Time-resolved Spectral Investigations of Pulsed Atmospheric Dielectric Surface Flashover Discharges*

In an attempt to identify the mechanisms leading to pulsed dielectric surface flashover in atmospheric conditions, a surface flashover event occurring on a magnesium fluoride (MgF$_{2})$ window was studied. The electrode configuration and the applied pulsed voltage level were chosen such that the generated electric field was symmetric with respect to the centerline between the electrodes. Sharpened stainless steel electrodes (estimated tip radius of 200$\mu $m) are attached to springs which press down onto the MgF$_{2}$ surface a distance of 8 mm apart. Diagnostics include time resolved emission spectroscopy in the VUV range and gated ICCD optical imaging of streamer progression during the first 30 nanoseconds of breakdown (with 3 nanosecond resolution) in the visible wavelength range. One important parameter on which the streamer formation and the subsequent breakdown strongly depends is the gas type and/or the gas composition. That is why the streamer formation was recorded for gated intervals from 3 to 50 ns in lab air, standard nitrogen, oxygen and SF$_{6}$. The results are compared to those ones in purified air, oxygen and nitrogen.