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<sup>-7</sup> mbar) with incident irradiance of 21 GW/cm<sup>2</sup> 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<sub>e</sub>) and temperature (T<sub>e</sub>) 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<sub>e</sub> and T<sub>e</sub> respectively declined from 10<sup>23</sup> to 10<sup>21</sup> m<sup>-3</sup> and 10 to 0.1 eV since the plume expansion. The time-gated emission images and the spatial and temporal variation of the N<sub>e</sub> and T<sub>e</sub> 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.
The irradiation effects of LD laser on thin aluminum alloy plates are studied in experiments characterized by relatively large laser spot and the presence of 0.3Ma surface airflow. A high speed profilometer is used to record the profile change along a vertical line in the rear surface of the target, and the history of the displacement along the direction of thickness of the central point at the rear surface is obtained. The results are compared with those without airflow and those by C. D. Boley. We think that it is the temperature rise difference along the direction of thickness instead of the pressure difference caused by the airflow that makes the thin target bulge into the incoming beam, no matter whether the airflow is blown or not, and that only when the thin aluminum target is heated thus softened enough by the laser irradiation, can the aerodynamic force by the surface airflow cause non-ignorable localized plastic deformation and result a burn-through without melting in the target. However, though the target isn’t softened enough in terms of the pressure difference, it might have experienced notable deformation as it is heated from room temperature to several hundred degree centigrade.
The lethality effect of high power laser on target is simulated with CFD method under different conditions of supersonic air flow on the surface of the target. Materials used in the experiments are 2cm aluminum plate. With the Mach number changing from 1 to 5, the lethality effects of the high power laser can be obtained from the simulations under these conditions of supersonic air flow. The flow-structure-laser coupling impact on the failure time of the target is discussed based on the simulation. Results show that with the increase of mach number, the effect on the aluminum plate is increase first and then decrease by the pressure. Because that it is obvious that the maximum area of pressure is away from the center of deformation region when the mach number is bigger than 5 . At the same time, when mach number is increase, the aerodynamic heating play more important role than the convective heat transfer on the temperature field of aluminum plate. there are two impacts from the supersonic flow. Firstly , the flow can produce the pressure on the surface of the aluminum plate. Secondly, the flow can produce aerodynamic heat on the aluminum plate.