A passive method for determining plasma dissociation degree using vacuum UV self-absorption spectroscopy

摘要

There has been a continued interest in utilizing streamer and spark discharges for new technologies that require low temperature plasma generation at atmospheric pressure, but diagnostics of these plasmas typically require external sources of probing radiation such as UV lamps or laser systems. Specifically, it is desired to measure the dissociated gas density from pulsed surface flashover plasmas, without the use of potentially invasive optical techniques such as two-photon absorption laser induced fluorescence (TALIF) spectroscopy, which may artificially increase the dissociation degree. We demonstrate a method for determining the dissociated gas density of N and H atoms in an N2/H2 surface flashover plasma by passively monitoring the self-absorption of intrinsic radiation produced by the 2s2 2p2 3s→2s2 2p3 NI transition(s) at 120.0 nm, and the 2p→ls HI Lyman-a transition at 121.57 nm. This radiation is partially trapped by the spark plasma, assumed to be of Gaussian cylindrical shape with 500 μπι diameter. The resulting emission line shapes can be calculated by inferring the plasma temperature, gas mixture, and the estimated dissociated atom density of each species in the plasma volume of measurement. For example, 80%/20% N2/H2 discharges with a measured electron temperature of ∼3.0 eV produce peak dissociated concentrations of 2% and 9% for atomic N and H, respectively, during the spark phase ∼100 ns after voltage collapse. By assuming the quasi-contiguous approximation of the Holtsmark micro-field due to local electron perturbation of the HI radiators, the Stark line width of Lyman-a radiation yields electron densities on the order of 1018 cm3 during the spark phase. This self-absorption method has been extended to provide density information of surface flashover plasmas in air environments by pas- ively monitoring the 2s2 2p3 3s→2s2 2p4 OI transitions) at 130.2 nm / 130.5 nm / 130.6 nm, which yield peak dissociated concentrations of 20% and 7% for atomic O and N, respectively.