Study of the Effect of Reflections on High-Power, 110-GHz Pulsed Gyrotron Operation

The effect of reflection is studied experimentally and theoretically on a high-power 110-GHz gyrotron operating in the TE22,6 mode in 3 mu s pulses at 96kV, 40A. The experimental setup allows variation of the reflected power from 0 to 33% over a range of gyrotron operating conditions. The phase of the reflection is varied by translating the reflector along the axis. Operating at a higher efficiency point, at 4.40T with 940kW of output power, reflected power exceeding 11% causes a switch from operation in the TE22,6 to simultaneous operation in the TE22,6 and TE21,6 modes with a large decrease of the total gyrotron output power. This switching effect is in good agreement with simulations using the MAGY code. Operating at a more stable point, 4.44T with 580kW of output power, when the reflection is increased, the output power remains in the TE22,6 mode but it decreases monotonically with increasing reflection, dropping to 200kW at 33% reflection. Furthermore, at a reflection above 22%, a power modulation at 25 to 30MHz is observed, independent of the phase of the reflected wave. Such a modulated signal may be useful in spectroscopic and other applications.