Understanding electrode plasma formation on wires and thin foils via vacuum ultraviolet spectrsocopy of desorbed surface contaminants*

Power flow studies on the 30-MA, 100-ns Z facility at Sandia National Labs (SNL) have shown that plasmas in the facility's magnetically insulated transmission lines (MITLs) can result in a loss of current delivered to the load. During the current pulse, thermal energy deposition into electrodes (ohmic heating, charged particle bombardment, etc.) causes neutral surface contaminants layers (water, hydrogen, hydrocarbons, etc.) to desorb, ionize, and form plasmas in the anode-cathode (AK) gap. Shrinking typical electrode thicknesses (~1 cm) down to that of thin foils (5−200 μm) produces observable amounts of plasma on smaller pulsed power drivers (≤1 MA). We suspect that as the electrode material bulk thickness decreases relative to the skin depth of the current pulse (50−100 μm for a 100−500-ns pulse in aluminum), the thermal energy delivered to the neutral surface contaminant layers increases, and thus more surface contaminants desorb from the current carrying surface. In this talk, we review our efforts to develop a thin-foil-based platform to study electrode plasma formation on smaller-scale facilities (≤ 1 MA) and present results from a vacuum ultraviolet (VUV) spectroscopy system developed to measure the hydrogen Lyman-α line (121.6 nm) from wires and foils with varying thicknesses (5−200 μm). We use the VUV measurements to compare hydrogen inventories to those predicted to be released via thermal processes in the electrode. The VUV range (100−200 nm) was chosen due to the expectation of low levels of background continuum emission relative to the visible range; this expectation is supported by preliminary simulations with PrismSPECT.