Reflections in gyrotrons with axial output

Influence of reflections on operation of gyrotrons with axial output is studied both theoretically and experimentally. By way of example the Fukui large orbit gyrotron with a permanent magnet operating in third harmonic at frequency 89 GHz is considered. In the case of strong reflection (\R\ = 0.6), extreme sensitivity of output power on the reflection phase is found. A qualitative agreement between theory and experiment is observed.