Millimetre wave gyrotron operation at high efficiency and power levels

Gyrotrons have reached impressive levels of performance such as pulse power output of a megawatt at frequencies higher than 100 GHz. Their applications include plasma heating for controlled fusion, high precision radar, RF source for accelerators, and ceramic heating. Operation in fundamental mode at millimeter wave frequencies requires the use of superconducting magnets. Harmonic operation offers the advantage of a lowered requirement of magnetic field intensity though at a lowered RF efficiency. Large-orbit gyrotrons are particularly suited for high harmonic operation. Techniques for enhancement of efficiency by energy recovery from spent beams are delineated for large as well as small-orbit gyrotrons. Achievement of high power levels in small-orbit gyrotrons is discussed along with related issues of complex cavities as well as cavities operating in 'whispering gallery' modes, and their mode stability. Quasi-optical gyrotrons have reduced mode density. However, mode competition still exists from longitudinal modes. Current research on single mode stability is outlined with reference to beam parameters