Experimental Study of the Propagation of an Ionizing Wave in a Coaxial Plasma Gun

Abstract

A coaxial plasma gun experiment is described. The gun has an azimuthal bias magnetic field, which is strong compared to the field from the discharge current. The discharge voltage is shown to depend linearly on the bias field, thus defining a velocity that is found to be almost independent of the pressure and the discharge current. This velocity is close to the ``critical velocity'' which has been found in rotating-plasma experiments. The velocity of the current layer is also measured; it is always smaller than or equal to the critical velocity and it decreases with increasing pressure and decreasing magnetic field.

Critical voltage of a rotating plasma

“…It was postulated to take place as soon as the relative velocity between the clouds approaches the critical value v c -(2 e <Z>i/rai)i, where 0i is the ionization potential and mi the ion mass. This suggestion has been supported by a number of experiments, both with homopolar devices [2,3,4] and with coaxial plasma guns [5,6]. Within a wide range of the experimental parameters, the observed plasma voltage was limited to a value close to that calculated from v c .…”

“…In these equations n denotes the particle density, v is the plasma velocity, E is the electric field, i is the current density, m = mi-\-m e , p=Pi+Pe, Pi and p e are the ion and electron pressures, rj = m c vdle 2 n is the resistivity, and v t \ is the electron-ion collision frequency. In a cylindrical frame of reference (r, cp, z), the momentum losses are represented by the force F = -[A* V x V x v -w m m V,, = F,, + F L (5) where…”

Simulation of geophysical phenomena in the laboratory

“…(8). More recent experiments by Wilcox et al, 89 designed to simultaneously study ionization processes and the dynamics of fluid flow in magnetic fields, have been carried out in a linear geometry avoiding the complicating centrifugal forces of the homopolar devices. Again, a limiting velocity near to the critical velocity that is virtually independent of the pressure and discharge current is found.…”