3-DIMENSIONAL DENSITY PROFILES OF ARGON METASTABLE ATOMS IN A DIRECT-CURRENT GLOW-DISCHARGE - EXPERIMENTAL-STUDY AND COMPARISON WITH CALCULATIONS

Three-dimensional density profiles of argon metastable atoms (Ar-m) h ave been measured by laser-induced fluorescence in a direct current gl ow discharge with flat cathode, for a range of voltages, pressures and currents (i.e. 700-1200 V, 0.7-1.6 torr, 0.6-3.3 mA). The primary exc itation line was taken to be the 794.818 nm line, whereas the 852.144 nm line was used as the fluorescence line. The profile is characterize d by two distinct peaks, at 2-4 mm and about 12 mm from the cathode re spectively. These peaks are explained as being caused by local Ar-m p roduction and loss processes, giving rise to local maxima which are no t completely spread out by diffusion. The experimental data have been compared with the results of a mathematical model consisting of a bala nce equation with many production and loss terms. The theoretical prof ile also shows two peaks, but at somewhat different positions, and the first peak is much more intense. This suggests that the model is not yet able to describe the behavior of metastable atoms exactly, and tha t the glow discharge is hence more complex than often assumed. Neverth eless, comparison of the overall Ar-m number density in the rest of t he discharge volume indicates to us that a generally reasonable agreem ent has been reached between experiment and theory. (C) 1997 Elsevier Science B.V.