A phenomenological model of wire array Z-pinch implosions, based on the analysis of experimental data obtained on the mega-ampere generator for plasma implosion experiments (MAGPIE) generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)], is described. The data show that during the first ∼80% of the implosion the wire cores remain stationary in their initial positions, while the coronal plasma is continuously jetting from the wire cores to the array axis. This phase ends by the formation of gaps in the wire cores, which occurs due to the nonuniformity of the ablation rate along the wires. The final phase of the implosion starting at this time occurs as a rapid snowplow-like implosion of the radially distributed precursor plasma, previously injected in the interior of the array. The density distribution of the precursor plasma, being peaked on the array axis, could be a key factor providing stability of the wire array implosions operating in the regime of discrete wires. The modified “initial” conditions for simulations of wire array Z-pinch implosions with one-dimension (1D) and two-dimensions (2D) in the r–z plane, radiation-magnetohydrodynamic (MHD) codes, and a possible scaling to a larger drive current are discussed.

We present the first measurements by x-ray radiography of the development of instabilities during the implosion phase of wire array Z pinches. The seeding of perturbations on the dense core of each wire is provided by nonuniform sweeping of the low-density coronal plasma from the cores by the global JxB force. The spatial scale of these perturbations ( approximately 0.5 mm for Al and approximately 0.25 mm for W) is determined by the size of the wire cores ( approximately 0.25 mm for Al and approximately 0.1 mm for W). A qualitative change in implosion dynamics, with transition to 0D-like trajectory, was observed in Al arrays when the ratio of interwire gap to wire core size was decreased to approximately 3.

Measurement and modeling of the implosion of wire arrays with seeded instabilitiesa) Phys. Plasmas 13, 056313 (2006); Detailed measurements of the dynamics of aluminum wire array Z pinches from immediately prior to implosion until stagnation and dissipation on axis are presented. Before implosion, the ϳ0.5 mm axial modulation seen in earlier laser probing images is observed as ablation on the surface of the wire cores facing away from the array axis. This results in the complete ablation of sections of the wire cores and a redistribution of current at the start of implosion. The dynamics of implosion are then strongly influenced by the number of wires in the array. With only eight wires, discrete snowplough bubbles expand from each wire toward the precursor. There is little, if any, correlation between the bubbles from adjacent wires, and a large temporal spread over which the bubbles arrive at the precursor is observed, along with a long rise time, low power soft x-ray pulse. With 32 or more wires, bubbles from adjacent wires merge close to the array edge to form an imploding sheath. The front edge of the sheath is well defined with a small spatial spread, and upon reaching the precursor, the start of a fast rising high power soft-x-ray pulse is seen. As x-ray emission increases, the stagnating column on axis starts to decrease in diameter, reaching a minimum at peak x-ray emission, which also coincides with the time when the rear edge of the snowplough reaches the column. Thereafter, the stagnated column is seen to go unstable, and trailing mass left behind during the implosion is accelerated toward the axis. Intense x-ray emission ends as this mass becomes cleared out.

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Wire core and coronal plasma formation and expansion in wire-array Z pinches with small numbers of wires have been studied on a 1 MA, 100 ns rise time pulsed power generator and a 500 kA, 50 ns generator. Two-frame point-projection x-ray imaging and three-frame laser optical imaging and interferometry were the principal diagnostic methods used for these studies. The x-ray images show that dense coronal plasma forms and is maintained close to each dense wire core in the array. A less dense, rapidly expanding ͑ϳ10 m/ns͒ coronal plasma, best seen in the laser images, surrounds the ϳ100 m radius dense corona. These results are in agreement with computer simulations and modeling carried out by Yu et al. ͓Phys. Plasmas 14, 022705 ͑2007͔͒. Results are also presented for the dependence of the wire core and coronal plasma expansion rates on the wire diameter, number of wires and current through individual wires and the overall configuration for Al, Cu, and W wire arrays. For example, the W wire dense core expansion rate increases with increasing initial wire diameter from 5.1 m ͑0.1 m/ns͒ to 12.7 m diameter ͑0.3 m/ns͒.