Experimental Investigation of 1064-nm IR Laser-Induced Air Plasma Using Optical Laser Shadowgraphy Diagnostics

Experimental measurements and analysis of pulsed 1064-nm Nd:YAG laser-induced air breakdown plasma at 760 torr has been carried out using high-speed and high-resolution laser shadowgraphy and optical diagnostics. Three different experimental laser energies and pulsewidths such as 170 mJ at 8 ns, 130 mJ at 7 ns, and 65 mJ at 12 ns are studied. The laser pulses were focused down to a ~ 7-μm spot size in air and the resulting laser flux densities range from 4 to 14 TW/cm². A 532-nm laser shadowgraphy coupled with high-speed and high-resolution image capturing diagnostics has been established to investigate spatiotemporal evolution and hydrodynamic behavior of the 1064-nm laser-induced plasma and neutral-density shock during the formation, expansion, and collapsing stages. The observed plasma formations were aspherical due to absorption translation during the initial laser-energy coupling. The aspherical feature seeded the hydrodynamic instability leading to the ultimate destabilization of the hot gaseous core after approximately 10 μs. The active plasma lifetime through plasma self-luminescence measurements indicate variations from 200 to 500 ns for the three laser pulses. Shock propagation velocity and plasma volume for three laser pulse series indicate similarly shaped profiles at different expansion velocities. Early plasma expansion velocities of 20 km/s were measured, and using Hugoniot relations, the neutral shock pressures and temperatures were inferred, and the results at the early plasma expansion stage were found to be over 1000 atm and 4 eV.