Experimental study of a 1.5 MW, 110 GHz gyrotron with a single-stage depressed collector

A single-stage depressed collector has been installed on a 1.5 MW, 110 GHz gyrotron and tested in 3 microsecond pulse operation at 96 kV and 40 A. The overall efficiency with the depressed collector was 50 % at 1.5 MW of output power with a voltage depression of 25 kV. Theory predicts up to 35 kV of depressed voltage for these operating conditions. The discrepancy between the experiment and the theory has been explained by the effect of an aftercavity interaction (ACI). In this paper a theoretical study using MAGY code will be compared with the experimental results. Low frequency oscillations due to reflected electrons bouncing in the adiabatic trap are identified on the 3 microsecond pulse, for accelerating voltages in the range of 45 kV to 90 kV, with observed oscillation frequencies between 100 MHz and 170 MHz. A characterization of the oscillations is performed, through the scaling of the oscillation frequency with cathode voltage and beam current. The experimental results are compared with existing theories.