Ultrafast heating and oxygen dissociation in atmospheric pressure air by nanosecond repetitively pulsed discharges

A detailed experimental investigation of the mechanism of ultrafast heating and oxygen dissociation produced by nanosecond repetitively pulsed discharges in atmospheric pressure air preheated at 1000 K is presented. The ultrafast mechanism creates excited electronic states of nitrogen, which then dissociate molecular oxygen through quenching reactions, with the remaining energy dissipated as heat. Optical and electrical diagnostic techniques have been applied to provide a self-consistent set of experimental data for a reference test-case with well-defined discharge and gas conditions. The pulses have a duration of 10 ns, an amplitude of 5.7 kV, a repetition frequency of 10 kHz and the pin-to-pin electrodes are separated by 4 mm. We present measurements of the gas temperature during and after the discharge using optical emission spectroscopy of the second positive system of nitrogen, spatially resolved profiles of the absolute densities of excited electronic states, determined using Abel-inverted spectra of the first and second positive systems of nitrogen, as well as the temporal evolution of the absolute densities of N-2(A), N-2(B), N-2(C), electrons and atomic oxygen. These measurements are synchronized with electrical measurements of pulse current, voltage, and energy. The discharge is found to dissociate about 50% of molecular oxygen and to produce a temperature increase of about 900 K within 20 ns, corresponding to an ultrafast heating rate of about 5 x 10(10) Ks(-1). Comparisons with numerical simulations show good agreement with the measurements and validate the ultrafast mechanism. About 35% of the electric energy deposited into the gas goes into O-2 dissociation, and about 21% into gas heating. Finally, the dissociative quenching rates of N-2(B) and N-2(C) with O-2 at 2200 K were measured and found to be 2.8(+/- 0.6) x 10(-10) cm(3) s(-1) and 5.8(+/- 0.9) x 10(-10) cm(3) s(-1), respectively. Combining these measurements with the literature values at 300 K, we propose a functional temperature dependence in the range 300-2200 K of 3.0 x 10(-10)(T/300)(0.3) cm(3) s(-1) for the C state, and a constant value of 3.0 x 10(-10) cm(3) s(-1) for the B state.