Numerical investigations of nanosecond surface streamers at elevated pressure

Single-pulse nanosecond surface dielectric barrier discharges operated in synthetic air and pure nitrogen at elevated pressure (6 bar) have been numerically studied by the classical fluid method. The aim of this work is to provide the necessary basis for analyzing the surface streamer-to-filament transition phenomenon. The electrical parameters, discharge morphology and propagation dynamics, as well as the possible influence of photoionization, kinetics and gas heating on the surface streamer stage at elevated pressure are discussed in a combined numerical-analytical way. A good agreement between measurement, numerical simulation and analytical estimation is achieved. The streamer thickness is derived to be inversely proportional to the pressure, the average reduced electric field in the streamer channel ranges from 75-150 Td, and the average electron density in the channel is similar to 10(22) m(-3) for both polarities. The characteristic time of electron decay in the positive streamer channel is only on the order of similar to 1 ns due to high charge exchange and electron-ion recombination rates. Photoionization provides an ionization source of 10(27)-10(28) m(-3) s(-1) in front of the streamer head for both gases. Stepwise ionization/dissociation significantly affects the discharge dynamics of negative polarity SDBD, while for positive polarity this influence is negligible. The fast gas heating could lead to +/- 0.15 cm change of the positive streamer length. The secondary surface ionization wave is numerically observed, providing a time duration 0.25-0.5 ns of high electric field (230 Td), making it possible for streamer-to-filament transition kinetics to take place.