Thermal and hydrodynamic effects of nanosecond discharges in atmospheric pressure air

We present quantitative schlieren measurements and numerical analyses of the thermal and hydrodynamic effects of a nanosecond repetitively pulsed (NRP) discharge in atmospheric pressure air at 300 and 1000K. The plasma is created by voltage pulses at an amplitude of 10kV and a duration of 10ns, applied at a frequency of 1-10kHz between two pin electrodes separated by 2 or 4mm. The electrical energy of each pulse is of the order of 1mJ. We recorded single-shot schlieren images starting from 50ns to 3μs after the discharge. The time-resolved images show the shock-wave propagation and the expansion of the heated gas channel. Gas density profiles simulated in 1D cylindrical coordinates have been used to reconstruct numerical schlieren images for comparison with experimental ones. We propose an original method to determine the initial gas temperature and the fraction of energy transferred into ultrafast gas heating, using a comparison of the contrast profiles obtained from experimental and numerical schlieren images. This method is found to be much more sensitive to these parameters than the direct comparison of measured and predicted shock-wave and heated channel radii. The results show that a significant fraction of the electric energy is converted into gas heating within a few tens of ns. The values range from about 25% at a reduced electric field of 164Td to about 75% at 270Td, with a strong dependance on the initial gas temperature. These experiments support the fast heating processes via dissociative quenching of N2(B³Πg, C³Πu) by molecular oxygen.