Ion-acoustic disturbances stimulated by truncated source distributions

Abstract

Plasma disturbances resulting from a modulated non-Maxwellian ion distribution emanating into a half-space plasma are discussed. The non-Maxwellian ion distribution is formed experimentally by truncating the distribution entering an energy analyzer situated at one end of a plasma column. The lower energy ions are modulated and reflected back into the plasma. When all the ions are absorbed by the analyzer, no propagating ion disturbance is observed. On reflecting some low energy ions, an ion-acoustic wave is detected. The wave amplitude initially increases when a larger segment of the ion distribution is reflected. However, when ions with velocities comparable to the phase velocity of the wave are reflected, the wave amplitude decreases. The signal velocity of the wave is also observed to increase as a greater number of ions are reflected. A modified fluid analysis to explain the observations is presented. In the analysis care is taken to include correct nonsymmetric velocity-space source functions derived from a ballistic model.