Plasma Decay of Nanosecond Pulsed Laser-Produced Ar and Ar-H2O Sparks at Atmospheric Pressure

Abstract: Time-resolved diagnostics were applied to investigate free-electron properties in nanosecond laser-produced discharges sustained at atmospheric pressure in Ar and in Ar with 3% H2O. The discharges were generated using 23 ns, 1064 nm laser pulses. Broadband plasma imaging and laser Thomson scattering were combined with optical emission spectroscopy, with particular emphasis on Stark broadening of the Halpha and Hbeta lines. The plasma exhibited a bright emission that persisted for up to 30--40 us after breakdown, followed by a very weak glow lasting up to 19 ms. Peak electron number density of about 2 x 10^17 cm-3 and electron temperature of about 7 eV were measured. Excellent agreement between both techniques was obtained for absolute electron number densities. The inferred temporal decay of free electrons is consistent with processes dominated by ambipolar expansion and two- and three-body electron-ion recombination. These results provide benchmark data for modeling nanosecond laser discharges and demonstrate the reliability of combining Thomson scattering with Stark broadening in atmospheric laser sparks.