Electric field in Ns pulse and AC electric discharges in a hydrogen diffusion flame

Time-resolved electric field is measured in ns pulse and AC sine wave dielectric barrier discharges sustained in an atmospheric pressure hydrogen diffusion flame, using picosecond second harmonic generation. Individual electric field vector components are isolated by measuring the second harmonic signals with different polarizations. Electric field measurements in a ns pulse discharge are self-calibrating, since the field follows the applied voltage until breakdown. Electric field is measured in a ns pulse discharge sustained both in the hydrogen flow below the flame and in the reaction zone of the flame. Peak electric field in the reaction zone is lower compared to that in the near-room temperature hydrogen flow, due to a significantly lower number density. In hydrogen, most of the energy is coupled to the plasma at the reduced electric field of E/N approximate to 50-100 Td. In both cases, the electric field decreases to near zero after breakdown, due to plasma self-shielding. The time scale for the electric field reduction in the plasma is relatively long, several tens of ns, indicating that it may be controlled by a relatively slow propagation of the ionization wave over the dielectric surfaces. In the AC discharge, the electric field is put on the absolute scale by measuring a Laplacian electric field between two parallel cylinder electrodes. The measurement results demonstrate that a strong electric field in the plasma-enhanced flame is produced during the entire AC voltage period, without correlation with the random micro-discharges detected in the plasma images. The measurement results indicate consistently higher peak electric field during the negative AC half-period, as well as a significant electric field offset. Both the asymmetry and the offset of the electric field are likely responsible for the ion wind resulting in the flame distortion. The results suggest that at the present conditions the ion wind is dominated by the transport of negative ions generated in the ambient air plasma near the flame. The results demonstrate a significant potential of ps second harmonic generation diagnostics for non-intrusive measurements of the electric field in atmospheric pressure flames enhanced by electric discharge plasmas. (C) 2018 Published by Elsevier Inc. on behalf of The Combustion Institute.