Long pulse operation of 170 GHz ITER gyrotron by beam current control

In a pulse extension of a high-power gyrotron, a pulse duration was limited by the shift of the oscillation mode caused by a beam current decrease during the shot. The cause of the current decrease is the cathode cooling by the electron beam extraction. To keep the beam current constant, pre-programming control of the input power to the heater was applied. As an initial result, the stable and steady state operation of 1000 s with the output power of 0.2 MW was demonstrated on the 170 GHz gyrotron. Temperatures of the inner components of the gyrotron and cooling waters were constant, and the vacuum pressure was less than 10−5 Pa for 1000 s operation, which gives a prospect for CW operation at the higher power.