OBSERVATION OF PLASMA SHIELDING BY MEASURING TRANSMITTED AND REFLECTED LASER-PULSE TEMPORAL PROFILES

Mass ablation rate increases with laser power density following a powe r law dependence and a significant change occurs at 0.3 GW/cm(2). A re flected laser temporal profile was measured from a brass sample. When the power density is greater than 0.3 GW/cm(2)+, the temporal profile changes. The transmitted laser-pulse temporal profile through a glass sample also was measured. When the power density is greater than 0.3 G W/cm(2), the later part of laser pulse becomes truncated. The power de nsity at which the laser temporal profile changes for each case is sam e as the power density that the mass ablation rate coefficient changes . The ablated mass can absorb incoming laser radiation through inverse Bremsstrahlung. The mass becomes thermally ionized and opaque to the incident radiation, preventing laser light from reaching the surface. A model based on thermal evaporation and inverse Bremsstrahlung absorp tion was developed. Calculations show that plasma shielding occurs at approximately 0.3 GW/cm(2). The experiments and model suggest that the significant change observed in mass ablation rate coefficient is caus ed by plasma shielding.