Operation of a step tunable, megawatt gyrotron

A pulsed (3µsec) gyrotron oscillator with a single cylindrical cavity has generated high output powers at frequencies between 126 and 243 GHz. Peak powers of 645 kW and total efficiencies of 28% were measured. By vaying the cavity magnetic field, step tuning through a sequence of TEm,p,1modes with p = 2, 3 and 4 was observed. Single mode emission was achieved in each of these modes, which were separated by about 7 GHz. The highest order mode observed was the TE22,4,1at 243 GHz, which corresponds to a cavity diameter of 12 free space wavelengths. An output power of 470 kW was measured in this mode at 80 kV and 35 A. Future experiments include the investigation of novel cavity concepts with reduced mode competiton that should allow us to generate powers in excess of 1 MW at 140 GHz, and a high power gyrotron oscillator designed to operate at 280 GHz. Although these experiments are pulsed, they can be scaled to cw operation.