The effect of inter-pulse coupling on gas temperature in nanosecond-pulsed high-frequency discharges

Abstract

The application of nanosecond-pulsed high-frequency discharges across a pin-to-pin air gap was investigated to determine the gas temperature evolution over a burst series of 10 pulses with approximately 10 ns duration at a frequency varying from 1 kHz to 250 kHz. The benefits of high pulse frequency (>10 kHz) for combustion applications has been previously noted, and there exists some experimental evidence that the temperature increases with pulse frequency, but the phenomenon has not been explored rigorously. In this study, discharges in quiescent air initially at 99 kPa and 293 K were explored at various pulse frequencies using optical emission spectroscopy. The gas temperature was measured via spectroscopy of the N2(C → B) emission during each discharge pulse with the assumption that the N2(C) state rotational distribution maps the ground state N2(X) distribution. The results show within a burst of repetitive pulses, that the remnant pulse energy caused a significant gas temperature increase between the end of the previous pulse and the initiation of the subsequent pulse, if the time between pulses was less than 100 µs. This result indicated that the discharges within the burst were thermally ‘coupled’ for frequencies greater than 10 kHz. Analysis of the temperature evolution after the first pulse in a burst indicated a rapid temperature rise of several thousand Kelvin over a few microseconds. The heating rate increased further if repetitive pulses were applied at higher frequencies. It could be inferred from the data that the temperature rise following the first pulse had peaked by 40 µs and that the air in the discharge zone was cooling by the initiation of the second pulse, even at the highest frequency of 250 kHz, presumably due to a gas recirculation effect associated with the first pulse.