S. Hiranai

Development and achievements on the high power ECRF system in JT-60U

An output power of 1.5 MW for 1 s was achieved at 110 GHz in a recent gyrotron development using the JT-60U ECRF system. It is the worlds highest power oscillation for a pulse duration of at least 1 s. The achievement was enabled by, in addition to the carefully designed cavity and collector, necessary because of thermal stress, an RF shield for the adjustment bellows and a low-dielectric-loss dc break. The way the power was modulated was improved upon by controlling the anode voltage, with high modulation frequency of 5 kHz being achieved in NTM stabilization experiments. Moreover, as a developmental step to realizing a reliable ECRF system for use in future fusion experiments, a long pulse demonstration of 0.4 MW and a 30 s injection into the plasma was achieved with real time control of the anode/cathode-heater. Confirmation was made that the temperature of the cooled components had been saturated with no evidence of any damage being discovered in the waveguides and antenna without forced cooling. An innovative antenna with a relatively wide range of beam steering capabilities utilizing a linearly moving-mirror concept was also designed for use as an active cooling antenna with longer pulses in the future, e.g. for JT-60SA. The beam profile and mechanical strength analyses proved the feasibility of the antenna.

Development of a Real‐time Modulation Control System on the JT‐60U ECRF System

A real‐time modulation control system was developed for studying the effectiveness of modulated Electron Cyclotron Current Drive (ECCD) on the stabilization of Neoclassical Tearing Modes (NTMs). The phase and duty cycle tracking was enabled with high accuracy even if the NTM frequency varied during ECCD. A characteristic of an anode voltage modulation for modulating the gyrotron oscillation that caused the time variation of the phase and duty cycle was also taking into account. It was shown that the phase and duty cycle could be kept the target values within 50 ms even if the frequency varied by 1 kHz/s or higher at a frequency of around 5 kHz. Then, NTM stabilization experiments with modulated ECCD for m/n = 2/1 NTM was successfully performed in JT‐60U.