An evaluation of equilibrium conditions and temperature-dependent speciation in a laser-produced air plasma

Journal Article · 08 August 2018 · Physics of Plasmas

DOI: https://doi.org/10.1063/1.5041987 · OSTI ID:1491739

Harilal, S. S.  ^[1]; Brumfield, B. E. ^[1]; Phillips, M. C. ^[1]

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1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

A 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 μs and persists for 100s of μs. Here, 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 μs, the molecular temperatures obtained from different species show a discontinuity at times ~ 30 μs 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. Finally, the measured mole fractions from laser-induced air plasma are within a factor of 2 - 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.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA), Office of Defense Nuclear Nonproliferation (NA-20)
• Grant/Contract Number: AC05-76RL01830
• OSTI ID: 1491739
• Report Number(s): PNNL-SA--133663
• Journal Information: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 8 Vol. 25; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English