Operating characteristics of a continuous-wave-relevant quasioptical gyrotron with variable mirror separation

Results from a quasioptical gyrotron experiment with a 20-28 cm mirror separation are presented, showing operation at powers up to 150 kW and efficiencies up to 12%. The output coupling would be varied from 0.4%-3% by changing the mirror separation and operating frequency. Operation was obtained over frequencies ranging from 95-130 GHz by changing the axial magnetic field, limited on the low end by waveguide cutoff in the diagnostics and at the high end by the maximum magnetic field achievable. The output power varied by approximately a factor of 2 over this range. Frequency variation of 4% was achieved by varying only the electron gun voltage; however, the output power also varied substantially due to the fact that the electron beam power was changing dramatically. Efficiency optimization by variation of output coupling and tapering of the magnetic field has been demonstrated. Regions of single-mode operation at powers up to 125 kW have been characterized and compared to recently developed theory.