Generation of high power sub terahertz radiation from a gyrotron at second harmonic resonance

A high power sub terahertz pulse gyrotron is under development in FIR-FU for application to collective Thomson scattering from a high density plasma in the large helical device. As a step to the final goal, a second harmonic gyrotron using a newly designed electron gun was fabricated aiming at 50 kW at around 400 GHz. Oscillation modes were carefully selected from the view point of mode separation. The electron optics was also considered in detail. Then, experiments have proved single mode oscillation of second harmonic modes and oscillation power of 50 kW at 349 GHz and 40 kW at 390 GHz.