Theory of Continuum Flush Probes

Abstract

Methods are discussed for determining both the density gradient at the surface and the free‐stream density of charged particles in a weakly ionized flowing gas by means of flush electrostatic probes. It is shown that the values of the density gradients of the ions and electrons at the surface can be determined by current‐voltage characteristics. The determination of the free‐stream charged‐particle density is studied by means of an analysis in the sheath region which, for highly negative probe potentials, gives sufficient boundary conditions to, in general, overspecify the density solution in the quasineutral region. This overspecification makes possible the determination of the boundary of the quasineutral region for a large class of continuum model equations describing the region. Using this information, current‐voltage characteristics can be drawn for various values of free‐stream charged‐particle density, and the density thus determined by comparison with experiment. These characteristics are drawn for two models.