Invited paper Observation of plasma shielding by measuring transmitted and reflected laser pulse temporal profiles

Abstract.

Mass ablation rate increases with laser power density following a power law dependence and a significant change occurs at 0.3 GW/cm$^2$. A reflected 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 GW/cm$^2$, the later part of laser pulse becomes truncated. The power density at which the laser temporal profile changes for each case is same 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 absorption 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 caused by plasma shielding.