Quasioptical Gyrotrons And CARMs for Tokamak

Currently planned tokamaks will require several megawatts of average rf power in the 100-300 GHz range for ECH. For example, the Compact Ignition Tokamak (CIT) design calls for 30 MW of 280 GHz radiation and the International Thermonuclear Experimental Reactor (ITER) design requires 20 MW of 120 GHz rf power. The leading candidate to fill these needs is the waveguide cavity gyrotron. However, as the power and frequency are raised, ohmic heating constraints in the cavity force the use of a larger, more overmoded cavity leading to difficulties with mode competition, output coupling, and electron beam collection. An alternative cyclotron resonance maser concept, in which the waveguide cavity is replaced by an open Fabry-Perot resonator, is currently under development at the U.S. Naval Research Laboratory (NRL). The inherent separation of the radiation and electron beams simplifies both radiation output coupling and the use of a depressed collector for spent electron beam energy recovery. Other important advantages include low ohmic heating density on the resonator mirrors and wide tunability. This paper will review recent experimental progress on the quasioptical gyrotron (QOG) and discuss the design of quasioptical gyrotrons and CARMs capable of meeting the needs of ITER and CIT.