Study of millimeter wave high-power gyrotron for long pulse operation

The development of a high-power millimeter wave source, gyrotron, is under way for fusion application. A performance of ∼10 s oscillation has been attained at 1 MW level output for 170 GHz frequency. Although the pulse extension was interrupted by a sudden outgassing in the gyrotron, the cause was confirmed to be the local heating of the internal component due to stray RF deposition. To suppress the heating, the inner surface of the component (bellows for RF beam steering mirror) was coated with copper, which reduced the Ohmic loss to 110 of the original one. Moreover, forced water cooling for the bellows was incorporated. As a result, no sudden pressure increase was observed, and a quasi-steady-state oscillation of 100 s with 0.5 MW power level was demonstrated at 170 GHz. The temperature of the major components of the gyrotron stabilized, which indicates a prospect for a 1 MW-CW, 170 GHz gyrotron.