THEORY OF PHASE-LOCKED GYROTRONS OPERATING AT CYCLOTRON HARMONICS

The theory of phase-locked gyrotrons operating at cyclotron harmonics is developed, with particular attention given to the effects of electr on velocity spread. Extensive studies are made for operation at the se cond harmonic in which phase locking is achieved by subharmonic prebun ching. One result of these studies is that we are able to predict, for a range of velocity spreads, the parameters that yield the highest ef ficiency. More important, we find that for electron beams with velocit y spread and a finite transit angle in the drift region, the bandwidth of stable operation is drastically reduced. This effect can be mitiga ted by properly tapering the external magnetic field there. Such taper ing is necessary in devices where the bandwidth is required to be a re asonable fraction of 1/Q.