Wall-induced inhomogeneities in the low-pressure Hg-Ar positive column

Radial variations of the Hg(63P/sub 0,1,2/) densities are measured in a low-pressure Hg-Ar discharge and the results are compared with a model. The 34-mm-diam discharge tube is operated in the vicinity of 400-mA current, 6.4-mTorr Hg pressure, and 2.81-Torr Ar pressure. A tunable dye laser beam passes axially through the discharge tube, and densities are measured using both a saturated absorption technique and an interference technique. The model incorporates continuity equations for Hg excited and ionic states, and a two-temperature approach to parametrizing the electron energy distribution function. The shapes of the radial density distributions of the various species are predicted rather than assumed. Considerable attention is given to the role of tail electrons above the energy threshold for Hg inelastic scattering. Theoretical considerations and the measured variations of the 63P0 and 63P2 metastable densities suggest that the tail electron temperature is lower near the wall than at the center of the discharge