Split-cavity monotrons achieving 40 percent electronic efficiency

The simplest of the microwaves tubes, the monotron consists of a circular cylindrical cavity driven by a linear electron beam. Without needing an externally applied magnetic field, and operating in circular TM_0n0 modes, the device yields the maximal electronic efficiency of 20.0%. However, an electronic efficiency of 40% can be achieved by two monotrons in tandem as this paper demonstrates. This is accomplished by using, as in RF linacs and split-cavity oscillators, a cylindrical cavity bisected by a conducting grid with RF fields oscillating p radians out of phase in the twin partitions. Specifically discussed are the design and operating characteristics of split-cavity monotrons driven by 10 keV, 50-70-A current beams. Dictated by the injection beam energy, the optimum cavity aspect ratio of length_radius=0.37 is central to achieving single p-mode operation while suppressing competing TM_0n0 modes. Rooted in one-dimensional analysis, a 9.2-GHz split-cavity monotron is synthesized with the cavity capacitively coupled to an output TM₀₁ waveguide for external utilization of the RF power internally generated at 40.0% conversion efficiency. Numerical particle-in-cell simulation with a 0.4-cm thick, 70-A current beam crossing a transparent grid gives the overall efficiency of 35.7% relative to 700-kW input beam power.