Pulsed Volume and Surface Discharges in an SF6 Environment

Summary form only given. Volume and surface discharges in SF₆ are investigated to support refurbishment of the Z-machine´s laser triggered gas switch, LTGS, at Sandia National Laboratories (SNL). The recently upgraded LTGS has exhibited a failure mode which results in surface flashover on the inside of the dielectric switch envelope. These flashover events degrade the LTGS performance and cause a pre-fire in the successive shot. The principal physical mechanisms involved in surface flashover at various pressures of SF₆ are therefore vital to the understanding of this specific switch failure mode. A 340 kV system was designed which replicated conditions found in the LTGS between two adjacent backbone electrodes. Besides measurement of flashover delay times for different dielectric materials (Lexan, acrylic, epoxies, etc.), emphasis was put on the detailed characterization of the flashover phenomenology. Hence, in addition to voltage and current diagnostics, fast optical imaging of the flashover along with the acquisition of optical emission spectra (~200 nm to 700 nm) was employed. One possible reason for the LTGS failure was thought to be ultraviolet (UV) radiation emitted from the volume discharge between the backbone electrodes, which could induce surface flashover on the inside of the envelope. Our present setup is utilized to gather evidence that would confirm this possibility by adding a field-stressed dielectric surface directly exposed to the volume arc produced between the model backbone electrodes. With a current amplitude as low as ~ 2 kA, the occurrence of sulfur and fluorine ions as well as hydrogen and carbon during dielectric surface flashover was revealed with optical emission spectroscopy. From the spectra, the plasma temperature was determined to be between 1.5 and 3 eV, largely depending on the SF₆ pressure. The presence of carbon and hydrogen is of significance because it indicates removal of material from the die- lectric surfaces during discharge. X-ray fluoroscopy has also confirmed sulfur deposits on the surfaces of the dielectrics tested. Experimental results with higher current amplitudes will be discussed as well as the results showing the UV´s influence on inducing surface flashover in an SF₆ environment.