Ignition of the Beam-Plasma-Discharge and its Dependence on Electron Density

摘要

A cold electron beam, propagating through a weakly ionized plasma will, under proper conditions, produce a modified beam-plasma state known as the Beam-Plasma-Discharge (BDP). As the subject of a continuing series of experiments in a large facility chamber it was previously determined that the BPD had an abrupt ignition threshold as the beam current I sub B was increased at fixed beam energy. While a specific empirical relationship was established among the controlling parameters of beam current, energy and length as well as ambient pressure and magnetic field, a dependence of the BPD on plasma density of the from, omega sub p approximates omega sub c, was suggested. We have since conducted a survey of various beam-plasma conditions covering beam currents from 8 to 85 ma, beam energies from 0.8 to 2.0 keV and magnetic fields at 0.9 and 1.5 gauss. This survey includes full determinations of radial profiles of electron density for each of the selected conditions extending from a low-density pre-BDP state to a strong BPD condition. At BPD threshold N sub e max was determined and omega sub p calculated as the density dependent threshold condition for BPD. The experimental results are shown to compare favorably with a developing theoretical model that considers BPD to be triggered by electron plasma wave excitation of a beam-plasma instability. (Author)