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.

The evolution of laboratory produced magnetic jets is followed numerically through three-dimensional, non-ideal magnetohydrodynamic simulations. The experiments are designed to study the interaction of a purely toroidal field with an extended plasma background medium. The system is observed to evolve into a structure consisting of an approximately cylindrical magnetic cavity with an embedded magnetically confined jet on its axis. The supersonic expansion produces a shell of swept-up shocked plasma which surrounds and partially confines the magnetic tower. Currents initially flow along the walls of the cavity and in the jet but the development of current-driven instabilities leads to the disruption of the jet and a re-arrangement of the field and currents. The top of the cavity breaks-up and a well collimated, radiatively cooled, "clumpy" jet emerges from the system. 2

Resistive magneto-hydrodynamic (MHD) simulations are used to evaluate the influence of three-dimensional inhomogeneities on x-ray power production in wire array Z-pinches. In particular, we concentrate on simulations of wire array Z-pinch experiments on the MAGPIE generator at Imperial College. An initial temperature perturbation is used to stimulate variations in wire core ablation rates that result in a highly non-uniform final implosion. Results indicate that x-ray power production is governed by the symmetry of the implosion surface and by the rate at which current can transfer to the axis through a threedimensional debris field that trails behind the main implosion. The peak power is ultimately limited by the growth of MHD instabilities in the stagnated pinch. The individual contributions of the implosion kinetic energy, compression of the stagnated pinch, ohmic heating and MHD instabilities to the radiation yield are quantified. The onset of m = 1 instabilities is found to provide an efficient mechanism for dissipation of the magnetic energy surrounding the stagnated pinch. The formation of a helical plasma column not only allows the magnetic field to do work in driving an expansion of the helix but also enhances the ohmic heating by elongating the path of the current through the pinch. The effect of these energy sources combined is to increase the radiation yield to typically 3 1 2 times the kinetic energy of the implosion. Simulations of arrays with different wire numbers, wire material and with nested arrays are used to examine the mechanisms that influence the peak soft x-ray power. In the simulations, peak power can be increased by: increasing the number of wires (which improves the implosion symmetry), by increasing the atomic number of the material (which increases the compressibility of the plasma) and by using a nested inner array (which brings the mass and the current to the axis more efficiently than a single array).

We present the first results of high energy density laboratory astrophysics experiments which explore the evolution of collimated outflows and jets driven by a toroidal magnetic field. The experiments are scalable to astrophysical flows in that critical dimensionless numbers such as the Mach number, the plasma β and the magnetic Reynolds number are all in the astrophysically appropriate ranges. Our experiments use the MAGPIE pulsed power machine and allow us to explore the role of magnetic pressure in creating and collimating the outflow as well as showing the creation of a central jet within the broader outflow cavity. We show that currents flow along this jet and we observe its collimation to be enhanced by the additional hoop stresses associated with the generated toroidal field. Although at later times the jet column is observed to go unstable, the jet retains its collimation. We also present simulations of the magnetic jet evolution using our two‐dimensional resistive magnetohydrodynamic laboratory code. We conclude with a discussion of the astrophysical relevance of the experiments and of the stability properties of the jet.

We present the Ðrst results of astrophysically relevant experiments where highly supersonic plasma jets are generated via conically convergent Ñows. The convergent Ñows are created by electrodynamic acceleration of plasma in a conical array of Ðne metallic wires (a modiÐcation of the wire array Z-pinch). Stagnation of plasma Ñow on the axis of symmetry forms a standing conical shock e †ectively collimating the Ñow in the axial direction. This scenario is essentially similar to that discussed by and colCanto laborators as a purely hydrodynamic mechanism for jet formation in astrophysical systems. Experiments using di †erent materials (Al, Fe, and W) show that a highly supersonic (M D 20), well-collimated jet is generated when the radiative cooling rate of the plasma is signiÐcant. We discuss scaling issues for the experiments and their potential use for numerical code veriÐcation. The experiments also may allow direct exploration of astrophysically relevant issues such as collimation, stability, and jet-cloud interactions.