Beam--plasma instabilities and the beam--plasma discharge
Using a new electron gun, a number of measurements bearing on the generation of beam--plasma discharge (BPD) in WOMBAT (waves on magnetized beams and turbulence) (R. W. Boswell and P. J. Kellogg, Geophys. Res. Lett. 10, 565 (1983)) have been made. A beam--plasma discharge is an rf discharge in which the rf fields are provided by instabilities (W. D. Getty and L. D. Smullin, J. Appl. Phys. 34, 3421 (1963)). The new gun has a narrower divergence angle than the old, and comparison of the BPD thresholds for the two guns verifies that the BPD ignition current is proportional to the cross-sectional area of the plasma. The high-frequency instabilities, precursors to the BPD, are identified with the two Trivelpiece--Gould modes (A. W. Trivelpiece and R. W. Gould, J. Appl. Phys. 30, 1784 (1959)). Which frequency appears depends on the neutral pressure. The measured frequencies are not consistent with the simple interpretation of the lower frequency as a Cerenkov resonance with the low-Trivelpiece--Gould mode; it must be a cyclotron resonance. As is generally true in such beam--plasma interaction experiments, strong low-frequency waves appear at currents far below those necessary for BPD ignition. These low-frequency waves are shown to control the onset of the high-frequency precursors to the BPD. A mechanism for this control is suggested, which involves the conversion of a convective instability to an absolute one by trapping of the unstable waves in the density perturbations of the low-frequency waves. This process greatly reduces the current necessary for BPD ignition.
- Research Organization:
- Plasma Research Laboratory, Research School of Physical Sciences, Australian National University, Canberra, A.C.T. Australia
- OSTI ID:
- 5955554
- Journal Information:
- Phys. Fluids; (United States), Vol. 29:5
- Country of Publication:
- United States
- Language:
- English
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