Design of a 100 kW-384 GHz second harmonic gyrotron

A novel high power sub-terahertz gyrotron operating at second harmonic resonance was designed. This gyrotron is intended for use as a probe beam source for collective Thomson scattering diagnostics in nuclear fusion plasma research. The resonant cavity mode, cavity radius, and its length were optimized from various aspects such as resonant frequency, coupling between the electron beam and the RF-electric field in the cavity, so-called perpendicular efficiency and radiation power. Then, the risk of mode competition was analyzed by using the CS-MMTD code. The second harmonic co-rotating TE_15,2 mode was selected as the optimum operating cavity mode because the results of the numerical experiments indicate that it is less susceptible to the mode competition even for high beam currents. By such optimization, it was found that a single mode second harmonic radiation with power of 100 kW at the frequency of 384 GHz can be well expected at a cathode voltage of 65 kV and beam current of 10 A.