The breakdown process in an atmospheric pressure nanosecond parallel-plate helium/argon mixture discharge

The breakdown process in an atmospheric pressure nanosecond helium/argon mixture discharge with parallel-plate electrodes is investigated by temporally and spatially resolved optical emission spectroscopy (OES). The spatially resolved electric field is obtained from the Stark splitting of the He i 492.1 nm line. Using the emissions from the He ii 468.6 nm, He i 667.8 nm, and Ar i 750.4 nm lines and a collisional–radiative model, the spatially resolved T_{e, high} and T_{e, low} (representing the effective T _e in the high energy and low energy part of the EEDF, respectively) are obtained. It is found that, compared with the average electric field provided by the external pulser, the electric field is greatly enhanced at certain location and is significantly weakened at other places. This observation shows the effect of the ionization wave propagation, as predicted in [1, 2]. The value of T_{e, high} is much larger than that of T_{e, low}, which indicates that an elevated high energy tail in the EEDF is built up under the influence of strong electric field during the breakdown process. Initially, the spatial distribution of the T_{e, low} and the T_{e, high} generally follows that of the electric field. However, at the end of the breakdown period, the location of the highest T_{e, low} and T_{e, high} is shifted away from the cathode sheath, where the electric field is strongest. This indicates the existence of a non-local effect and is supported by the result from a simple Monte-Carlo simulation. (paper)