Recent Development in Diagnostic of Electronegative Plasmas Through Laser Pulse Induced Photo-Detachment

Abstract

Laser pulse induced photo-detachment is an indirect measurement technique devoted for the estimation of plasma electronegativity, i.e. ratio of negative ion density in proportion to electron density. The technique combines the use of a laser pulse and a positively biased Langmuir probe. The laser pulse converts negative ion into electron-atom pair, while the Langmuir probe records the increase of electron saturation current due to the electron density increase induced by negative ion photo-detachment. The negative ion proportion is thereafter deduced from the recorded laser photo-detachment signal which is nothing but the temporal evolution of probe current measured from the instant when the laser pulse is fired until the probe current recovers to its nominal value (prior the laser pulse). As in any indirect measurement technique, one needs a model describing the relationship between observables (electron saturation current) and investigated quantities (electronegative plasma properties). Such model relies on a set of assumptions that are subject of discussion in the present review.