Gyro-BWO experiments using a helical interaction waveguide based on a thermionic cathode

Summary form only given. Experimental operation of a gyrotron backward wave oscillator with a helically corrugated interaction region is presented. A thermionic cathode electron gun in a Pierce-type geometry was used to generate a rectilinear electron beam of 1 /spl mu/s pulse duration, 185 keV energy and /spl sim/2.5 A in current. Transverse velocity was imparted to the electron beam in a ´kicker´ consisting of 4 pairs of small rectangular multi-turn coils arranged in a Helmholtz-like configuration. When a pulsed current was applied to the kicker, the electron beam acquired a transverse velocity typically with a pitch factor of up to 1.6. The resulting electron beam described a helical, axis-encircling trajectory. The CRM instability provided the effective coupling between the radiation and the axis encircling electron beam with the interaction harmonic number equal to the azimuthal mode index of the radiation. In this experiment the operating mode was synthesised by coupling the TE/sub 2,1/ mode (azimuthal index, m/sub A/=2) and the counter-rotating TE/sub 1,1/ mode (m/sub B/=-1) on a three fold helical corrugation. The backward wave co-rotating with the electron beam resonantly interacted with the 2nd harmonic of the electron cyclotron wave and underwent a total reflection from the upstream anode side. The reflected forward wave propagated through the system without interaction with the electron beam. The output window, positioned at the other end of the cavity, consisted of 3 pieces of quartz and had a reflection coefficient of less than 1% in the frequency range of 8 GHz to 10 GHz. At a beam energy of 185 keV and current of 2 A, the output frequency and power from the gyro-BWO were measured as a function of the tuning cavity magnetic field. A 3 dB frequency tuning range of 8.0-9.5 GHz (i.e. 17% relative tuning range) was measured with a maximum output power of 62.4 kW and electronic efficiency of 17%. In addition, the interaction frequency could be tuned by - arying the electron beam energy. At a fixed cavity magnetic field of 0.195 T, the output frequency and power from the gyro-BWO were measured as a function of tuning electron beam energy while the beam current was maintained at 2-2.5 A. A 3 dB relative frequency tuning range of 8% was measured when the electron beam voltage was changed from 215 kV to 89 kV.