The role of magnetic topology change in development of ablation streams in wire arrays

This paper investigates the influence of the magnetic field topology and the global field penetration time on the ablation dynamics of cylindrical wire array Z-pinches. 3D resistive MHD simulation results with the GORGON code suggest that a change in the global magnetic field topology is critical in initiating wire array ablation. The simulations show a characteristic evolution in the development of streams of ablated material ejected from the wires toward the array axis. In simulations of tungsten arrays, the fundamental behavior occurs in two steps. The first is the development of coronal plasma that is trapped around the wire core in closed "local" magnetic flux. This coronal mass, together with the closed flux, is then accelerated inward after a certain "dwell" time, leaving behind a radially distributed current density with entirely open "global" magnetic field lines, producing smooth, distributed acceleration of ablated plasma inward from the wire core until the onset of the final implosion.These simulation results are investigated experimentally by using B-dot probes to track the evolution of the field over time and space in wire arrays on the Cornell COBRA accelerator.The field radially inward from a wire initially grows in the "local" direction, then after a dwell time reverses and grows in the global direction. The time of reversal is retarded at further inward radial positions. Thus the local field is advected inward, with an inferred velocity typical of ablation streams.