Design of multimegawatt millimeter-wave converters for operation at high gyroharmonics

This paper explores the theoretical feasibility of employing gyroharmonic conversion for production of multimegawatt level 100--300 GHz radiation for tokamak plasma heating and control. A summary is provided of a feasibility study for the production of multimegawatt RF power from a spatiotemporally modulated gyrating relativistic electron beam at a high harmonic of the frequency of modulation. Parameters are given for optimizing the production of power at 148.5 GHz, the thirteenth harmonic of an 11.424-GHz cyclotron autoresonance traveling wave interaction that accelerates the beam. With an injected 16-A 200-kV beam, to which 7.2 MW is added in the accelerator, it is predicted that a power output of 2.2 MW at 148.5 GHz can be realized, after taking into account ohmic wall losses. This requires an injected laminar beam with small initial axial velocity spread, and an output structure that effectively suppresses competing modes. In this device a tapered guide magnetic field no stronger than 8.2 kG is required. Means to produce the required high-quality electron beam, and to allow only the TE13.1 design mode to propagate in the output structure are discussed. A similar design for a fifteenth harmonic converter at 171.4 GHz is also described