Studies of relativistic backward-wave oscillator operation in the cross-excitation regime

We first reported the operation of a relativistic backward-wave oscillator (BWO) in the so-called cross excitation regime in 1998. This instability, w hose general properties were predicted earlier through numerical studies, r esulted from the use of a particularly shallow rippled-wall waveguide [slow wave structure (SWS)] that was installed in an experiment to diagnose puls e shortening in a long-pulse electron beam-driven high-power microwave (HPM ) source, This SWS was necessary to accommodate laser interferometry measur ements along the SWS during the course of microwave generation. Since those early experiments, we have studied this regime in greater detail using two different SWS lengths. We have invoked time-frequency analysis, the smooth ed-pseudo Wigner-Ville distribution in particular, to interpret the heterod yned signals of the radiated power measurements. These recent results are c onsistent with earlier theoretical predictions for the onset and voltage sc aling for this instability. This paper presents data for a relativistic BWO operating in the single-frequency regime for two axial modes, operating in the cross-excitation regime, and discusses the interpretation of the data, as well as the methodology used for its analysis. Although operation in th e cross-excitation regime is typically avoided due to its poorer efficiency , it may prove useful for future HPM effects studies.