Spectral signature of the beam plasma discharge

Abstract

Spectral measurements of the light emitted from a beam plasma discharge (BPD) in nitrogen show an abrupt increase in the ratio of N2 first positive emissions to the N2+ first negative emissions (red/blue ratio) at the onset of BPD. However, at beam currents substantially above BPD threshold, the red/blue ratio reverts to that produced by the beam itself (pre‐BPD). This is interpreted as the appearance, at BPD onset, of a large suprathermal electron population, followed by a hardening of the suprathermal energy spectrum at higher beam currents. The new results are used to interpret previous “white‐light” video images of BPD experiments. It is concluded that even larger red/blue ratios may occur at lower pressures and that a pre‐BPD glow associated with radiofrequency emissions at the upper hybrid frequency is produced by a softer electron population than that occurring in BPD. In addition to the red/blue ratio, it was found that the ratio of the intensities of the 4–1 band and the 6–3 band of the first positive emissions varies systematically with beam current. The overall behavior is similar to that of the red/blue ratio—an abrupt increase followed by a steady decrease to the pre‐BPD level. But the detailed dependence of the two ratios on current is different.