Drift Instability of a Non-Maxwellian Plasma

The authors investigate experimentally the instability of a spatially inhomogeneous plasma that incorporates beams of charged particles (electrons). This so-called drift-beam instability is based on the,same physical mechanism that causes the drift (''universal'') instability of a collisionless plasma with a Maxwellian electron velocity distribution. However, the drift-beam instability possesses a number of peculiarities that permit one to investigate the drift instability mechanism more effectively than is possible in the case of a Maxwellian plasma. According to the theory, the main differences between drift-beam instability and normal drift instability are: (1) The instability begins at some threshold (critical) beam density (in the case of a Maxwellian plasma there is no particle density threshold); (2) The critical beam density (n1c) is very strongly dependent on the electron velocity and beam geometry, the strength of the magnetic field, and the ratio of the electron density of the cold plasma to the beam density; (3) The nature of the dependence of n1c on the fundamental parameters of the system is determined by the spatial structure of the electron-ion oscillations under consideration. In particular, if the oscillations possess axial symmetry, then the dependence in question is the same as for ''normal'' (non-drift) beam instability. The above-mentioned peculiarities of drift-beam instability were the prerequisites for model experiments carried out to obtain information about the mechanism of drift instability. In these experiments the authors studied the excitation conditions (thresholds), dispersion properties and spatial structure of the drift-beam instability. They found good agreement with the fundamental propositions of the theory. It was shown experimentally that the electron-ion drift-beam instability expresses itself in phenomena such as the interruption of the beam current in the plasma and the heating of the plasma ions to energies of the same order as those of the beam electrons. The latter phenomenon was used successfully in filling magnetic mirror traps with a plasma having an ion temperature of about 1 keV and a density of about 1011 cm-3. (author)