Flush probe studies of plasma flow over a flat plate
Date
1978
Authors
Giles, Clinton Randy
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
A method of utilizing a planar geometry probe to measure flow properties of a plasma has been developed. No previous work exists in the literature on using a Langmuir probe to examine the viscous boundary layer around the probe. The technique involves applying a step voltage to a flat plate probe in a flowing continuum plasma and observing the relaxation of the ion current to flush probes mounted in the plate. The time it takes to relax to a new equilibrium ion current equals the flow time along the ion sheath edge from the flat plate's leading edge to the flush probe.
A simplified theory was developed to predict the relaxation phenomenon for the flat plate ion current when the ion sheath edge was outside the viscous boundary layer. The purpose of this theory was primarily to explain the ion current signal which was used in the relaxation measurements. The success of the model was evidenced by agreement (within 25%) between theory and experiment when measuring the amplitude of the ion current spike that resulted from the voltage step.
A rigorous theory proposed in this thesis, gave numerical results for t he ion heath thickness as a function of plate voltage when the ion sheath edge is inside the hydrodynamic boundary layer. This theory was based on Stahl and Su's technique of matching the quasineutral solution to the ion sheath solution of the equations governing the plasma behaviour around the flat plate. There is no earlier model for the ion sheath completely inside the viscous boundary layer when ion convection is an important transport mechanism. This new model allowed the step voltage method to be used in measuring flow times in the viscous boundary layer which were then compared to an approximate fluid model calculated using Von Karman's momentum integral. Agreement to within 10% between the flush probe relaxation times and the fluid approximation was observed. It is felt that this new method of plasma flow measurements is amenable to further development as a general diagnostic technique in flowing plasma studies.