Results from thermionic cathode gyro-TWA experiments

Experimental operation of a gyro-travelling wave amplifier with a helically corrugated interaction region is presented. Two thermionic cathode electron guns in Pierce-type geometries were used to generate rectilinear electron beams of 1 μs pulse duration. One diode was operated at 185 keV energy with a current of 7.5 A and the other was operated with an applied potential of 115 kV producing a current of 18 A. Transverse velocity was imparted to the electron beam in a ´kicker´. The electron beam acquired a transverse velocity typically with a pitch factor of between 1 and 1.2. The resulting electron beam described a helical, axis-encircling trajectory. The coupling between the second harmonic cyclotron mode of the gyrating electron beam and the radiation field occurred in the region of near infinite phase velocity over a broad frequency band using a cylindrical waveguide with a helical corrugation on its internal surface. With a beam energy of 185 keV, the amplifier achieved a maximum output power of 220 kW, saturated gain of 24 dB, saturated bandwidth of 8.4 to 10.4 GHz (21% relative bandwidth) and an efficiency of 16%. With a beam energy of 115 keV, the amplifier achieved a maximum output power of 160 kW and an efficiency of 8%.