Laser Induced Fluorescence Combined with Laser Absorption Spectroscopy for Optically Thick Plasma Diagnostics

Abstract

It is difficult to apply Laser Induced Fluorescence (LIF) method for optically thick plasma because its fluorescence profile is distorted by re-absorption. In this study, LIF was combined with Laser Absorption Spectroscopy (LAS) to measure spatially resolved translational temperature in an optically thick argon plume. By utilizing absorption data of a probe laser beam through the plume, distortion of LIF profile was corrected, and accurate translational temperature was obtained. Although translational temperature without correction was 2400 ± 300K, corrected translational temperature was estimated at 1300 ± 200 K. This crucial difference indicates that the effect of re-absorption must be in consideration in optically thick plasma. Nomenclature= probe laser diameter f c = correction factor I = laser intensity I 0 = initial laser intensity I t = transmitted laser intensity I s = saturation laser intensity K = integrated absorption coefficient Ks = integrated laser absorption coefficientof absorbance M A = atomic mass Q = quenching rate R plasma = radius of plume S f = measured fluorescence intensity S' f = corrected fluorescence intensity 2 T = translational temperature x = coordinate along the laser path y = coordinate along the line of sight  = invariable coefficient Ω = solid angle  = relative laser frequency  0 = laser frequency  D = Doppler width