Local Thermodynamic Equilibrium Of Laser-induced Plasma Based On Emission Spectroscopy Technology

Laser-induced breakdown spectroscopy（LIBS）,as a new atomic emission spectroscopy technique,has been widely used in many fields.However,the quantitative analysis of LIBS strongly relies on the local thermodynamic equilibrium（LTE）assumption,and plasma deviation from LTE will significantly reduce the accuracy of the quantitative results.This paper evaluated the temporal and spatial dependence of LTE of plasma generated by laser ablation in vacuum.The main research contents include:1.An emission spectrum analysis method for evaluating the thermodynamic state of laser-induced plasmas is presented.The method is based on the universal electron energy distribution function（EEDF）,which describes the relationship between plasma emission spectral intensity and the upper energy level of the corresponding spectral line transition.The deviation from LTE of plasma can be characterized by the degree to which the theoretical model deviates from the Maxwellian distribution.2.A spectral diagnostic system with high resolution spatial and temporal detection capability was established to observe the space-time evolution characteristics of plasma thermodynamic states.Moreover,a spectral analysis program suitable for this experimental system is designed to measure the spectral line intensity and find the best fitting parameters to characterize the thermodynamic states of plasma.3.An experimental study of 532 nm nanosecond pulsed laser and 532 nm picosecond pulsed laser ablation of silicon targets at 10^-3 Pa was respectively carried out to investigate the evolution of LTE boundary under different laser pulse widths.It is found that the plasma generated by nanosecond laser ablation meets LTE in the region close to the target surface（z<1.5 mm）at t_d<120 ns and away from the target surface（z>7 mm）at t_d>240 ns.However,for plasma produced by picosecond laser ablation,the LTE condition is satisfied only in the region far from the target surface（z>4 mm）at t_d>180 ns.Through the above research,we can better understand the physical mechanism of plasma thermodynamic state evolution to LTE,which is conducive to improving the quantitative analysis accuracy of LIBS technology.