Reflection of Metastable Atoms by a Glass Wall in a Positive Column Discharge Plasma

Abstract

The leser-induced-fluorescence-spectroscopy (LIFS) method has been applied to a dc discharge plasma, and the radial density distribution of metastable ( 2 ¹S) helium atoms has been measured. It was found that the density did not tend to zero at the discharge tube wall. Rather, the density was about 20-30% of the value on the tube axis. By using the collisional-radiative model, we interpreted the result. As the origin of this finite population at the wall we considered two possibilities, namely, 1 ) the helium ions which recombine at the wall converted partly into the metastable atoms and 2 ) the metastable atoms were not quenched completely at the wall, but were reflected by a certain amount. We found that the first process could not reproduce the experimental population distribution, and that the second process with the reflection coefficient of the metastable atoms of 80-90% accounted for the experiment.