Temporal and Spatial Evolution of a Laser Spark in Air

Abstract

Experiments were conducted to quantify the temperature and electron number density of a laser spark formed in air. The laser spark was created by focusing a 180-mJ beam from the second harmonic (wavelength of 532 nm) of a pulsed Nd:YAG laser with a 100-mm focal length lens. For early times, between 50 ns and 1 μs, images of the spark emission have been obtained to characterize the geometry. Also the temperature and electron number density were measured from the emission spectra between 490 and 520 nm where 46 N II lines are observed. The methodology of fitting the experimental data to the modeled spectra to deduce the temperature and electron number density is described. For the initial temporal range, the temperature peaks at approximately 50,000 K and decays over the first 1 μs. The electron number density peaks at approximately 10 19 cm -3 , decaying only slightly slower than t -1 . For the longer temporal evolution, from 20 to 1000 μs, planar temperature measurements were achieved using filtered Rayleigh scattering. The peak temperature starts at approximately 4100 K at 20 μs and decays to 580 K by 1 ms. The planar temperature images show a center jet propagating in the opposite direction as the initiating laser beam, which induces a vortex ring (or torus) propagating in the direction of the beam. The evolution of the position and radius of the torus structure is described and fit with a power law utilized by previous investigators. The temperature of the plasma created by laser-induced optical breakdown appears to fit quite well using a triple exponential over the four orders of magnitude of temporal range where measurements were conducted.