Effects of after cavity interaction in a 1.5 MW, 110 GHz gyrotron with a depressed collector

We report experimental results on a 1.5 MW, 110 GHz gyrotron operating in 3 microsecond pulses with a single-stage depressed collector. Our research is focused on the observation of the after cavity interaction (ACI), and its effect on re-absorption of microwave power by the electron beam and, more importantly, broadening of the spent beam energy distribution in the launcher, i.e., the region after the cavity. The second effect limits the efficiency of a depressed collector considerably. Simulations indicate that this reduced depressed collector capability deteriorates with higher initial velocity spread of the electron beam. Efforts to design a cavity or magnetic field profile that minimizes the ACI effects are under way. While ACI affects the depressed collector operation as mentioned above, we recently found experimental evidence of collector depression affecting the cavity interaction itself. The minimum amount of current required to sustain the design mode, TE_22,6 in the hard self excitation regime decreased by as much as 6 A as the collector was depressed by up to 30 kV. This has implications in being able to reach further into the high efficiency hard self excitation regime. The microwave frequency also increased by as much as 15 MHz.