MHz sampling rate measurements of N2(A3Σu+) population in a Ns pulse discharge in a heated plasma flow reactor

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Abstract Absolute, time-resolved population of a metastable electronic state of molecular nitrogen, N2( A 3 Σ u + , v = 0 ), is measured in a heated plasma flow reactor excited by a ns pulse discharge burst, by tunable diode laser absorption spectroscopy using a distributed feedback laser. Nitrogen or NO–N2 gas mixture in the reactor are heated by a tube furnace maintained at T = 800–1000 K, at pressures of P = 50–300 Torr. A diffuse plasma in the flow channel made of quartz is generated using a repetitively pulsed, double dielectric barrier, ns discharge in a plane-to-plane geometry. N2( A 3 Σ u + ) molecules in the plasma are generated by electron impact excitation of N2 ( B 3 Π g ) and N2 ( C 3 Π u ) states, followed by their cascade quenching. The laser is tuned across an absorption line near 1028 nm in a N2 B 3 Π g , v ′ = 0 ← A 3 Σ u + , v ′ ′ = 0 band at 100 kHz–1 MHz. The laser output wavelength and the absorption signal are measured by an etalon and two high bandwidth detectors. At all operating conditions, the absorption signal is fully resolved in time. The data points are taken every 500 ns–5 μs, with the temporal uncertainty of 50–500 ns, respectively. The data are used to infer the line pressure broadening coefficient and its temperature dependence. This study demonstrates the feasibility of this approach for the time-resolved N2( A 3 Σ u + ) measurements in pulsed hypersonic flow facilities, behind strong shock waves and in hypervelocity expansion flows.