Direct Laser Ablation Of Silicon as A Function Of Pulse Length at 1015 W/Cm2 Intensities

Laser ablation of a material progresses over nanosecond timescales to form a rapidly expanding plasma and a consequent thermomechanical shock with the ablation pressure depends on the laser intensity and wavelength [1] . Experiments are conducted at Omega-EP with a fixed laser intensity of 10 15 W/cm 2 and varying pulse durations from 100 ps to 10 ns to investigate laser energy coupling. Radiation-Hydrodynamics (RHD) simulations are leveraged to generate plasma density profiles that are processed for direct benchmarking with experimental 4ω probing of the coronal plasma blow-off. For pulses greater than 500 ps, the measured ablation front temperature is ~500 eV, and is associated with a hot dense layer immediately at the target surface. Interestingly, the measured shock velocity (in the quartz witness layer) decreases for shorter pulse lengths: ~35 km/s for 10 ns, ~22 km/s at for 1 ns, and <7 km/s for 100 ps -suggesting considerable shock decay. Such effects are investigated using RHD to reveal that there exists a critical time needed for the ablation pressure to accumulate up to ~30 Mbar near the target surface and that the decay rate of the shock pulse is proportional to twice the laser pulse length, demonstrating the crucial role of pulse length on shock generation.