Time-resolved optical emission spectroscopy of nanosecond pulsed discharges in atmospheric-pressure N ₂ and N ₂/H ₂O mixtures

In this contribution, nanosecond pulsed discharges in N ₂ and N ₂/0.9% H ₂O at atmospheric pressure (at 300K) are studied with time-resolved imaging, optical emission spectroscopy and Rayleigh scattering. A 170ns high-voltage pulse is applied across two pin-shaped electrodes at a frequency of 1kHz. The discharge consists of three phases: an ignition phase, a spark phase and a recombination phase. During the ignition phase the emission is mainly caused by molecular nitrogen (N ₂(C-B)). In the spark and recombination phase mainly atomic nitrogen emission is observed. The emission when H ₂O is added is very similar, except the small contribution of H _α and the intensity of the molecular N ₂(C-B) emission is less. The gas temperature during the ignition phase is about 350 K, during the discharge the gas temperature increases and is 1s after ignition equal to 750K. The electron density is obtained by the broadening of the N emission line at 746nm and, if water is added, the H _α line. The electron density reaches densities up to 4×10 ²⁴m ³. Addition of water has no significant influence on the gas temperature and electron density. The diagnostics used in this study are described in detail and the validity of different techniques is compared with previously reported results of other groups.