NONSTATIONARY THEORY OF MODE COMPETITION IN GYROTRONS WITH ALLOWANCE FOR SMALL INHOMOGENEITY OF THE GUIDING MAGNETIC-FIELD

High-power millimeter wave generators can induce electron cyclotron wa ves in a plasma which can fulfill many functions (plasma breakdown, he ating, current drive) in fusion plasma reactors including both tokamak and stellarator concepts. All these applications require tens of mega watts of power in the frequency range 100-300 GHz and power 0.5-1.5 MW per unit. Among various microwave sources gyrotrons are the most deve loped source. Because of the heat losses in resonator walls at such hi gh frequencies and powers oversized cavities must be used. In such cav ities the mode competition becomes a severe problem. The most advanced theory of mode competition in gyrotrons is the nonstationary nonlinea r theory. In the present article we extend this theory by allowing for inhomogeneity of the guiding magnetic field. The importance of this g eneralized theory is illustrated in the case of a 3 MW 140 GHz gyrotro n with a coaxial cavity working in the TE(21,13) mode.