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

Power flow studies on the 30-MA, 100-ns Z facility at Sandia National Labs have shown that plasma in the facility’s magnetically insulated transmission lines can result in a loss of current delivered to the load.¹ During the current pulse, thermal energy deposition into the electrodes causes neutral surface contaminant layers to desorb, ionize, and form plasma in the anode-cathode gap.^2,3 We suspect that as the electrode thickness decreases relative to the skin depth of the current pulse (50−100 µm for aluminum and 100-500 µm for stainless steel, for a 100−500-ns pulse), the thermal energy delivered to the surface contaminant layers increases, and thus, faster desorption rates.

We review experimental results on a smaller scaled facility (≤ 1 MA) to characterize the contaminants using the plasma temperature, density, and determination of the species and their ionized states during surface desorption. These results are collected from a vacuum ultraviolet spectrometer developed to measure the spectra from wires and foils of varying thicknesses (5 µm – 1 mm) and materials (aluminum and stainless steel). To quickly compare theoretical values to experimental results, a newly developed, semi-analytic desorption and heating model is then used.

1. W.A. Stygar et al., Phys. Rev. S.T.-A.B. 12, 120401 (2009)

2. M.R. Gomez et al., Phys. Rev. A.B. 20, 010401 (2017)

3. T.J. Smith et al., Rev. Sci. Inst. 92, 053550 (2021)