Electrode plasmas and Hall-conductivity effects on multi-MegaAmpere transmission-line current loss

The pulsed-power accelerators driving z-pinch and dynamic material-properties experiments achieve some of the highest current densities (>1 MA/cm²) in routine laboratory operation. While high current densities are required to melt and implode z-pinch targets, the transmission lines are also Ohmically heated and, approaching the load, exceed the threshold for electrode plasma formation. These plasmas may reduce accelerator efficiency by shunting current upstream from the load, referred to as “current loss” [Phys. Rev. Accel. Beams 22 120401 (2019), Phys. Rev. Accel. Beams 24 060401 (2021)]. Theory and simulation have been applied to this problem to create a comprehensive description of electrode plasma formation and transport at the multi-MA scale that invokes a Hall-related current [Phys. Rev. Accel. Beams 23 110401 (2020), Phys. Rev. Accel. Beams 26 040401 (2023)]. This theory has now been subject to experimental verification at Sandia National Laboratories’ Z Machine. We present results from a unique experimental configuration that has been designed to be able to discriminate between three competing theories for current loss on Z. This shot series fields a long (~10 cm?), narrow (~3 mm) final magnetically-insulated transmission line that heats without melting to generate low plasma densities (<10¹⁵ cm^-3) in the transmission line gap. These plasmas are predicted to completely fill the narrow transmission line gaps. This configuration shows a loss of up to 25% of the total current. The measured line and load currents are compared to theory and simulation. These data unambiguously confirm the expectations that current loss is limited by magnetized plasma transport model invoking Hall-conductivity. [MC1] Other data from spectroscopy and Faraday cups verify the predicted appearance and transport of the electrode plasmas, and charged particles. The implications for the scaling of current loss in future systems will be discussed.[MC2]

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