Investigation of the hydrodynamic expansion following a Nanosecond Repetitively Pulsed discharge in air

We report on an experimental study of the hydrodynamic expansion following a Nanosecond Repetitively Pulsed (NRP) discharge in atmospheric pressure air at 300 and 1000 K. The discharge is created by voltage pulses of amplitude 10 kV, duration 10 ns, applied at a frequency of 1-10 kHz between two pin electrodes. The electrical energy of each pulse is of the order of 1 mJ. We recorded single-shot schlieren images starting from 50 nanosecond after the discharge. The time-resolved images show the shock-wave propagation and the expansion of the heated gas channel. The temporal evolution of the gas temperature behind the shock front is estimated from the measured shock-wave velocity by using the Rankine-Hugoniot relations. The results show that the gas heats up by almost 1100 K within 50 ns after the pulse. This fast gas heating is consistent with a two-step mechanism involving electron-impact excitation of N₂ followed by the dissociative quenching of the excited electronic states of N₂ by O₂.