Study of sub-terahertz high power gyrotron for ECH&CD system of DEMO

Summary form only given. For the Electron cyclotron resonance heating and current drive (ECH&CD) system on fusion DEMO reactor, the high power mm-wave of 200 GHz ~300 GHz is expected. To increase the gyrotron frequency, extremely high-order oscillation mode should be adopted to suppress the Ohmic loss on the resonator wall less than ~20 MW/m<;sup>2<;/sup>. Here, we designed and fabricated a 300 GHz gyrotron of TE<;sub>32,18<;/sub> mode oscillation and started a short pulse experiment to investigate the oscillation characteristics of the high order mode at ~300 GHz. As a preliminary result, we demonstrated the power generation of ~0.5 MW at ~300 GHz oscillation at the oscillation mode of TE<;sub>32,18<;/sub>, and other modes. A height of the test gyrotron is ~2 m. The electron gun is diode type magnetron injection gun (MIG), to maximize the diameter of an electron emitter (d=74 mm). The resonator is a conventional open cavity. The diameter is 31.6 mm, which corresponds to the 300 GHz oscillation with TE32,18 mode. This mode is capable of 0.5 MW/300 GHz at CW operation. The distance from the emitter to the resonator is ~520 mm. The oscillation power is transmitted by the collector as a waveguide, and outputted as the oscillation mode through the sapphire window of 102 mm in diameter. A super-conducting magnet (SCM) is a 13 T liquid-He-free-magnet, which has a room temperature bore diameter of 110 mm. A dummy load is put on the top of the gyrotron to absorb and measure the output power. In the experiment, we demonstrated the power generation of ~0.5 MW at ~300 GHz oscillation at the TE32,12 mode. The applied beam voltage is 80 kV, and the beam current is 36.8 A. Pulse duration is ~2 msec. A MIG field is optimized to oscillate the target mode. The power peak was around 12.0 T, which corresponds to the TE32,18 mode. By decreasing the magnetic field to ~11.73 T, lower adjacent mode TE31,18 appeared. Detailed studies of- the mode competition, etc., will be carried out.