Experimental Observation of the Effect Of Aftercavity Interaction in a Depressed Collector Gyrotron Oscillator

We present the experimental observation of the effect of an aftercavity interaction (ACI) in a depressed collector gyrotron oscillator. The gyrotron generates an output power of 1.5 MW at 110 GHz in 3 microsecond pulses with a 96 kV and 40 A electron beam and has a single-stage depressed collector. The ACI arises from cyclotron resonant interaction between the microwave beam traveling out from the cavity and the gyrating electron beam. The interaction occurs in the uptaper of the launcher, immediately downstream from the cavity, where the magnetic field is slightly lower than its value in the cavity region. The ACI results in a reduction in efficiency since the electron beam tends to extract power from the wave. There is also a broadening of the spent beam energy profile, which in turn limits the overall efficiency of a gyrotron. Measurements of the maximum depression voltage of the collector vs. beam current at 96 kV are compared with simulations from the MAGY code. Excellent agreement is obtained between theory and experiment but only if the aftercavity interaction (ACI) is included. These results verify the role of the ACI in reducing the efficiency of the gyrotron interaction.