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

To Prevent Surface Erosion During High Gradient Operation and to Assist in Studying RF Breakdown, the Gradient Hardened 17 GHz Linac Structurewas Designed to have To Prevent Surface Erosion During High Gradient Operation and to Assist in Studying RF Breakdown, the Gradient Hardened 17 GHz Linac Structure was Designed to have Brazed and Machined Stainless Steel Surfaces in the High Field Regions of the Circuit. AAC 08 H AIMSON R ESEARCH C ORPORATION 2

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. H AIMSON R ESEARCH C ORPORATION 2. 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. 3. RF WAVEFORM DYNAMICS RECORDED WHILE RF PROCESSING AT HIGH SURFACE GRADIENTS AND LOW REPETITION RATES. AAC 08

To Prevent Surface Erosion During High Gradient Operation and to Assist in Studying RF Breakdown, the Gradient Hardened 17 GHz Linac Structurewas Designed to have To Prevent Surface Erosion During High Gradient Operation and to Assist in Studying RF Breakdown, the Gradient Hardened 17 GHz Linac Structure was Designed to have Brazed and Machined Stainless Steel Surfaces in the High Field Regions of the Circuit. AAC 08 H AIMSON R ESEARCH C ORPORATION PAC

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. AAC 08 H AIMSON R ESEARCH C ORPORATION 5

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

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

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NORMAL LINAC OPERATION RF Source Power Bridge Load Power Linac Input Power Linac Reflected Power High Power RF Waveforms of the Bridge Protected High Gradient 17 GHz Linac Frequency = MHz. Time Base = 50 ns/div. “A Linear Accelerator Power Amplification System for High Gradient Structure Research,” in Advanced Accelerator Concepts, AIP Conf. Proc., No. 472, pp. 1003–1013, 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. WITH ARC IN LINAC H AIMSON R ESEARCH C ORPORATION 11 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. AAC 08 H AIMSON R ESEARCH C ORPORATION

AAC 08 H AIMSON R ESEARCH C ORPORATION 13 ARC SPARK SIZZLE 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.

AAC H AIMSON R ESEARCH C ORPORATION 17 GHz Data Attained After 1.5e05 Pulses Input Power from RF Source (160ns)... Linac Input Power E SM / E A E SM / E A Maximum Accelerating Gradient Maximum Surface Gradient SST/Cu SST/Cu 16 MW 16 MW 50 MW 50 MW MV/m 84 MV/m 186 MV/m 186 MV/m (RF Processing Still in Progress)

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Observations During High Gradient Testing of the 22 Cavity SST/Cu Linac H AIMSON R ESEARCH C ORPORATION A notable difference in the processing behavior of the ring driven linac structures was the absence of retrogressive breakdown thresholds, even after heavy discharges duringA 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 1.5x10 5 pulses,and RF processing is still in progress.A gradient of 84 MV/m has been achieved after 1.5x10 5 pulses, and RF processing is still in progress. AAC 08 17

Observations During High Gradient Testing of the 22 Cavity SST/Cu Linac 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.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. AAC 08 H AIMSON R ESEARCH C ORPORATION 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.

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

H AIMSON R ESEARCH C ORPORATION Ivan Mastovsky Roark MarshBrian Munroe 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. AAC 08 20

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