Nanosecond-scale spectroscopy of vacuum ultraviolet emission from pulsed atmospheric discharges

Abstract

This paper describes a 2nd-generation system for directly studying the emission of vacuum ultraviolet (VUV) light from pulsed dielectric surface flashover at atmospheric pressure. 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 virtually unexplored, and yet could be a significant factor in the physics of fast breakdown of directed energy systems (such as MW-class high power microwave devices) in the aerospace community. First generation experiments at Texas Tech University have shown that VUV emission corresponding to nitrogen and oxygen excitation in the energy range 8-10 eV is easily produced, but the use of MgF2 optics inhibited future work with existing hardware due to the transmission cutoff of this dielectric material and chromatic aberration if used as a lensing medium. In an effort to enhance the detection capabilities of our hardware in the wavelength range from 115-135 nm, the current system utilizes a custom designed set of off-axis parabolic MgF2-Aluminium coated mirrors as the primary focusing element. High resolution spectroscopy with the upgraded system resulted in the observation of the nitrogen doublet at 149.5 nm, leading to a better fit for the appropriate line broadening parameters for an approximate 10 eV Boltzmann electronic temperature. Evidence of self-absorption for HI (121.5 nm) provides new insight into the generation of space charge in these plasma structures, which has been investigated quantitatively in both SF6-H2 and N2-H2 mixtures.