Long-pulse operation of a gyrotron with beam/rf separation

Varian is carrying out the development of high-power, cw gyrotrons at frequencies ranging from 85--140 GHz. An experimental tube employing a TE{sub 22,2,1}-mode cavity and utilizing an output coupling concept where the collection of the spent electron beam is separated from the outgoing microwave power, has been designed and constructed. In initial tests on the tube, output powers of 450--500 kW have been obtained for pulse durations out to 2.5 seconds, in the desired TE{sub 22,2,1} mode at a frequency of 110 GHz. Electron cyclotron resonance heating (ECRH) requirements for planned magnetic fusion experiments such as the International Thermonuclear Experimental Reactor (ITER) will require millimeter-wave sources capable of output powers of at least 1 MW cw at frequencies in the range, 100--200 GHz. Initial megawatt-level development work at Varian resulted in a gyrotron design that was tested to output powers of 1 MW for short pulses ({approximately}1 ms) and 400 kW for 0.5-s pulses at a frequency of 140 GHz. Output powers up to 1 MW have been achieved for short pulses (1 ms). Future tests on this and subsequent experimental tubes will be aimed at achieving 1 MW power levels under long-pulse and ultimately, cw conditions.