Non-Invasive Diagnostics of Molecular Gas Plasmas with Quantitative Optical Emission Spectroscopy

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This project aims to develop a new non-invasive and easily-implemented optical diagnostic of an ionized gas, a plasma, for the particular class of low-pressure molecular gas plasmas. Industrial processes involving plasmas contribute to a long list of societal benefits that continues to grow at a rapid pace. The unique properties of low-pressure ionized gas plasmas enable the manipulation of materials in ways that cannot be achieved by other means. Specifically, their complex dynamics lead to non-equilibrium chemistry; energetic ions, radicals (atoms, molecules or ions that react strongly with substrate surfaces), and photons produced in the plasma through collisions with energetic electrons etch, modify or deposit thin films on materials surfaces. Process outcomes are thus critically sensitive to the abundances of electrons of different energies and other plasma parameters. This project will also celebrate plasma technology and enhance public 'plasma literacy' through the development of a new hands-on outreach exhibit highlighting plasma thrusters for spacecraft propulsion. The centerpiece of the display will be a working plasma thruster operating at atmospheric pressure. The display will be created in partnership with the Rocket Club at Madison West High School and will become part of the Rocket Club's popular traveling exhibit, viewed by thousands of people each year at science festivals and school visits.The spectrum of light emitted by the plasma carries an encoded mapping of plasma parameters. Probing plasma properties by measuring the intensity of plasma light emissions at characteristic wavelengths is made possible through a model that predicts emission spectra by accounting for the excitation and de-excitation processes that govern light emission by the plasma. This project's innovation is the creation of a quantitative emission model for molecular gases, with oxygen as an initial focus. Through careful consideration of known excitation rates, the study will identify spectral features for inclusion in the model, choosing some that show a strong quantitative dependence on the electron energy distribution function and others that are strong functions of the dissociation fraction, i.e., the fraction of molecules broken up into constituent atoms. This approach will allow plasma parameters to be determined as the values for which the model produces a best fit to the observed spectral intensities, measured using optical emission spectroscopy (OES). To demonstrate and explore the capabilities of the molecular OES diagnostic, it will be applied to selected discharge phenomena (i) that exhibit complex plasma dynamics not fully understood, (ii) for which new molecular OES is likely to contribute new insights and (iii) for which the new insights obtained have relevance for development of industrial processes.