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.
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 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.
A series of experiments has been conducted in order to investigate the azimuthal structures formed by the interactions of cylindrically converging plasma flows during the ablation phase of aluminium wire array Z pinch implosions. These experiments were carried out using the 1.4 MA, 240 ns MAGPIE generator at Imperial College London. The main diagnostic used in this study was a two-colour, end-on, Mach-Zehnder imaging interferometer, sensitive to the axially integrated electron density of the plasma. The data collected in these experiments reveal the strongly collisional dynamics of the aluminium ablation streams. The structure of the flows is dominated by a dense network of oblique shock fronts, formed by supersonic collisions between adjacent ablation streams. An estimate for the range of the flow Mach number (M ¼ 6.2-9.2) has been made based on an analysis of the observed shock geometry. Combining this measurement with previously published Thomson Scattering measurements of the plasma flow velocity by Harvey-Thompson et al. [Physics of Plasmas 19, 056303 (2012)] allowed us to place limits on the range of the ZT e of the plasma. The detailed and quantitative nature of the dataset lends itself well as a source for model validation and code verification exercises, as the exact shock geometry is sensitive to many of the plasma parameters. Comparison of electron density data produced through numerical modelling with the Gorgon 3D MHD code demonstrates that the code is able to reproduce the collisional dynamics observed in aluminium arrays reasonably well. V C 2013 American Institute of Physics. [http://dx
Wire array Z-pinch dynamics are studied in experiments with 16-mm diameter arrays of between 8 and 64, 15-μm diameter aluminum wires, imploded in 200–260 ns by a 1.4-MA current pulse. Side-on laser probing shows early development of noncorrelated m=0-like instabilities with an axial wavelength ∼0.5 mm in individual wires. End-on interferometry (r-θ plane) shows azimuthal merging of the plasma with a density of 1017 cm−3 in 90–65 ns for 8–64 wires, respectively. At the same time low-density plasma reaches the array axis and forms a precursor pinch by 120–140 ns. At 0.7–0.85 of the implosion time a global m=0 instability with a wavelength of 1.7–2.3 mm was detected in soft x-ray gated images, laser probing, and optical streaks. The time when the instability reaches the observable level corresponds to the number of e-foldings for the growth of the classical Rayleigh–Taylor instability of ∫γ dt∼5.6–7. The scaling of this number with the number of wires is consistent with the instability growth from the seed level determined by the averaging of uncorrelated density perturbations in individual wires. Preliminary results from a 4×4 array permit the simultaneous observation by laser probing of the characteristic bubble and spike structure of the magneto Rayleigh–Taylor instability.
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