Electrode plasma transport into the gap of high-current Z-pinch accelerators

In the pulsed-power accelerators that drive Z-pinch implosions, multi-MA currents rapidly heat the electrodes above the threshold for plasma formation. When these surface plasmas are transported into the transmission line gap, they shunt some portion of the driver current away from the load, referred to as “current loss.” In highly magnetized systems, current loss is well approximated by a Hall current proportional to the minimum plasma density in the gap and the electric field [N. Bennett, D. R. Welch, G. Laity, D. V. Rose, and M. E. Cuneo, Magnetized particle transport in multi-MA accelerators, ]. This paper explores the mechanism that drives the initial plasma transport into the gap, which is difficult to isolate in large-scale power-flow simulations but is instrumental in seeding the current-loss mechanism. The stability and dynamics of an electron sheath emitted from a resistive cathode plasma in the presence of large magnetic fields are investigated using semianalytic theory and simulation. A dispersion analysis determines that the instability growth rate scales inversely to ion mass and magnetic field strength. This scaling is confirmed in kinetic models of an electron sheath and resistive plasma using the relativistic electromagnetic particle-in-cell code CHICAGO. The simulations are initiated with field magnitudes representative of the final feed of a Z-pinch driver and demonstrate that a resistive plasma instability grows on ns timescales, generating perturbed electric fields of sufficient strength to rapidly accelerate plasma ions into the gap. Although the resulting plasma density in the gap is 2 orders of magnitude below the surface density, it is sufficient to generate the published current losses.