Wave phenomena preceding and during a beam‐plasma discharge

Abstract

The waves and associated phenomena which are generated prior to and during the beam‐plasma discharge (BPD) in the waves on magnetized beams and turbulence (WOMBAT) apparatus are described. These range in frequency from below the ion gyrofrequency of a few kilohertz to strong harmonics of the electron plasma frequency above 1 GHz. The low‐frequency ion waves appear to be driven by E × B forces arising from the nonneutrality of the electron beam‐plasma column at low beam currents (Ib) or low gas pressures (P). As Ib increases, the motion becomes chaotic after a sequence of wave period doublings resulting in the generation of subharmonies. Windows of coherent motion are observed within the chaotic regions. Further increases in Ib lead to increases in ionizing collisions and when the background plasma density becomes sufficiently high, waves around the electron plasma frequency can be generated. Initially, these waves are modulated by the gross density perturbations of the large‐amplitude ion waves, but subsequently they also increase to large amplitudes: eϕ/kT ∼ 1. This is sufficient to accelerate the background electrons to ionizing energies and produce the BPD. At low pressures this occurs for fpe/fce ∼ 0.5 (BPD1) with the process occurring in short bursts (≲100τpe). Increasing Ib produces a sudden jump in the frequency to ∼1.5fce and a further increase in the plasma density and luminosity, which we term BPD2. A sequence of BPDs then occurs as Ib is further increased with characteristic frequencies between gyroharmonics up to fpe ≤ 6fce.