Recent results from the NRL X-band magnicon amplifier experiment

Summary form only given, as follows. In this paper, we present a progress report on the NRL program to develop a high power second harmonic magnicon amplifier at 11.4 GHz. The experiments are being carried out on the single-shot NRL Long-Pulse Accelerator Facility using a plasma cathode to create the electron beam. Typical beam parameters are 500-700 kV, 200-300 A, 5.5 mm diameter, with a 300 nsec voltage flattop. We have previously reported the results of an initial two-deflection-cavity experiment, and of a simulation-based design study for the complete magnicon circuit. Following the design study, the complete magnicon circuit was fabricated, tuned, and cold-tested. It includes a drive cavity, two gain cavities, and a two-section penultimate cavity, all operating at 5.7 GHz, followed by an output cavity designed to operate in the TM/sub 210/ mode at 11.4 GHz. In preliminary experiments, a low power (/spl sim/1-10 kW) saturation effect was observed in the deflection cavities, apparently due to plasma formation caused by the diode X-ray flux and by inadequate vacuum conditions. Recent experiments have shown that it is possible to "burn through" this low power saturation effect, and achieve high fields in the deflection cavities in narrow gain spikes. These gain spikes lead to rf breakdown of the deflection cavities, typically in a temporal cascade that begins in the penultimate cavity and ends in the first gain cavity. In addition, moderate power level emission (/spl sim/1 MW) is observed from the output cavity. Experiments are in progress to characterize the output radiation, and to improve the overall magnicon performance.