Quantitative Schlieren Diagnostics Applied to a Millisecond Pulsed-DC Hybrid Discharge in Atmospheric Pressure Air

The gas temperature of a hybrid discharge in atmospheric pressure air is investigated by using quantitative schlieren imaging. The discharge is stabilized in a pin-to-plate electrode geometry and operated in a millisecond pulsed-DC regime with current amplitudes up to 75 mA and a duration of 10 ms, applied at a frequency of 100 Hz. An equilibrium composition model is considered to account for the production of N, O, and NO, which influence the Gladstone–Dale coefficient of air at high-gas temperatures. Also, a procedure is described which allows the determination of the errors introduced in the time-average gas refraction index due to gas temperature fluctuations. The results show that the axial values of the gas temperature profiles span a large range from ~ 1000 to 5000 K, nearly following the evolution of the discharge current. The temperature values found agree well with those reported in the literature for atmospheric pressure air plasmas, ranging from micro-glow to hybrid discharges.