Present status of gyrotron development for ITER and prospect of advanced gyrotron for fusion reactor

Kasugai, Atsushi; Takahashi, Koji; Kajiwara, Ken; Kobayashi, Noriyuki; Sakamoto, Keishi

In Japan Atomic Energy Agency, the development of the 170 GHz gyrotron with the oscillation mode of TE31,8 for ITER have been carried out aiming at the achievement of the high power, the long pulse and the high efficiency operation. It was cleared that the enhancement of the gyrotron power and the extension of the pulse length were restricted by unusual oscillation due to a beam current decrease and local heating due to stray radiation loss inside the gyrotron. To solve these problems, the improvement of a radiator and an output circuit on the internal mirror were modified. Furthermore, the beam current was controlled by pre-programming of the cathode heater voltage. And, the oscillation efficiency was enhanced by improvement of the electron beam quality. As the result of these improvements, the steady state operation of 1 hour (3600 s) with the output power of 0.6 MW, which significantly exceeded the typical operation time 400 s of ITER was demonstrated. The operation of a 0.6 MW/1 hour could be obtained by the usual operation method which fixed the cavity magnetic field and the pitch factor of the electron beam. However, it was possible that the state of oscillation is controlled by changing these operation parameters during the shot in the long pulse operation. By the advanced operation mode, the gyrotron output power increased to 0.82 MW with the same beam power (the beam current and the beam voltage) of the 0.6 MW/1 hour operation. The oscillation characteristics in the long pulse operation was clarified, and the advanced operation mode of the gyrotron to the high efficiency oscillation was newly established by controlling a pitch factor of the electron beam and the cavity magnetic field in the long pulse operation. As a result, the high efficiency operation of 56 % with the output power of 0.82 MW was succeeded beyond 600 s (10 min).