Scaling of 1064 nm laser induced plasmas in air and other phase boundaries

Abstract

Summary form only given. 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 pulse widths 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 4-14 TW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . A 532 nm laser shadowgraphy coupled with high speed and high resolution image capturing diagnostics has been established to investigate spatio-temporal 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 10s. The active plasma lifetime through plasma self-luminescence measurements indicate variations from 200 - 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 atmospheres and 4 eV. We also present the theoretical and experimental measurements and analysis of the optical breakdown threshold for dry air by 1064 nm infrared laser radiation and the significance of the multiphoton and collisional cascade ionization process on the breakdown threshold measurements over pressures range from 10 Torr to 2000 Torr. Comparative analysis of the laser air breakdown results at 1064 nm with corresponding results of a shorter laser wavelength (193 nm) and a longer microwave wavelength (108 nm). A universal scaling analysis of the breakdown threshold measurements provided a direct comparison of breakdown threshold values over a wide range of frequencies ranging from microwave to ultraviolet frequencies.