Time-resolved line emission spectroscopy and the electrical currents in the plasma jet generated by dielectric barrier discharge for soft ionization

Abstract

The spatially and temporally resolved optical emission from the helium capillary dielectric barrier discharge driven by a square wave high voltage was measured simultaneously with the discharge currents during the positive and negative voltage periods. The He and N2+ emission intensities were monitored at 501nm and 391nm, respectively. The plasma jet was found to be formed only during the positive voltage period, and it preceded the discharge ignition in the capillary. Thus, the measurements of the difference between the capillary discharge currents during the positive and negative voltage periods enabled quantification of the charge, which was transported through the plasma jet. The analysis of the spatiotemporal behavior of the He emission intensities in the plasma jet yielded the velocity of the helium excitation propagation and the effective plasma jet length. The combination of these measurements enabled determination of the absolute values of the plasma jet currents. The linear relationship between the He emission intensities in the plasma jet and the plasma jet currents was found to exist over the emission intensity range of two orders of magnitude. The N2+ emission intensity was also found to be linearly dependent on the plasma jet current. The fast electrons, producing the excited He atoms, were found to be located mostly within the estimated effective plasma jet length. In this region a cascade of processes involving excited He atoms, N2+ and H2O molecules lead to establishment of the conditions necessary for the plasma jet to be used as soft ionization source. The production of reactive species can be related to the plasma jet current, which can serve as a measure of the soft ionization efficiency.