Radial neutral gas temperature and density profiles in low-pressure argon discharges

Abstract

In this paper we describe the direct measurement of the neutral gas density and the determination of the neutral temperature profiles in a low‐pressure weakly ionized argon discharge. The measurements were made using a single‐wavelength coupled cavity laser interferometer to study the neutral density variations in the afterglow of a crowbarred dc (steady‐state) discharge. The temporal and radial variations of the neutral gas density in the afterglow were used to infer the dc steady‐state gas density and temperature profiles, assuming an ideal gas. It is shown that the profiles are strongly current and pressure dependent and that the axial depletion of neutrals can be as high as 25–30%. Long neutral relaxation times (2–3 msec) were encountered in order to reach thermal equilibrium in the afterglow for the pressures and currents studied (1–10 Torr, 50–350 mA). Radial profiles of the neutral gas density for a fixed pressure and current are in agreement with the hypothesis that neutral heating is caused primarily by collisions with the electron gas, resulting in a convective flow of the hot neutrals radially outward towards the cooler walls.