Radio and Plasma Wave Generation by an Electron Beam in a Laboratory Plasma

Abstract:

The generation of waves by electron beams is a fundamental problem highly relevant to active experiments in space. For example, compact high-energy electron beam sources may be used on future spacecraft to map magnetic field lines in the Earth's magnetosphere (provided the beam is not disrupted by wave generation) or to generate waves for radiation belt remediation. We present results from a 20 keV beam experiment on the Large Plasma Device (LAPD) at UCLA conducted in support of active experiments and aimed at addressing the wave generation mechanisms, wave properties, and the dependence on plasma parameters. In these laboratory experiments, electromagnetic emission between the plasma and upper hybrid frequencies is observed by both in-situ probes and by an antenna outside of the plasma. The parallel phase speed of the excited waves is measured to be consistent with generation via a resonance process, while estimates of the radiated power are consistent with incoherent Cherenkov emission. Kinetic modeling suggests that the apparent absence of strong instabilities is due to velocity dispersion imposed by the beam injection conditions. Signatures of nonlinear interactions between fluctuations above the plasma frequency and observed whistler modes are also observed. Future experiments will extend our parameter space to a ~1 MeV electron beam.