The role of magnetic topological change in the onset of ablation of cylindrical wire arrays

The formation of a dense Z-pinch from a cylindrical wire array follows a process through which the discrete wires expand, ablate plasma onto a common geometric axis, and then implode, forming the pinch. We numerically investigate the transition from the initial core-corona structure to ablating wires and provide an analytic model which predicts the coronal radius over time and the time for onset of ablation. We find that for ablation to occur, the initially compressive vacuum magnetic field topology around each wire, with global field on the outside of the array and local field on the inside of the array, must undergo a transition to a configuration where the JxB force is due entirely to global field contributions around each wire. This only occurs if current is carried toward the center of the array creating a plasma precursor. In these low magnetic Reynolds number flows, the fast wave is damped and an acoustic-like shock carries this initial current to the array axis. Using resistive MHD simulation, we present results using array parameters which verify our analysis, indicating when ablation occurs and when discrete wire implosion without ablation occurs.