Homogenous power distribution along gyrotron collector walls

EC RF power sources will play a major role in the future operation of large scale tokamaks (ITER, DEMO). A 1MW/170GHz gyrotron is the standard RF power unit considered for ITER. Even though the efficiency of such devices has been significantly increased thanks to the depressed collector technique, the power deposited on the collector by the spent electron beam is still above 1MW. As a consequence, this power shall be spread along the collector walls to avoid overheating of the collector internal surface. A novel solution, developed and tested at the Swiss Plasma Center (SPC), to flatten the power deposition profile is described in this paper. Based on an AM modulation of the vertical collector sweeping coil current, it allows for a more homogeneous power deposition profile and a substantial decrease of the peak load. This solution relies on a commercial off-the shelf (COTS) power supply and a common PLC controller, so it is a relevant solution for all MWclass gyrotrons presently in operation and for future applications.