PNNL

Abstract

We investigated the lifecycle of laser-generated air sparks or plasmas using multiple plasma diagnostic tools. Sparks are generated by focusing fundamental radiation from an Nd:YAG laser in air and studies include early and late time spark dynamics, decoupling of shock wave from the plasma core, emission from the spark kernel, cold gas excitation by UV radiation and shock waves produced by air spark, spark’s final decay and turbulence formation. The shadowgraphic and self-emission images showed similar spark morphology at earlier and late times of its lifecycle, however, significant differences are seen in the midlife images. Spectroscopic studies in the visible region showed intense blackbody-type radiation at early times followed by clearly resolved ionic, atomic and molecular emission. The measured spectrum at late times contained emission from both N2+ and CN. Additional spectral features have been identified at late times due to emission from O and N atoms, indicating some degree of molecular dissociation and excitation. Detailed spatial and temporally resolved emission analysis provide insight about various physical mechanisms leading to molecular and atomic emission by air sparks, including spark plasma excitation, heating of cold air by UV radiation emitted by the spark and shock-heating.