X-BAND RF STRUCTURES, BEAM DYNAMICS AND SOURCES WORKSHOP – XB-10 Cockcroft Institute, Daresbury, UK, Nov. 30 – Dec. 3, 2010 EXPERIMENTAL STUDIES OF ACTIVE RF PULSE COMPRESSION* A.L. Vikharev, 1,2 O,A. Ivanov, 1,2 A.M. Gorbachev, 2 M.A. Lobaev, 2 S.G. Tantawi, 3 J.R. Lewandowski, 3 J.L. Hirshfield, 1,4 1 Omega-P, Inc., New Haven, CT Institute of Applied Physics, Nizhny Novgorod, Russia SLAC, Menlo Park, CA Department of Physics, Yale University, New Haven, CT _______________ *Research sponsored by US DoE and Russian Foundation for Basic Research.

Q: What is active, as opposed to the usual passive, RF pulse compression? A: Elements in the RF circuit have properties that can be rapidly switched from one state to another. OUTLINE OF TALK: Reminder of 2005 world record “GW-level” SLED-II passive pulse compression result at SLAC. Reminder of the basics of active switching of a resonant delay line. Plasma switch as an active element in X-band pulse compression experiments. Plasma-activated diffraction grating in a Ka-band pulse compression experiment. Recent experimental results with plasma switches as active elements in X-band SLED-II at SLAC. Explosive cold cathode as an active element in an X-band switch for RF pulse compressor. Summary

compression ratio (in time) R - during filling R - for dischargi ng efficiency  efficiency for passive SLED II  = 85%  = 75%  = 84%  = 65%  = 83%  = 50%  = 83%  = 41% To increase efficiency of SLED II, S.Tantawi, R. Ruth, and A. Vlieks [Nucl. Instr. Meth. Phys. Res. A370, pp (1996)] suggested to switch R, the reflection coefficient to the resonant delay lines, since optimum coupling during delay line filling differs from optimum coupling during discharge. Table below shows comparative efficiencies for active and passive SLED-II’s, for the case of discharging just before the last time bin (lossless line).

Efficiency for active SLED-II will of course degrade as losses in the delay lines are taken into account. Further, the switching time may be chosen to maximize efficiency, e.g., to switch before or after the last time bin. From Tantawi, Ruth, and Vlieks (1996).

1 -waveguide 38 mm in diameter, 2-stepped widening, 3-ring-shaped quartz discharge tube, 4 -iris. Plasma switch as the active element for an active pulse compressor quartz gas-discharge tubeenergy storage Reflection coefficient vs frequency in the passive state (a), and the switched state (b).

TE 01 mode storage cavities, power gain > 10:1, P peak = 53 MW, η = 56%.

Plasma switching using TE 01 / TE 02 dual mode storage cavities

Dependence upon electron density of the transmission coefficient across the switch at different positions of a following adjusting short, which will change the mode conversion factor. Note the number of adjustments needed to optimize switching.

TE 01 / TE 02 dual mode storage cavities – self-triggered switching P peak = 68 MW, G = 8.1, η = 61%

Schematic diagram of the experimental set- up: 1 - synthesized signal generator, 2 - circulator, 3 - mode converter, 4 - circular waveguide, 5 - plasma switch, 6 - gas discharge tube, 7 - smooth tapered transition, 8 - delay line, 9 - short circuit, 10 - high-voltage pulse generator, 11 - phase shifter, 12 - detector, 13 - pulse generator, 14 - trigger generator, 15 - peak power analyzer, 16 - rectangular waveguide. Envelope of the compressed pulse with the plasma switch (pressure of nitrogen of 200 mTorr), with cw excitation of the storage resonator. Low-power test of switch used in experiments on SLED-II at SLAC

1 – microwave generator, 2 – phase shifter, 3 – TWT, 4 – crystal, 5 – klystron, 6 – combiner, 7 – plasma switch, 8 – delay line, 9 – high voltage generator, 10 – trigger generator, 11 – load, 12 – oscilloscope, 13 – power meter, 14 – computer, 15 – movable short circuit, 16 – ion pump Schematic diagram and photos of the experimental set-up at SLAC

Recent results from experiments with active SLED-II (Nov. 2010) 1 – incident pulse; 2 – passive compression; 3 – passive & active compression passive power gain 4.2 passive power gain 3.9 passive & active power gain 5.7 passive & active power gain 4.3 passive efficiency 52 % passive efficiency 45% passive & active efficiency 72 % passive & active efficiency 50% P incident = 10.2 MW, P output = 58 MW P incident = 25.5 MW, P output = 110 MW

From AAC-2010 High Power Microwave Switch Employing Electron Beam Triggering O.A. Ivanov, V.A. Isaev, M.A. Lobaev, A.L. Vikharev, J.L. Hirshfield

Summary Active switching at low power has been demonstrated with power gains of the order of 10:1 using plasma switches in several SLED-like configurations, namely 1. single-mode energy storage cavities; 2. dual-mode energy storage cavities; 3. quasi-optical storage cavity. Active switching at high power (>50 MW) has been demonstrated at X- band in a SLED-like configuration, but self-triggering dominated in the dual-mode configuration. Active switching with power gain >10:1 has been demonstrated at the MW- level at Ka-band, using a quasi-optical resonator with a plasma-switched Bragg reflector. Active switching of SLED-II at SLAC has demonstrated a marked increase in efficiency, as compared with passive switching, but failure of quartz discharge tubes to condition posed a serious limitation. Electron beam switching with power gain of 20:1 and without quartz tubes has been demonstrated at X-band at low power. High power tests are scheduled to begin in January STAY TUNED!