Simulation of high power CW gyrotron collectors including the effect of secondary emission

High power CW gyrotrons (as the 1 MW, 140 GHz conventional cavity gyrotron for the stellarator W7-X at IPP Greifswald, Germany or the 2 MW, 170 GHz coaxial cavity gyrotron for ITER) require a sophisticated depressed collector design that is capable of absorbing the spent electron beam. The simulation program ESRAY, originally developed at FZK/IHM for electron gun design, is also capable of simulating gyrotron collectors, including the effects of eddy currents in the collector wall. At the moment, work is in progress to include secondary emission models in the ESRAY code. The effect of secondary emission has the advantage of a slightly reduced peak power density on the collector wall. It may also have drawbacks like secondary electrons reaching parts of the collector structure that are insufficiently cooled or electrons traveling back to the resonator region where they may have negative influence on the gyrotron interaction. In this paper we present the first results obtained with the enhanced version of our simulation tool ESRAY showing the effects of secondary emission on the collector wall of the 2 MW 170 GHz coaxial cavity gyrotron for ITER.