Spatiotemporal structure of electron density inside atmospheric pressure glow discharge measured by spectroscopic methods

We have investigated discharge mechanisms in an atmospheric pressure glow discharge (APGD) by spectroscopic diagnostics. The APGD was a parallel-plate electrode configuration and generated in He gas with small N$_{2}$ impurity. We measured electron density ($n_{e})$ distributions by using a combination method of a CO$_{2}$-laser heterodyne interferometry and a millimeter-wave transmission. To discuss the formation mechanisms of $n_{e}$ distribution, we also measured spatiotemporal structures of He metastable atom density by a laser absorption spectroscopy (LAS) method, and calculated a dependence of ionization frequency on the N$_{2}$ impurity ratio. The measurement results of the combination method revealed that the spatial $n_{e}$ distribution was localized near the dielectric barrier, and the distribution was more broaden only at 0.25{\%} of N$_{2}$ impurity ratio. In the LAS measurement, we observed a typical structure of glow discharge in the APGD. This result indicates that the localization of $n_{e}$ distribution is due to the formation of negative glow region. The calculation results of the ionization frequency give us the explanation for the broaden $n_{e}$ distribution at 0.25{\%} of N$_{2}$ impurity ratio, which is the enhanced ionization in the range of lower reduced electric field.