High-efficiency, long-pulse operation of MW-level dual-frequency gyrotron, 84/126GHz, for the TCV Tokamak

The first unit of the dual-frequency gyrotron, 84-126GHz/1MW/2s, for the upgrade of the TCV ECH system has been delivered and is presently being commissioned. During a first phase, long-pulse operation (T
_RF
>0.5s) has been achieved and powers in excess of 0.93MW/1.1s and 1MW/1.2s have been measured in the evacuated RF-load at the two frequencies, 84GHz (TE
_17,5
mode) and 126GHz (TE
_26,7
mode), respectively. Considering the different rf losses in the experimental setup, the power level generated in the gyrotron cavity is in excess of 1.1MW and 1.2MW, with a corresponding electronic efficiency of 35% and 36%. These values are in excellent agreement with the design parameters and would likely lead to a gyrotron total efficiency higher than 50% in case of implementation of a depressed collector. The gyrotron behavior is remarkably reliable and robust with the pulse length extension to 2s presently only limited by external auxiliary systems.