Space Charge Instabilities in Synthesized Plasmas

Abstract

A theoretical and experimental investigation of instability phenomena in synthesized plasmas is carried out. In the theoretical analysis the space charge and potential distributions are obtained for an idealized one-dimensional model. The model consists of two face-to-face electrodes each emitting ions and electrons in any ratio and with a Maxwellian velocity distribution. The calculations show that no stable zero-field solution is possible; instead, distributions with either a potential maximum or minimum in the center are obtained. For a range of values of the ratio β of injected ion to electron space-charge density near unity (0.81 ≤β<1.235), double-valued solutions to the problem are obtained. The actual solution into which the system settles depends on whether this range of β values is approached from above or below. Transitions from one such state to the other can occur at the limits of this β range. In the experimental investigation of a synthesized plasma, these instabilities, or state-to-state transitions, were found to be in good agreement with the theory. Oscillations triggered by state-to-state transitions were observed with a frequency corresponding to the ion transit time through the interelectrode space. These oscillations are not described by the steady-state analysis.