEW WIDTH TO ERASE THE SURFACE MEMORY OF THE PRIOR PROCESSING HOLD -OFF LEVEL AND FOR THE BREAKDOWN TO DEFAULT TO A MUCH LOWER LEVEL. ARC SPARK 893 894 895 896 SIZZLE HAIMSONRESEARCHCORPORATION

  14. 14 AAC 08 17 GHz Data Attained After1.5e05 Pulses SST/Cu 16 MW 50 MW 2.22 84 MV/m 186 MV/m (RF Processing Still in Progress) Input Power from RF Source (160ns) . . . Linac Input Power . . . . . . . . . . . . . ESM / EA . . . . . . Maximum Accelerating Gradient. . . . . . Maximum Surface Gradient . . . . . . . HAIMSONRESEARCHCORPORATION

  15. 15 AAC 08 HAIMSONRESEARCHCORPORATION

  16. 16 AAC 08 HAIMSONRESEARCHCORPORATION

  17. 17 AAC 08 Observations During High Gradient Testing of the 22 Cavity SST/CuLinac • A notable difference in the processing behavior of the ring driven linac structures was the absence of retrogressive breakdown thresholds, even after heavy discharges during • wide pulse operation. • A gradient of 84 MV/m has been achieved after 1.5x105 pulses,and RF processing is still in progress. HAIMSONRESEARCHCORPORATION

  18. AAC 08 Observations During High Gradient Testing of the 22 Cavity SST/CuLinac • Because the resonant ring automatically limits energy deposition during RF breakdown events, it is possible to continue operating at threshold values of surface gradient without trip interruptions. Thus, by also operating at a low pulse repetition rate, the pulse by pulse progression of the RF processing mechanism can be continually monitored and studied as the RF pulse parameters are varied. • This low pulse repetition mode of operation continues to provide interesting data related to structure BD and is influencing the planning and direction of future R&D efforts. HAIMSONRESEARCHCORPORATION

  19. 19 DC – High Directivity Directional Coupler FC – Faraday Cup M – Motor Drive PSh – Phase Shifter W – Ceramic RF Window R F DC W AAC 08 17 GHz TWRK FC TRIPLE HYBRID SPECTROMETER FC F R R F R F PSh DC LOAD DC HYBRID (A) DC 22 CAVITY TEST LINAC M M INJECTOR 94 CAVITY TW LINAC DC (B) F R LOAD F R HYBRID LOAD DC HAIMSONRESEARCHCORPORATION

  20. 20 AAC 08 This is to acknowledge contributions of the MIT Plasma Science and Fusion Center Staff, especially the assistance of Ivan Mastovsky,Roark Marsh and Brian Munroe with the Ongoing RF Testing of the Gradient Hardened 17GHz Linac Structure. HAIMSONRESEARCHCORPORATION

  21. AAC 08 21 DESIGN FEATURES AND INITIAL RF PERFORMANCE OF A GRADIENT HARDENED 17 GHz LINAC J. Haimson and B. Mecklenburg Haimson Research Corporation Work performed under the auspices of the U.S. Department of Energy SBIR Grant No. DE-FG02-05ER84362

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