Initial operation of an X-band magnicon amplifier experiment

We present a progress report on a program to develop a high-power second harmonic magnicon amplifier at 11.4 GHz for linear accelerator applications. The experiments are being carried out on the NRL Long-Pulse Accelerator Facility using a plasma cathode to create the electron beam. Typical beam parameters are 500-700 kV, 170-250 A, 5.5 mm diameter, with a ~300 nsec voltage flattop. The accelerator operates single pulse with a ~10^-2 Hz repetition rate. A complete five cavity magnicon circuit was designed via computer simulation, fabricated, and cold tested. In early tests, a low power 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. Following a major effort to improve the vacuum and surface conditions, recent experiments have shown that it is possible to “burn through” this low power saturation effect, and achieve high fields in the 5.56 GHz penultimate cavity when the drive cavity is excited by a 10 kW input signal. Synchronous with the penultimate cavity signal, a 100-200 nsec multi-MW frequency-doubled output pulse is observed at 11.12 GHz