Experimental studies of a wiggler-focused sheet electron beam high power millimeter-wave free-electron laser amplifier

The detailed results of experiments with a short period (9.6 mm) wiggler sheet electron beam (1.0 mm x 2.0 cm) millimeter-wave free electron laser amplifier are presented.; The sheet electron beam was produced from an explosive field emission diode, which was driven by a pulse-line accelerator. A variety of different cathode materials with different emitting surface shapes were studied experimentally. The cathode material tungsten graphite yielded a better overall diode performance than the conventional velvet or reactor graphite. The effective emittances of the sheet electron beams were measured using a slit-pinhole method, modified as appropriate for a sheet beam configuration.; This free electron laser (FEL) amplifier utilized a strong wiggler field for sheet beam confinement in the narrow beam dimension and an offset-pole side-focusing technique for the wide dimension beam confinement. The beam analysis herein includes finite emittance and space-charge effects, as well as beam matching and other related issues. The analysis shows that a matched sheet beam is achievable. High beam current was successfully propagated through a waveguide inside the wiggler channel as a result of extensive analytical and simulation studies and experimental optimization.; The initial device was successfully operated at 94 GHz with a waveguide having a 2.8 mm x 4.0 cm interior cross-section. A maximum gain of 17 dB was achieved with a 56 period wiggler, 9 A beam current, 550 kV beam voltage, and a 3.9 kG wiggler magnetic field, at an injected power level of several watts. A successive high power experiment (1 kW injected power) at 86 GHz with the same device resulted in a maximum gain of only 6 dB. The discrepancy was attributed to device structure errors, electron beam quality, and device design parameters.; An improved design resulted in a reduction of sensitivity to structure errors and beam velocity spread, a reduction of required beam energy, as well as a much higher propagated beam current. The device was studied over a broad range of experimental parameters. A maximum gain of 24 dB was achieved with a 1 kW injected signal power at 86 GHz, a 450 kV beam voltage, 17 A beam current, 3.8 kG wiggler magnetic field, and a 74 period wiggler length. The maximum gain with a 1 watt injected millimeter-wave power was observed to be over 30 dB. The lower gain at higher injection power level indicates that the device has approached saturation. The experimental results have a good agreement with expectations from a one-dimensional simulation code. The successful operation of this device has proven the feasibility of the original concept and demonstrated the advantages of the sheet beam FEL amplifier.