Shock Front Detachment during Pulsed Laser Ablation of Graphite

Pulsed laser ablation of pyrolytic graphite with a 5.7 J/cm2 frequency-doubled Nd:YAG laser in backgrounds of argon, nitrogen, and mixed gas at pressures from 3 to 180 Torr was performed to study the dynamics of the ablation shock wave and plume emissive contact front. White light schlieren shock wave imaging and optical emission imaging with a 2.88–40 ns gated ICCD camera was used to determine shock wave and emissive plume trajectories to find the location of shock detachment from the plume and for blast energy characterization by Sedov-Taylor theory. The shock detachment points are used to limit emissive contact front Sedov-Taylor fits to the portion of the plume which exhibits a shock-like trajectory, resulting in improved laser-plume coupling energy estimates compared to standard fits. The emissive plume expands with initial Mach numbers up to M ~ 54 at t = 62 ns, decreasing to M ~ 7 as the emission becomes too weak to detect after several microseconds. The shock wave expands with initial Mach numbers up to M ~ 55 at t = 62 ns, decreasing to M ~ 1 at t = 20 µs. The shock waves exhibit spherical shock fronts, but the dimensionality, n, decreases as pressure and mass of the background gas increase, while the plumes exhibit an opposite trend. The Sedov-Taylor energy released in the sudden ablation is typically 55–75% of the laser pulse energy. The detachment-limited blast energy calculations for the emissive plume agree to within 3–5% of the shock wave energy values. Shock detachment points are nearer the target at higher pressure and scale with the mean free path