Spatially and temporally resolved thermometry of laser-induced plasmas.

摘要

Laser generated plasmas are widely employed as consistent sources of localized high temperatures and high electron densities. Current applications include Laser Induced Breakdown Spectroscopy, laser ignition of combustible gases, laser fusion, and plasma etching. In my experiments, A 1064 nm, 20 ns pulse from a Nd:YAG laser with pulse energies ranging from 175 mJ to 500 mJ is tightly focused to produce a spark at various, near atmospheric pressures. Spatially resolved Rayleigh scattered light from a time-delayed, 355 nm Nd:YAG laser pulse traversing the spark at right angles is collected with a linear diode array. At a delay time of 30 mus, the laser plasma remnant appears as a nearly spherically symmetric region with a center temperature of about 4500 K.; After around 100 mus, the hot gas starts to change into a toroidal shape expanding radially at an average speed of a few meters per second. The final torus size increases with decreasing pressure and increasing laser power. Also, at about the same time, an axial jet emerges approximately from the toroidal center which is much cooler and which propagates towards the plasma ignition laser. This general behavior of the plasma afterglow can be reconciled with a recently developed numerical model, where in the aftermath of the spark shock wave, a pair of vortices is produced which in turn moves the residual hot gas into the observed toroidal geometry.