Fast-Randomization Phenomena in an Unstable Plasma-Sac Configuration

Results of a detailed experimental study of conditions in and about a plasma sac formed at a constriction in a discharge column are reported. An evolution of phenomena is initiated by the application to the discharge column of a high-current pulse, which in combination with a low steady-state pressure produces a marked depletion of neutral-gas atoms. This depletion causes a high voltage drop to develop across the double sheath which bounds the plasma sac and a resultant high acceleration of the electron gas toward the constriction. The Mach number increases; when it approaches a critical value, of the order of or slightly greater than unity, extremely rapid randomization of the electron velocities ensues. Concurrently, the Mach number is stabilized near unity over a substantial range of increase of the voltage drop which is accelerating the electrons. A tentative explanation is included.