INFLUENCE OF SIDE-BAND OSCILLATIONS ON GYROTRON EFFICIENCY

We report the observation of sideband mode effects on the efficiency o f overmoded gyrotron operation. Two cavities were designed and operate d which differed in the presence of sideband modes. In one version of the cavity, parasitic backward waves modes were observed and efficienc ies were approximately 22%. at 40 A beam current. With the use of a mu lti-mode self-consistent nonlinear code, a modified design was generat ed which eliminated the sideband modes. Experiments were conducted wit h this new cavity which produced efficiencies of approximately 33% at 30 A and 27% at 40 A beam current, but with a slightly higher velocity ratio than seen with the earlier cavity. An additional cavity, also w ith no sideband modes but with a longer cavity length and therefore hi gher Q obtained powers up to 1.3 MW with an efficiency of 39% at a 40 A beam current.