Recent results from a long pulse, relativistic vacuum and plasma-filled backward wave oscillator experiment

The University of New Mexico (UNM) long pulse backward wave oscillator (BWO) experiment has been investigating three issues: (i) the observation of axial mode switching during the course of microwave generation in vacuum, (ii) the use of laser interferometry to correlate the evolution of wall plasma with pulse shortening during operation in vacuum, and (iii) the effects of a controlled plasma prefill from a cathode-mounted plasma source on BWO output characteristics. Results-to-date from our investigations indicate that a cross-excitation instability is observed under certain operating conditions during vacuum operation. This instability depends on three parameters: (i) the normalized slow wave structure length, (ii) the ratio of electron beam current to start-oscillation current, and (iii) the reflection coefficient at the downstream end of the electrodynamic system. Furthermore, a HeNe laser interferometer indicates that plasma appears in two phases during the course of vacuum operation. The initial low density phase I plasma is attributed to beam scrape-off from the cutoff neck region at the input to the electrodynamic system. A significantly higher phase II plasma is measured after the occurrence of pulse shortening, and the magnitude of this plasma is correlated with the radiated microwave power level. We believe this plasma is attributed to a catastrophic discharge occurring during very high power excitation. Finally, the intentional prefill of the slow wave structure with a preionized plasma emanating from the cathode is found to both enhance microwave generation efficiency, and quench the radiated power, depending on the density of the plasma prefill.