PNNL

Abstract

laser-induced air breakdown plasma is a dynamical system and its fundamental parameters change significantly during its lifetime. The temporal evolution of the spectral features from an air plasma shows emission from bremsstrahlung followed by discrete electronic transitions from ions, atoms, and rotational and vibrations bands from molecules. The molecular band emission from air plasma typically appears at times ?5 s and persists for 100’s of s. In this article, an evaluation is made on the temperature evolution and speciation in a laser-produced air plasma. The air plasmas are generated by focusing 1064 nm, 6 ns pulses from an Nd:YAG laser. High- resolution emission spectra of various molecules such as N2, N2+, CN, OH, NH, NO are acquired for fits to spectral models. Fitting of atomic and molecular emission features permits tracking of the air spark temperature evolution from 1 to 200 µs. Though the excitation and molecular temperatures show good overlap at times ? 10 s, the molecular temperatures obtained from different species show a discontinuity at times ~ 30 s and this is related to shock collapse and subsequent changes in hydrodynamics and chemistry of the plume. The fitting of multiple species in broadband spectra has permitted calculation of the relative concentrations of various molecules as a function of temperature that provides insight into the air spark speciation. The measured mole fractions from laser-induced air plasma are within a factor of 2 to 6 of the values estimated by speciation model under the assumption of thermal and chemical equilibrium across the temperature ranges studied and probable reasons for the observed deviation are described.