Diode laser absorption measurements of metastable helium in glow discharges

Abstract

A laser-based technique for accurate measurement of the population density profiles of the helium metastable in a parallel-plate, glow discharge is described. The system utilizes a distributed-Bragg-reflector diode laser operating at 1083 nm to perform absorption measurements on the transitions of helium. The narrow bandwidth (<3 MHz) of the laser permits simultaneous measurement of the line-integrated metastable density and gas temperature. Axial and radial distributions of the triplet line-integrated metastable density were measured in normal and abnormal glow discharges. Current densities ranging from (normal glow) to (abnormal glow) were used at static gas pressures of 2 and 5 Torr. Abel inversion of the data provided a spatial distribution of the metastable density that shows a region that has near one-dimensional character. The location of the peak triplet metastable density in the plasma was displaced from the peak of the negative glow emission towards the anode. Axial profiles of triplet metastable density gave values from to for current densities of 8 to , respectively.