Computational simulation of initiation and implosion of circular arrays of wires in two and three dimensions

We have performed realistic two-dimensional (2-D) r-θ resistive MHD simulations of high-current aluminum wire array initiation and implosion. These show only a moderate differentiation of wire plasma into a warm dense core and a hot diffuse corona. Wire plasmas in 28-wire array simulations implode without forming a shell; those in 56-wire simulations first merge but then separate. As both implode, thread-like plasmas settle into valleys formed across the field lines by the thread mass. Thus, shell formation does not smooth the initial wire asymmetry, because the r-θ Rayleigh-Taylor instability amplifies it. This argues against shell-formation as the primary explanation for the observed effect of increased wire number on radiation power. We have also performed three-dimensional (3-D) ideal MHD simulations that continue those 2-D simulations; they start with a fully consistent MHD state. These simulations, perturbed between the 2-D and 3-D phases, show that azimuthally uncorrelated 3-D perturbations-appropriate for wires-grow more slowly than fully azimuthally correlated 2-D r-z perturbations. Further, the uncorrelated perturbation growth rate is smaller for 56 wires than for the 28, as the magnetic field couples more plasma threads over the same distance. These 3-D effects may explain the observed radiation power improvement with increased wire number.