Quasi-optical mode converter for a multi-frequency D-band gyrotron

A design of a quasi-optical mode converter operating at nine modes is described in this paper. The main application for high-power gyrotrons is electron cyclotron resonance heating (ECRH) and plasma stabilization in nuclear fusion devices like tokamaks. The absorption of the microwave in the plasma depends on its frequency and the plasma parameters. Thus having a multi-frequency high-power source would give more flexibility to plasma stabilization. Experimental results with a fast step-tunable gyrotron are also demonstrated. At the ASDEX Upgrade tokamak at IPP in Garching there is an interest in step-tunable gyrotrons operating at frequencies between 105 GHz and 143 GHz. For this purpose a multi-frequency gyrotron is under investigation at the Institute of Applied Physics in Nizhny Novgorad, Russia, in cooperative development with the Forschungszentrum Karlsruhe (FZK). Here we deal with the quasi-optical mode converter of this gyrotron. To enhance the efficiency and minimize the losses due to stray radiation, an optimization of the quasi-optical mode converter for all operating modes is important.