Laboratory simulation of the injection of energetic electron beams into the ionosphere-Ignition of the beam plasma discharge

Experiments, which somewhat simulate the injection of monoenergetic (several keV) electron beams into the ionosphere, have been performed in the very large (17 m × 26 m) vacuum chamber at Johnson Space Center. Typical operating ranges were: Beam current, I (0-130 mA), beam energy, E (0.5-3 kV), magnetic field, (0.3-2 G), path length, L (10-20 m), and injection pitch angle, α(0-80°). Measurements were carried out in both steady state and pulsed modes. In steady state and for constant V, B, p, L, α, the beam plasma discharge (BPD) is abruptly ignited when the beam current is increased above a critical value; at currents below critical, the beam configuration appears grossly consistent with single particle behavior. If it is assumed that each of the experiment parameters can be varied independently, the critical current required for ignition obeys the empirical relationship at p < 2 × 10^-5 torr: I{A figure is presented} E^3/2 B^0.7pL. The BPD is characterized by 1) a large increase in the plasma production rate manifested in corresponding increases in the 3914 Å light intensity and plasma density, 2) intense wave emissions in a broad band centered at the plasma frequency and a second band extending from a few kHz up to the electron cyclotron frequency, 3) scattering of the beam in velocity space and 4) radial expansion and pitch angle scattering of the primary beam leading to the disappearance of single particle trajectory features. Measurements of the BPD critical current have been carried out with an ion thruster (Kaufman engine) to provide a background plasma, and these indicate that the presence of an ambient plasma of typical ionospheric densities has little effect on the critical current relation. Measurements of wave amplitudes over a large frequency range show that the amplitude of waves near the plasma and electron cyclotron frequencies are too small to cause or sustain BPD, and that the important instabilities are at much lower frequency (∼ 3 kHz in these measurements).