Laser shadowgraphy, two-wavelength laser interferometry, schlieren imaging and optical emission spectroscopy diagnostics of laser induced plasmas in different phases and at phase boundaries

摘要

Fast gating and high resolution laser shadowgraphy, two wavelength laser interferometry, Schlieren imaging and optical emission spectroscopy diagnostics were carried out to experimentally investigate pulsed 1064 nm Nd:YAG laser-induced breakdown plasma in air at 760 Torr1 and in liquid (water) and in liquid-metal phase boundaries. 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 micron spot size in air and the resulting laser flux densities range from 4–14 TW/cm2. The detailed experimental arrangements and results will be presented. 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 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. Laser induced plasma breakdown and its resulting shockwave generation and bubble formation in water was also investigated and characterized. The results of these investigations will be presented. Two wavelength laser interferometry based plasma density measurements will be presented. Plasma temperature measurements using optical emission spectroscopy diagnostics will be presented.