Electron density measurements during microwave generation in a high power backward-wave oscillator

Laser interferometry is used for the first time to measure plasma electron density along the slow wave structure (SWS) wall during microwave generation in a vacuum, long pulse, high power backward-wave oscillator (BWO). The University of New Mexico long pulse backward-wave oscillator, which displays the characteristic pulse shortening phenomenon, is investigated in these studies. Although pulse shortening is observed across a wide class of high power microwave devices, its origin is not definitively understood. Many hypotheses suggest that the unintentional introduction of plasma into the interaction region near the walls of the SWS is one of several likely causes of pulse shortening in intense electron beam driven slow wave devices. This article presents initial measurements of the line-integrated, temporally resolved plasma density between an intense, relativistic, annular electron beam and SWS walls for a variety of radiated microwave peak power levels. Line-integrated electron densities, ⟨n_eL⟩, between 9.10¹⁵ and 2.410¹⁶ cm^-2 for radiated microwave powers between 20 and 120 MW have been measured. The two main sources of the measured electron density are postulated to be (i) plasma generated from the cutoff neck due to beam scrape off, and (ii) material removed and ionized from the SWS walls during microwave generation