Physics investigations of vacuum ultraviolet emission from pulsed atmospheric discharges

Summary form only given. The role of self-produced VUV emission (i.e. energies greater than 7 eV) on photo-ionization processes during the early nanoseconds of pulsed discharges is a new area of interest in the aerospace community, and could play a significant role in the theoretical understanding of plasma generation at short (nanosecond) timescales. Our previously reported experiments have shown VUV emission from atomic species of oxygen and nitrogen, which were excited during breakdown, but detailed analysis of emission in the range 115 - 135 nm was difficult due to chromatic aberration of VUV transparent optics in this regime. These limitations were recently alleviated by fitting the spectral apparatus with a custom set of MgF₂-Aluminum coated off-axis parabolic mirrors used in conjunction with a high resolution vacuum monochromator. VUV emission is observed by either VUV sensitive intensified CCD or photomultiplier sensors, with additional current/voltage monitors and externally focused fast-frame (nanosecond capable) imaging complimenting the diagnostic setup. High resolution spectroscopy has been achieved in the excitation range of interest (8 - 10 eV), where species of atmospheric gases and electrode metal have been identified during the early nanoseconds of plasma generation. Temporal studies have shown that most VUV emission occurs during the time before voltage collapse and subsequent power flow, while the majority of visible emission is released after breakdown when the electron energy distribution has shifted to lower energies. This experiment also allowed for direct emission imaging of VUV radiation, where the spatial profile relative to plasma position is still intact. While the observed metal species are only emitted in the regions close to the electrodes as expected, significant differences were observed for species of NI (which is released throughout the plasma volume) and OI (which is released strongly in regions of high electric field- atoms during breakdown. This is most likely due to the accumulation of ionic space charge from photo-ionization in the gas volume, coupled with the high absorption cross section of molecular oxygen in the VUV range. These measurements will be discussed in context to current physics models of electric breakdown and plasma generation at atmospheric pressure.