Temporal and spatial diagnosis of nanosecond laser ablation and laser induced plasma

摘要

We present a versatile method to diagnose method to diagnose nanosecond laser induced plasma (LIP) plume with good temporal (10 ns here) and spatial (here sub-millimeter) resolution, without requiring the assumption of local thermodynamic equilibrium (LTE). The spatially resolved emission images from plasmas formed by 532 nm laser ablation of a silicon target in vacuum (10~(-7) mbar) with incident irradiance of 21 GW/cm~2 were recorded at different time delays using a time-gated iCCD camera attached to a spectrograph and image optics. The spectroscopic emission lines associated with different charged species are assigned in the NIST Atomic Spectra Database. The further analysis of Stark broadened line shapes of those emission images allows tracking the plume dynamics and provides insight into the early time (i.e. within several tens of nanoseconds) mechanism of laser-target interaction and the subsequent laser-plasma coupling. The electron density (N_e) and temperature (T_e) values and their variations with space and time are obtained from best-fitting model to the observed line shapes based on a non-LTE electron energy distribution function (EEDF) rather than a Maxwellian EEDF. The value of N_e and T_e respectively declined from 10~(23) to 10~(21) m~(-3) and 10 to 0.1 eV since the plume expansion. The time-gated emission images and the spatial and temporal variation of the N_e and T_e values both highlight the inhomogeneity of the LIP plume, and provide the future analysis and possible derivation of the electron emitting model from target surface after laser-lattice interaction within sub-nanosecond.