Beam-Generated Waves in a Large Plasma Chamber

Plasma waves generated in a large vacuum chamber (20 x 30 m) by an electron beam of energy 0.5 to 2 keV, are measured. Both pre-beam plasma discharge (pre-BPD) and beam plasma discharge (BPD) states are investigated. The measured waves fall roughly into three categories: 1) a low-frequency spectrum with f ≪ f_ce, f_p; 2) a whistler mode spectrum; and 3) a high-frequency spectrum. The extremely low frequency waves are shown to be surface waves on a (nonneutral) plasma column but a higher frequency part of the low-frequency spectrum was not thoroughly investigated. It is speculated that it also represents surface waves on a nonneutral column. Its measured properties are consistent with the lower hybrid drift instability. The whistler mode spectrum seems to be a Cerenkov resonance with the lower Trivelpiece-Gould mode. It does not seem to play an important role in modifying the particle distributions. The high frequency spectrum is, as suggested by Rowland et al. [21], a Cerenkov resonance with the upper Trivelpiece-Gould mode. It is responsible for the major perturbation of the beam energy in BPD.