<title>Pulse shortening in high-power backward wave oscillators</title>

Abstract

Pulse shortening is a phenomenon common to all high-power microwave devices. Whereas in electron beam driven sources the electron beam propagation in the device may be for several microseconds or more, the microwave pulse duration is typically no greater than approximately 100 ns. Specific reasons for pulse shortening may vary among devices, but all explanation of the phenomenon put forth involve the introduction of plasma into the interaction region near the walls and/or the degradation of the beam quality. To gain a better understanding of pulse shortening in high power backward wave oscillators (BWO's), an investigation is being conducted at the University of New Mexico (UNM) on the UNM Long-Pulse BWO Experiment. Recent experiments have involved monitoring the beam current in the slow-wave structure (SWS) at different radii as a function of time. The current waveforms are correlated with the time histories of microwave pulses measured in separate experiments. The results reveal the appearance of electrons between SWS ripples at times corresponding to when the microwave signal peaks. A drop in the main beam current is observed shortly thereafter. Coatings of TiO 2 and Cr have been placed on the inner surface of the SWS in an effort to suppress electron emission. Initial results with the TiO 2 coatings have shown a measurable increase in microwave pulse width.