High power operation of 110 GHz gyrotron at 1.2 MW on the JT-60 ECRF system

A high power operation of 1.2 MW for 4 s was obtained by a 110 GHz gyrotron with depressed collector on the JT-60 ECRF system, where the cathode voltage is −58 kV without voltage regulation and the acceleration voltage is +85 kV with regulation. The gyrotron is featured by a diamond output window and a RF absorber in its beam tunnel. The RF absorber suppresses the parasitic oscillation, which limited the performance of the gyrotron without the RF absorber less than 1 MW×2 s. The operation was done with the JT-60 tokamak plasma as a dummy load, where the EC wave was transmitted through a diamond antenna window on the corrugated waveguide. The edge temperature of the antenna window without cooling was in proportion to the pulse duration. The maximum efficiency of the gyrotron was about 42% in the depressed collector operation on the JT-60 ECRF system.