EXPERIMENTAL-STUDY OF A MEGAWATT 200-300 GHZ GYROTRON OSCILLATOR

A detailed experimental study is presented of a pulsed megawatt gyrotr on oscillator operating in the 200-300 GHz range, whose design is cons istent with continuous operation for electron cyclotron resonance heat ing (ECRH) of fusion plasmas. Two different radii beams produced by ma gnetron injection guns (MIG's) were used to excite the cylindrical wav eguide cavity. The emission was found experimentally to be single mode , single frequency with a single rotation, which can be mode converted for transmission. The highest poser reached with the larger radius el ectron beam was 1.2 MW at 230 GHz in the TE34,6 mode with an efficienc y of 20% and beam parameters of 59 A and 100 kV. The highest power rea ched with the smaller radius electron beam was 0.78 MW at 280 GHz in t he TE25,13 mode with an efficiency of 17% and beam parameters of 51 A and 92 kV. The smaller radius beam gave a peak efficiency of 18% at 0. 72 MW, 290 GHz in the TE25,14 mode. Efficiencies obtained in this expe riment are about half that of less highly overmoded gyrotrons. Analysi s of the experiment suggests that the low efficiency is primarily caus ed by azimuthal mode competition, in agreement with multimode theory f or a tapered cavity. These experimental results show that megawatt pow er levels can be generated in continuous wave (cw) gyrotron oscillator s at 200-300 GHz with efficiencies approaching 20%.