Experimental study of plasma parameters in nanosecond surface dielectric barrier filamentary discharge

Nanosecond surface dielectric barrier discharges (nSDBDs) at atmospheric pressure have been studied extensively over the last two decades for flow control. About ten years ago, the nSDBD at high pressures was suggested as a source for plasma assisted ignition of combustible mixtures. During last six years, it was found that a severe transformation of a single-shot nanosecond surface streamer discharge is observed at increasing gas pressure and/or voltage.The present thesis is devoted to study of streamer-to-filament transition in a single shot high pressure surface nanosecond barrier discharge in non-reactive gases (nitrogen, oxygen and their mixtures). Literature review presents detailed analysis of streamer discharges and of transitory nanosecond sparks widely studied during last 4 years.The results are presented in three parts. The first part shows the parameters of streamer-to-filament transition in the high pressure nSDBD for different gas mixture composition. For both negative and positive polarities, the transition is a function of pressure and of the voltage amplitude. For positive polarity, the effect of molecular oxygen addition on the transition is extremely strong. The influence of different dielectrics and different electrode materials of the start and development of the filaments is studied experimentally. The micro-images of discharge propagation on three electrode configurations at three different stages – streamer, transition to filament and filamentary regime – are compared.In the second part, plasma properties in the filaments are studied with the help of the energy measurements, optical emission spectroscopy (OES) and Particle Image Velocimetry (PIV). In the streamer and transition regime, the OES spectra mainly contains second positive system of molecular nitrogen while in the filamentary regime continuum (CW) emission and a few atomic lines are observed. The results of measured plasma parameters, namely synchronized in time specific deposited energy, electron temperature and electron density are included. The value of specific deposited energy in the filaments is as high as 6-8 eV/particle; the electron density is in the range of 10 to 18-19 power per cubic centimeter, and the electron temperature stays at the level of 1.5-2 eV in the near afterglow. Plasma at this stage is found to be close to the LTE demonstrating slow (tens of nanoseconds) electron density decay linked to the temperature relaxation. The results of the measurements are compared with the results of numerical modeling explaining the main experimentally observed features. In the model, stepwise ionization and dissociation from electronically excited states of molecular nitrogen leads to fast increase of the electron density, dissociation degree and gas heating at 6 bar on the time scale of parts of nanoseconds.The third part is devoted to detailed study of streamer-to-filament transition in the micro-scale with the spatial resolution 7.6 µm/px. At a given time instant, the surface ionization wave front, composed from the merged streamers, is broken by a few plasma channels, moving with a higher velocity (we call them “protrusions”). Their radii are 10-20 times smaller comparing to a typical streamer radius; they form, within a few nanoseconds, a regular structure of plasma channels around the high voltage electrode. Inside each of these channels, a backward emission of the second positive system of N2 propagates from the “protrusion” head back to the high-voltage electrode. Continuous spectra, atomic lines and high electron density first appear when the backward emission approaches the high-voltage electrode, at the distance about 500 µm from the edge. The calculated results of emission intensity, electron density and electric field are given for studying the influence of oxygen admixtures on the transition.