Spatially and temporally resolved evaluation of local thermodynamic equilibrium for laser-induced plasma in a high vacuum

An analytical method to validate local thermodynamic equilibrium (LTE) in laser-induced plasmas is reported in this article. A more universal and general than Maxwellian electron energy distribution function (EEDF) is used to describe the emission intensity as a function of plasma temperature and transition levels. Spatially and temporally resolved spectra recorded in laser ablation experiments at a pressure of 10 −5 mbar provide access to the insight into the actual EEDF of electrons at individual positions in the local plasma plume, and the evolution of the thermodynamic state in plasma is therefore estimated by using the EEDF departure from the Maxwellian distribution. The consistency of similar EEDFs derived from emission lines associated with differently charged ions at the same position and time delay justifies the method as a reasonable LTE criterion. This figure evaluates the spatial and temporal dependence of LTE deviation. ( v = 1, satisfies LTE; v > 1, departure from LTE.)