Microwave emission from a REB propagating in air, 10−5−760 Torr

Our analysis of microwave emissions from a Y = 2.75, 128 kA, ∼100 ns, annular REB propagating through air of pressure 10⁻⁵ − 2 Torr supports a radiative model of A. High-power, broadband (at least 6 – 40 GHz) reflexing radiation of the beam at low pressures (P < ∼20 mTorr), with transition to B. Rapidly-evolving plasma radiation at moderate pressures (20 mTorr < P < ∼Torr), with peak powers that diminish linearly with pressure as electron-neutral collisions damp radiative plasma instabilities. The data suggest that radiation arises from two possible processes: 1. Radiation from electrostatic waves scattered off of ion inhomogeneities, and 2. Radiation from inverse-Compton-boosted electrostatic waves impacted by bunched relativistic beam electrons.