Modeling and experimental verification of plasmas induced by high-power nanosecond laser-aluminum interactions in air

It has been generally believed in literature that in nanosecond laser ablation, the condensed substrate phase contributes mass to the plasma plume through surface evaporation across the sharp interface between the condensed phase and the vapor or plasma phase. However, this will not be true when laser intensity is sufficiently high. In this case, the target temperature can be greater than the critical temperature, so that the sharp interface between the condensed and gaseous phases disappears and is smeared into a macroscopic transition layer. The substrate should contribute mass to the plasma region mainly through hydrodynamic expansion instead of surface evaporation. Based on this physical mechanism, a numerical model has been developed by solving the one-dimensional hydrodynamic equations over the entire physical domain supplemented by wide-range equations of state. It has been found that model predictions have good agreements with experimental measurement for plasma front location, temperature, and electron number density. This has provided further evidence (at least in the indirect sense), besides the above theoretical analysis, that for nanosecond laser metal ablation in air at sufficiently high intensity, the dominant physical mechanism for mass transfer from the condensed phase to the plasma plume is hydrodynamic expansion instead of surface evaporation. The developed and verified numerical model provides useful means for the investigation of nanosecond laser-induced plasma at high intensities.

Figure 1

Schematic diagram of the model setup.

Figure 2

(Color online) Experimental system for high-speed photography.

Figure 3

(Color online) Experimental system for emission spectroscopy.

Figure 4

(Color online) Images of plasma plume produced during laser ablation of aluminum in air at $8.1\mathrm{GW}∕{\mathrm{cm}}^2$.

Figure 5

Transient front location for plasma induced by laser ablation of aluminum in air at $5\mathrm{GW}∕{\mathrm{cm}}^2$.

Figure 6

Transient front location for plasma induced by laser ablation of aluminum in air at $8.1\mathrm{GW}∕{\mathrm{cm}}^2$.

Figure 7

Transient temperature of plasma induced by laser ablation of aluminum in air at $3.8\mathrm{GW}∕{\mathrm{cm}}^2$.

Figure 8

Transient electron temperature for plasma induced by laser ablation of aluminum in air at $8\mathrm{GW}∕{\mathrm{cm}}^2$.

Figure 9

Transient electron number density for plasma induced by laser ablation of aluminum in air at $3.8\mathrm{GW}∕{\mathrm{cm}}^2$.

Figure 10

Transient electron number density for plasma induced by laser ablation of aluminum in air at $8\mathrm{GW}∕{\mathrm{cm}}^2$.

Figure 11

The ratio of plasma spectral radiation energy density over blackbody radiation energy density $U_v∕U_{bv}$ as a function of frequency $v$ at $t=8\mathrm{ns}$ and $z=40\mathrm{\mu }\mathrm{m}$ (laser power density: $8.1\mathrm{GW}∕{\mathrm{cm}}^2$).