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The aim of this work is to produce and study a high energy density laboratory plasma relevant to astrophysical accretion disks. To this end, an experimental setup based on a modified cylindrical wire array was devised, which employs a cusp magnetic field to introduce angular momentum into the system. The setup was studied numerically with the three-dimensional, resistive magneto-hydrodynamic code GORGON. Simulations show that a differentially-rotating flow is formed, with typical rotation velocity and Mach number values of 60 km/s and Mφ ∼ 5 respectively. The plasma is radiatively cooled and presents a Reynolds number higher than 107. In addition, the magnetic Reynolds number and the plasma β are >1. Such a plasma is of interest for the study of hydrodynamic and magneto-hydrodynamic instabilities, and turbulence generation in differentially-rotating plasma flows.

Absorption spectroscopy was applied to wire-array Z-pinches on the 1 MA pulsed-power Zebra generator at the Nevada Terawatt Facility (NTF). The 50 TW Leopard laser was coupled with the Zebra generator for X-ray backlighting of wire arrays at the ablation stage. Broadband X-ray emission from a laser-produced Sm plasma was used to backlight Al star wire arrays in the range of 7–9 A. Two time-integrated X-ray conical spectrometers recorded reference and absorption spectra. The spectrometers were shielded from the bright Z-pinch X-ray burst by collimators. The comparison of plasma-transmitted spectra with reference spectra indicates absorption lines in the range of 8.1–8.4 A. Analysis of Al K-shell absorption spectra with detailed atomic kinetics models shows a distribution of electron temperature in the range of 10–30 eV that was fitted with an effective two-temperature model. Temperature and density distributions in wire-array plasma were simulated with a three-dimension magneto-hydrodynamic code. Post-processing of this code’s output yields synthetic transmission spectrum which is in general agreement with the data.

Coiled arrays, which are cylindrical arrays in which each wire is formed into a helix, suppress the modulation of ablation at the fundamental wavelength. Outside the vicinity of the wire cores, ablation flow from coiled arrays is modulated at the coil wavelength and has a two-stream structure in the r, thetas plane. Within the vicinity of the helical wires, ablation is concentrated at positions with the greatest azimuthal displacement, and plasma is axially transported from these positions such that the streams become aligned with the sections of the coil furthest from the array axis. The GORGON MHD code accurately reproduces this observed ablation structure, which can be understood in terms of J times B forces that result from the interaction of the global magnetic field with a helical current path as well as additional current paths suggested by the simulations. With this ability to control where the ablation streamers occur, large wavelength coils were constructed such that the breaks that form in the wires had sufficient axial separation to prevent perturbations in the implosion sheath from merging. This produced a new mode of implosion in which the global instability can be controlled, and perturbations correlated between all wires in an array. For large-wavelength eight-wire coiled arrays, this produced a dramatic increase in X-ray power, equaling that of a 32-wire straight array. These experiments were carried out on the mega ampere generator for plasma implosion experiments (1 MA, 240 ns) at Imperial College London, London, U.K.

Collimated flows ejected from young stars are believed to play a vital role in the star formation process by extracting angular momentum from the accretion disk. We discuss the first experiments to simulate rotating radiatively cooled, hypersonic jets in the laboratory. A modification of the conical wire array $z$-pinch is used to introduce angular momentum into convergent flows of plasma, a jet-forming standing shock and into the jet itself. The rotation of the jet is evident in laser imaging through the presence of discrete filaments which trace the rotational history of the jet. The presence of angular momentum results in a hollow density profile in both the standing conical shock and the jet.

Jets and outflows are thought to be an integral part of accretion phenomena and are associated with a large variety of objects. In these systems, the interaction of magnetic fields with an accretion disk and/or a magnetized central object is thought to be responsible for the acceleration and collimation of plasma into jets and wider angle flows. In this paper we present three-dimensional MHD simulations of magnetically driven, radiatively cooled laboratory jets that are produced on the MAGPIE experimental facility. The general outflow structure comprises an expanding magnetic cavity which is collimated by the pressure of an extended plasma background medium, and a magnetically confined jet which develops within the magnetic cavity. Although this structure is intrinsically transient and instabilities in the jet and disruption of the magnetic cavity ultimately lead to its break-up, a well collimated, knotty jet still emerges from the system; such clumpy morphology is reminiscent of that observed in many astrophysical jets. The possible introduction in the experiments of angular momentum and axial magnetic field will also be discussed., Comment: 15 pages, 4 figures, accepted by Astrophysics and Space Science for Special Issue High Energy Density Laboratory Astrophysics Conference

The SPHINX facility (with 1-mus rise time and 5 MA) has been developed for soft X-ray production at the Centre d'Etudes de Gramat, Gramat, France. The first experiments were performed with aluminum nested wire arrays Z-pinches for K-shell production. A K-shell yield of up to 25 kJ and the full-width at half-maximum of an X-ray pulse of about 50 ns were obtained with an 800-ns implosion time and a 4-MA peak current. The specific features observed during these experiments are discussed. Inner-array dynamics highlights the fact that resistivity should not be neglected in the analysis. The effect of the return-current housing on the array dynamics, as well as the ways to mitigate it and control its impact on the radiation pulse, is also examined

A review of recent experiments on the MAGPIE generator (1 MA, 250 ns) aimed at studying the implosion dynamics of wire array Z-pinches is presented. The first phase of implosion is dominated by the gradual ablation of stationary wire cores and gradual redistribution of the array mass by the precursor plasma flow. It is found that the rate of wire ablation depends on the magnitude of the global (collective) magnetic field of the array, and increases with the field. The existence of the modulation of the ablation rate along the wires leads to the presence of a 'trailing' mass left behind by the imploding current sheath. The trailing mass provides an alternative path for the current, reducing the force available for compression of the pinch at stagnation. The observed dependence of the ablation rate on inter-wire separation suggests an explanation for the existence of the optimal wire number maximizing the x-ray power. Axially resolved spectroscopy shows the presence of the x-ray 'bright' spots (< 150 µm) emitting intense continuum radiation.

We report on experiments on the MAGPIE generator (1 MA, 250 ns) to study the physics of plasma formation in wire array Z-pinches. It is shown that the gradual redistribution of the array mass by the precursor plasma flow from wire cores plays a very important role in implosion dynamics. It is found that the rate of wire ablation depends on the magnitude of the global (collective) magnetic field of the array and increases with the field. Due to this dependence, the azimuthal variations of the global magnetic field affect the implosion dynamics. The existence of the modulation of the ablation rate along the wires leads to the presence of a 'trailing' mass left behind by the imploding current sheath. The trailing mass provides an alternative path for the current, reducing the force available for compression of the pinch at stagnation. The observed dependence of the ablation rate on inter-wire separation suggests an explanation for the existence of the optimal wire number maximizing the x-ray power.

Progress in understanding the physics of dynamic-hohlraums is reviewed for a system capable of generating 13 TW of axial radiation for high temperature (>200 eV) radiation-flow experiments and ICF capsule implosions.

Jet research has long relied upon a combination of analytical, observational and numerical studies to elucidate the complex phenomena involved. One element missing from these studies (which other physical sciences utilize) is the controlled experimental investigation of such systems. With the advent of high-power lasers and fast Z-pinch machines it is now possible to experimentally study similar systems in a laboratory setting. Such investigations can contribute in two useful ways. They can be used for comparison with numerical simulations as a means to validate simulation codes. More importantly, however, such investigations can also be used to complement other jet research, leading to fundamentally new knowledge. In the first part of this article, we analyze the evolution of magnetized wide-angle winds in a collapsing environment. We track the ambient and wind mass separately and describe a physical mechanism by which an ionized central wind can entrain the ambient gas giving rise to internal shells of molecular material on short time scales. The formation of internal shells in molecular outflows has been found to be an important ingredient in describing the observations of convex spurs in P-V diagrams (Hubble wedges in M-V diagrams). In the second part, we present astrophysically relevant experiments in which supersonic jets are created using a conical wire array Z-pinch. The conically convergent flow generates a standing shock around the axis which collimates the flow into a Mach ∼ 30 jet. The jet formation process is closely related to the work of Canto et al. (1988) for hydrodynamic jet collimation. The influence of radiative cooling on collimation and stability is studied by varying the wire material (A1, Fe, and W).

Experiments on the implosion dynamics of single and nested nickel wire arrays driven by the MAGPIE generator with a 1-MA, 240-ns current pulse are reported. Like aluminum and tungsten wire discharges, the plasma formed from individual wires has a core-corona structure. In a single nickel array, a wire core size of 75 /spl mu/m before the implosion was measured by X-ray radiography. The precursor plasma is formed on axis and is m=1 unstable. The implosion of the array occurs later than predicted by the 0-D model, indicating that a fraction of current flows through the precursor plasma. This is in contrast with copper arrays (copper has the similar radiation properties as nickel, Z/sub nickel/=28, Z/sub copper/=29) where the precursor plasma is uniform and implosion time is as predicted by 0-D model. Experiments with nickel nested arrays show that the expansion of the wires in the outer and the inner array is similar and a precursor plasma is formed on the axis earlier than in single arrays. It appears from the experimental data that a significantly larger fraction of the current is flowing through the inner array.

Supersonic jet production in conical wire array Z-pinches is modeled using a two-dimensional (2D) resistive magneto-hydrodynamic (MHD) code. In conical wire arrays, the converging plasma ablated from the wires stagnates on axis, forming a standing conical shock which redirects and collimates the flow into a jet. As the jet exits the collimator shock, it is radiatively cooled and accelerated by the steep thermal gradients present. Purely hydrodynamic simulations using conditions relevant to the MAGPIE facility show good agreement with the experiments (Lebedev et al., 2002), indicating that narrow, high Mach number (M ∼ 20), radiatively cooled tungsten jets of astrophysical relevance can be obtained. To investigate the effects of lower radiative cooling on jet collimation, we modeled an aluminum conical wire array. When radiative losses are less significant, lower Mach number (M ∼ 10), less collimated jets are obtained. MHD simulations relevant to the “Z” facility were carried out to investigate the scaling of jet parameters. The resulting hypersonic (M ∼ 40), high density jets should allow the investigation of a wider range of astrophysical jet conditions.

Experiments to study plasma formation and implosion dynamics of wire array z pinches performed on MAGPIE generator (1.4 MA, 240 ns) at Imperial College are reviewed. Data from laser probing and X-ray radiography show that heterogeneous plasma structure with dense wire cores surrounded by low-density coronal plasma persists in wire arrays for a significant part of the implosion. Early implosion of the coronal plasma produces a precursor plasma column on the array axis, parameters of which depend on the rate of radiative cooling. 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 J X B force. The spatial scale of these perturbations ( 0.5 mm for Al, 0.25 mm for W) is determined by the size of the wire cores ( 0.25 mm for Al, ∼0.1 mm for W). A qualitative change in implosion dynamics, with transition to 0-D-like trajectory, was observed in Al arrays when the ratio of interwire gap to wire core size was decreased to ∼3. In experiments with nested wire arrays, two different modes of operation were identified, both giving significant sharpening of the X-ray pulse ( 10 ns) in comparison with a single array, despite the small number of wires in the arrays (16 outer, 16 inner) and the long implosion time (260 ns).

A series of specialized multidimensional resistive magnetohydrodynamic (MHD) models have been developed to tackle the different phases of evolution of wire array z-pinch implosions. Two-dimensional (r–z) “cold-start” or “wire initiation” simulations of single wires indicate the persistence of a two-component structure with a cold, dense core embedded within a much hotter, low density, m = 0 unstable corona. Cold-start simulations with similar conditions to wires in an array show a general trend in the plasma structure from discrete wires with large m = 0 perturbation amplitudes to partially merged wires with smaller perturbation amplitudes as the number of wires is increased. Two-dimensional (r–θ) simulations then show how the persistence of dense wire cores results in the injection of material between the wires into the interior of the array, generating radial plasma streams which form a precursor plasma upon reaching the axis. Higher-resolution 2-D (r–θ) simulations show similar behavior for large number wire arrays in use at Sandia National Laboratories. This model is also used to predict which modes of implosion are in operation in nested wire array experiments. Separate r–θ plane simulations of the flux of plasma imploding towards the axis from the outer array and the bombardment of the inner array by this flux are presented. Finally, 2-D (r–z) simulations of the Rayleigh–Taylor instability during the final implosion phase are used to illustrate the effect upon the power and duration of the radiation output pulse. The results of low-resolution 3-D resistive MHD simulations are also presented. The need for much higher resolution 3-D simulations of certain aspects of wire array evolution is highlighted.

Two different modes of nested wire array implosion driven by a 1-MA, 240-ns current pulse were observed, determined by the fraction of total current induced in the inner array. Penetration by the outer array through the inner with switching of current occurred if current in the inner array was initially suppressed. Simultaneous implosion of arrays with apparent compression of magnetic flux between the arrays was observed if $\ensuremath{\sim}20%$ of the current was in the inner array. In both cases the x-ray pulse rise time of $\ensuremath{\sim}10\mathrm{ns}$ (for 260-ns implosion time) was considerably smaller than for a single array.

A series of experiments has been carried out on the Mega Ampere Generator for Plasma Implosion Experiments (MAGPIE) generator in order to study the dynamics of carbon fiber Z-pinches. The generator was operated at 1.4 MV, with a peak current of 1 MA, and a rise time of 150 ns. In some shots, a current prepulse of about 30 kA was provided to study its influence on the dynamics of the fiber pinch. Carbon fibers of 7, 33, and 300 /spl mu/m diameter were used during these experiments. The diagnostics employed were a self-referencing interferometer, a two-frame Schlieren system, an optical streak camera, and a four-frame X-ray framing camera. A novel feature of these measurements is the employment of an optical streak camera with a set of four slits arranged along the fiber axis and displaced in the radial direction. This permitted the study of the temporal evolution (axial and radial) of the plasma regions emitting in the visible part of the spectra. Correlation between these regions of the plasma and the location of X-ray hot spots is discussed. In carbon fibers of 33 pm diameter, the radial expansion velocity measured from Schlieren images was 3.6/spl times/10/sup 6/ cm/s and 5.5/spl times/10/sup 6/ cm/s for shots with and without prepulse, respectively. The dominant axial wavelengths of instabilities in the coronal plasma were between 0.05 and 0.2 cm, which correspond to ka values between 10 and 20, where k is the wavenumber of the instability and a is its amplitude. The dynamics of carbon fibers of different diameters are compared.

An experimental technique for the detection and time-resolved characterisation of hard X-radiation from a Z-pinch is described. The detectors employed consist of combinations of plastic scintillators and photomultipliers tubes coupled through optical fiber. The scintillators were fitted with thick aluminium and lead filters and cross filtering is then used to obtain the energy of the X-rays. It was found that the X-rays can reach energies of up to 4 MeV, much higher than the applied voltage. The observed radiation intensity varied with the disruption time lag.

The structure of magnetic fields was investigated in stagnated wire-array $Z$ pinches using a Faraday rotation diagnostic at the wavelength of 266 nm. The distribution of current in the pinch and trailing material was reconstructed. A significant part of current can switch from the main pinch to the trailing plasma preheated by x-ray radiation of the pinch. Secondary implosions of trailing plasma generate kinetic energy and provide enhanced heating and radiation of plasma at stagnation. Hot spots in wire-array $Z$ pinches also provide enhanced radiation of the $Z$ pinch. A collapse of a single hot spot radiates 1$%$--3$%$ of x-ray energy of the $Z$ pinch with a total contribution of hot spots of 10$%$--30$%$.

X-pinch radiography was used to analyze the interaction between streams of coronal plasma and on-axis foam targets in wire array z-pinch experiments on the MAGPIE generator (1 MA,240 ns). The implosion of the x-pinch, used in place of a current return conductor to the load, provided a short (

High-resolution laser probing diagnostics at a wavelength of 266 nm allow observation of the internal structure and instabilities in dense stagnated $Z$ pinches, typically hidden by trailing material. The internal structure of the 1-MA $Z$ pinch includes strong kink and sausage instabilities, loops, flares, and disruptions. Mid- and small-scale density perturbations develop in the precursor and main pinch. The three-dimensional shape and dynamics of the wire-array $Z$ pinch are predetermined by the initial configuration of the wire array. Cylindrical, linear, and star wire-array $Z$ pinches present different sets of instabilities seeded to the pinch at the implosion stage. Prolonged implosion of trailing mass can enhance x-ray production in wire arrays. Fast plasma motion with a velocity g100 km/s was observed in the $Z$ pinch at stagnation with two-frame shadowgraphy. Development of instabilities in wire arrays is in agreement with three-dimensional magnetohydrodynamic simulations.

Determination of B-field structures in pulsed power driven exploding wire experiments is vital to recover detailed information about the evolution, driving mechanisms of ablation, and subsequent instability development, but is complicated by the presence of large volumes of hot, dense plasma. Optical and electrical probe diagnostics typically fail early in the experiment. We present progress on a new project, which examines the use of proton deflectometry to measure magnetic fields in pulsed power plasmas.

Summary form only given. A Thomson scattering diagnostic has been developed to probe plasmas produced using the MAGPIE generator (1.4MA, 240ns). The diagnostic uses a frequency doubled Nd:YAG laser delivering ∼2–4J in 8ns FWHM to the target plasma. The density and temperature of the wire array targets probed produces scattered light in the collective regime (α>1). Any bulk motion of the probed plasma Doppler shifts the scattered spectrum allowing the velocity of the probed plasma to be measured. Measurements of the ablation velocity and the implosion velocity in cylindrical wire arrays at different positions in the arrays and for different wire materials are presented. In addition, the ion acoustic features in this scattering regime can be used to take measurements of electron and ion temperatures in e.g. precursor plasmas.

Wire-array plasmas were investigated in the nonradiative ablation stage via x-ray absorption spectroscopy. A laser-produced Sm plasma was used to backlight Al wire arrays. The Sm spectrum was simultaneously observed by two spectrometers: one recorded the unattenuated spectrum and the other the transmission spectrum with 1.45-1.55 keV K-shell absorption lines. Analysis of absorption spectra revealed electron temperature in the range of 10-30 eV and the presence of F-, O-, N- and C-like Al ions in the absorbing plasma. A comparison of this electron temperature with the postprocessed absorption spectra of a 2D MHD simulation yields results in general agreement with the data analysis.

The magnetohydrodynamic behavior of tungsten wire ablating in wire-array Z pinch discharge on MAGPIE is simulated in a two-dimensional fine-grid domain using the GORGON code. A nonideal resistivity model has been implemented in the simulation to obtain plasma transport coefficients in the high density regime along with a screened hydrogenic model to calculate the radiative cooling. Starting from the initial state of warm dense plasma, the evolution of ablated wire is demonstrated to show its explosion and implosion dynamics as a function of discharge time and then the computed profile of electron density is compared with the contour lines reproduced from the measurement by a laser interferometer during the early stage of discharge. The comparison overall shows a fair agreement in terms of the magnitude and the profile shape while some discrepancies can be attributed to the simplified description of the internal wire core physics (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Radiative blast waves can be created by focusing intense laser pulses into highly absorbing clustered gases. By considering the plasma conditions these shocks can be categorized as optically thin radiative shocks, a regime of particular interest for laboratory astrophysics experiments. A periodic spatial modulation is introduced to the shock front in order to investigate instability and shock collisions. Hydrodynamic simulations are presented which are in qualitative agreement with the experimental results. A technique to perform a single shot measurement of the entire shock trajectory and the possibility to detect oscillations in the shock velocity is discussed. © 2009 Elsevier B.V.

We present experimental results on the formation of supersonic, radiatively cooled jets driven by pressure due to the toroidal magnetic field generated by the 1.5 MA, 250 ns current from the MAGPIE generator. The morphology of the jet produced in the experiments is relevant to astrophysical jet scenarios in which a jet on the axis of a magnetic cavity is collimated by a toroidal magnetic field as it expands into the ambient medium. The jets in the experiments have similar Mach number, plasma beta and cooling parameter to those in protostellar jets. Additionally the Reynolds, magnetic Reynolds and Peclet numbers are much larger than unity, allowing the experiments to be scaled to astrophysical flows. The experimental configuration allows for the generation of episodic magnetic cavities, suggesting that periodic fluctuations near the source may be responsible for some of the variability observed in astrophysical jets. Preliminary measurements of kinetic, magnetic and Poynting energy of the jets in our experiments are presented and discussed, together with estimates of their temperature and trapped toroidal magnetic field., Comment: 7 pages, 6 figures, accepted for publication in Astrophysics & Space Science

Collimated outflows (jets) are ubiquitous in the universe, appearing around sources as diverse as protostars and extragalactic supermassive black holes. Jets are thought to be magnetically collimated, and launched from a magnetized accretion disk surrounding a compact gravitating object. We have developed the first laboratory experiment to address time-dependent, episodic phenomena relevant to the poorly understood jet acceleration and collimation region (Ciardi et al., 2009). The experiments were performed on the MAGPIE pulsed power facility (1.5 MA, 250 ns) at Imperial College. The experimental results show the periodic ejections of magnetic bubbles naturally evolving into a heterogeneous jet propagating inside a channel made of self-collimated magnetic cavities. The results provide a unique view of the possible transition from a relatively steady-state jet launching to the observed highly structured outflows.

International audience; Astrophysical observations have demonstrated many examples of bow shocks, for example, the head of protostellar jets or supernova remnants passing through the interstellar medium or between discrete clumps in jets. For such systems where supersonic and super-Alfvénic flows and radiative cooling are all important, carefully scaled laboratory experiments can add insight into the physical processes involved. The early stage of a wire array z-pinch implosion consists of the steady ablation of material from fine metallic wires. Ablated material is accelerated toward the array axis by the J×B force. This flow is highly supersonic (M>5) and becomes super-Alfvénic (MA>2). Radiative cooling is significant in this flow and can be controlled by varying the material in the ablated plasma. The introduction of wires as obstructions in this steady flow leads to the formation of bow shocks, which can be used as a laboratory testbed for astrophysical bow shocks. The magnetic field associated with this obstruction wire can be controlled by varying the current through it. Differences in the shock for different cooling rates and different magnetic fields associated with the obstruction will be discussed, along with comparisons of dimensionless parameters in the experiments to astrophysical systems.

Radial wire arrays offer the potential for higher energy density of radiated x-rays compared to cylindrical arrays. Higher radiated energy density would allow for more compact hohlraums thus easing power requirements for ICF. In an effort to explore how radiated power scales with the rise-rate of the current we performed experiments on two 1 MA pulsed power generators with very different rise rates∔ MAGPIE at Imperial College with about 300 ns rise time and COBRA at Cornell with about 100 ns rise time. Comparisons of radiated power from 16-wire Al and Cu arrays over a range of masses are presented. Results from these initial experiments suggest that radiated power for a given material is similar when implosion times are matched to the current rise time. We also present comparisons of radiated power from modified array geometries such as wire length and cathode diameter. Some results from Ti/Ni and Fe/Ni arrays are also presented.

Magneto-hydrodynamic simulations provide a powerful tool for improving our understanding of the complex physical processes underlying the behavior of wire array Z-pinches. With the advent of large scale parallel computing and three dimensional models, it is now becoming possible to model the entire plasma volume with sufficient resolution to capture the important physical phenomena, from the end of the initiation phase right through stagnation. In this talk we use the 3D MHD code Gorgon to show how the growth of separate m=0 like instabilities on each wire eventually merge together to form a global Rayleigh-Taylor instability structure during the implosion of the array as a whole. Detailed comparisons with soft X-ray images and laser interferometry from experiments on the MAGPIE generator are used to bench-mark the model. Results from similar calculations for arrays on the Z generator are compared to the plasma structure observed in radiography and laser shadowgraphy data. To resolve all of the important features, these Z simulations require up to 700 million computational elements and typically run for 3 weeks on up to 400 processors. By analyzing the energy equation within the code, the relative contributions of kinetic energy, Poynting flux, etc. to the production of radiation can be assessed and the limitations imposed by opacity effects and losses in the driving circuit can be highlighted. The application of the code to other pulsed power configurations such as radial and helical wire arrays, X-pinches and laboratory astrophysics experiments is also discussed.

The limited understanding of the mechanisms driving the mass ablation rate of cylindrical wires arrays is presently one of the major limitations in predicting array performance at the higher current levels required for inertial confinement fusion (ICF) ignition. Continued investigation of this phenomenon is crucial to realize the considerable potential for wire arrays to drive both ICF and inertial fusion energy, by enabling a predictive capability in computational modeling. We present the first study to directly compare the mass ablation rates of wire arrays as a function of the current rise rate. Formation of the precursor column is investigated on both the MAPGIE (1 MA, 250ns [Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)]) and COBRA (1 MA, 100ns [Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)]) generators, and results are used to infer the change in the effective ablation velocity induced by the rise rate of the drive current. Laser shadowography, gated extreme ultraviolet (XUV) imaging, a...

An inverse (exploding) wire array configuration, in which the wires form a cylinder around a current carrying electrode on axis, was used to study the ablation phase of the wires. This configuration allows the parameters of the plasma from individual wires of the array to be measured as the ablated plasma streams propagate in the outward radial direction. The density distribution and the evolution of the natural mode of modulation of the ablation flow was measured with interferometry and soft x-ray imaging. Measurements of the voltage across the array, which in this configuration is determined by the private magnetic flux around the individual wires, allow information on the localization of the current to be obtained. Results are compared to three-dimensional magnetohydrodynamics simulations.

Coiled arrays, a cylindrical array in which each wire is formed into a helix, suppress the modulation of ablation at the fundamental wavelength. Outside the vicinity of the wire cores, ablation flow from coiled arrays is modulated at the coil wavelength and has a 2-stream structure in the r,thetas plane. Within the vicinity of the helical wires, ablation is concentrated at positions with the greatest azimuthal displacement and plasma is axially transported from these positions such that the streams become aligned with sections of the coil furthest from the array axis. The GORGON MHD code accurately reproduces this observed ablation structure, which can be understood in terms of axial JxB forces that result from the interaction of the global magnetic field with radial components of a helical current path as well as additional current paths suggested by the simulations. With this ability to control where ablation streamers occur, large wavelength coils were constructed such that the breaks that form in the wires had sufficient axial separation to prevent perturbations in the implosion sheath from merging. This produces a new, organised mode of implosion in which the global instability can be controlled and the perturbations correlated between all the wires in the array. For large wavelength 8-wire coiled arrays, this produced a dramatic increase in x-ray power, equaling the x-ray power of a 32-wire straight array. These experiments were carried out on the MAGPIE generator (IMA, 240ns) at Imperial College, London.

A new wire-array configuration has been used to control the modulation of ablated plasma flow for the first time. Cylindrical aluminum coiled arrays, in which each straight wire is replaced with a single helix, were driven by a 1 MA, 240 ns current pulse. Ablated plasma is directed away from the coiled wire cores in a manner that can be understood in terms of Lorentz forces that arise from a complex current path modeled by 3D magnetohydrodynamic simulations. Outside the diameter of the helix, the flow of ablated plasma is axially modulated at the wavelength of the coil.

Two- and three-dimensional MHD simulations are used to provide a theoretical description of 2 wire molybdenum X-pinch experiments. The initial evolution from solid wires to the formation of supersonic jets and a central micro-Z pinch is found to result from the slow rate of wire ablation and from the distribution of the Lorentz force. The growth of m=0 instabilities triggers the formation of micron sized regions of intense x-ray emission with plasma pressures in the Gbar range. A simple analytical model is used to predict how the maximum density and temperature scale with material and current.

This paper summarizes the present understanding of the processes leading to precursor column formation in cylindrical wire arrays on the $1\phantom{\rule{0.3em}{0ex}}\mathrm{MA}$ MAGPIE generator at Imperial College London. Direct experimental measurements of the diameter variation during the collapse and formation phase of the precursor column are presented, along with soft x-ray emission, and quantitative radiography. In addition, data from twisted cylindrical arrays are presented which give additional information on the behavior of coronal plasma generated in wire array $z$ pinches. Three stages in precursor column formation are identifiable from the data: broad initial density profile, rapid contraction to small diameter, and slow expansion after formation. The correlation of emission to column diameter variation indicates the contraction phase is a nonlinear collapse resulting from the increasing on-axis density and radiative cooling rate. The variation in the minimum diameter is measured for several array materials, and data show good agreement with a pressure balance model. Comparison of column expansion rates to analytical models allows an estimate of column temperature variation, and estimates of the current in the column are also made. Formation data are in good agreement with both fluid and kinetic modeling, but highlight the need to include collisionless flow in the early time behavior.

The twisting of magnetic fields threading an accretion system can lead to the generation on axis of toroidal field loops. As the magnetic pressure increases, the toroidal field inflates, producing a flow. Collimation is due to a background corona, which radially confines this axially growing “magnetic tower”. We investigate the possibility of studying in the laboratory the dynamics, confinement and stability of magnetic tower jets. We present two-dimensional resistive magnetohydrodynamic simulations of radial arrays, which consist of two concentric electrodes connected radially by thin metallic wires. In the laboratory, a radial wire array is driven by a 1 MA current which produces a hot, low density background plasma. During the current discharge a low plasma beta (β < 1), magnetic cavity develops in the background plasma (β is the ratio of thermal to magnetic pressure). This laboratory magnetic tower is driven by the magnetic pressure of the toroidal field and it is surrounded by a shock envelope. On axis, a high density column is produced by the pinch effect. The background plasma has ≳1, and in the radial direction the magnetic tower is confined mostly by the thermal pressure. In contrast, in the axial direction the pressure rapidly decays and an elongated, well collimated magnetic-jet develops. This is later disrupted by the development of m = 0 instabilities arising in the axial column.

We present new data from High-Energy Density (HED) laboratory experiments designed to explore the interaction of a heavy hypersonic radiative jet with a cross wind. The jets are generated with the MAGPIE pulsed power machine where converging conical plasma flows are produced from a cylindrically symmetric array of inclined wires. Radiative hypersonic jets emerge from the convergence point. The cross wind is generated by ablation of a plastic foil via soft-X-rays from the plasma convergence region. Our experiments show that the jets are deflected by the action of the cross wind with the angle of deflection dependent on the proximity of the foil. Shocks within the jet beam are apparent in the data. Analysis of the data shows that the interaction of the jet and cross wind is collisional and therefore in the hydrodynamic regime. We consider the astrophysical relevance of these experiments applying published models of jet deflection developed for AGN and YSOs. We also present results of 3-D numerical simulations of jet deflection using a new astrophysical Adaptive Mesh Refinement code. These simulations show highly structured shocks occurring within the beam similar to what was observed in the experiments.

Nested tungsten wire arrays (20-mm on 12-mm diam.) are shown for the first time to operate in a current-transfer mode at 16-19 MA, even for azimuthal interwire gaps of 0.2 mm that are the smallest typically used for any array experiment. After current transfer, the inner wire array shows discrete wire ablation and implosion characteristics identical to that of a single array, such as axially nonuniform ablation, delayed acceleration, and trailing mass and current. The presence of trailing mass from the outer and the inner arrays may play a role in determining nested array performance.

Summary form only given, as follows. Wire array Z-pinches are extremely efficient sources of high power soft X-ray pulses (up to 280 TW) for energising hohlraums for inertial confinement fusion and other high energy density physics applications. The main process affecting quality of the implosion and X-ray power at stagnation is development of the Rayleigh-Taylor instability in a plasma accelerated by the magnetic pressure. In this paper we present experimental results on Rayleigh-Taylor instability growth in wire array Z-pinches driven by a 1.4 MA, 240 ns current pulse. Al, W and Ti wire arrays of 16 mm diameter with between 8 and 64 wires were studied. Comparison of the instability development for different wire materials and wire number will be presented. The radial structure of the instability was observed by laser probing and soft X-ray imaging in implosions of arrays consisting of four groups of four closely spaced wires (4/spl times/4 wire array). Experiments to study the instability development in nested-wire arrays and effect of the B/sub z/ magnetic field induced by wires twisting will be presented.

Summary form only given, as follows. Recent experiments on the MAGPIE generator at Imperial College have demonstrated the complex three dimensional nature of wire array Z-pinch implosions. We have developed a 3D resistive MHD code, in order to model these experiments. Running the code in 2D(x,y) geometry with high spatial resolution, reproduces the collapse dynamics, radial plasma streaming and precursor formation observed experimentally. The persistence of cold, unionized, wire cores dominates the collapse dynamics. Lower resolution 3D runs illustrate the differences in the behavior of the Rayleigh-Taylor instability in wire arrays compared to uniform shell implosions. Results of simulations of single and nested wire array experiments at Sandia National Laboratories will also be presented.

Summary form only given. Details are presented of a new approach to indirect drive inertial confinement fusion which makes use of highly supersonic, radiatively cooled, slugs of plasma to energise a hohlraum. Such projectiles could be formed by, for example, the implosion of a conical tungsten wire array (in a similar fashion to radiatively cooled jets]) or by using a radial wire array accelerated along a coaxial plasma gun. In each case, substantial radiative cooling of the tungsten plasma at the launch point results in very high Mach numbers and low divergence. 2D resistive MHD simulations are presented which suggest that a 'Z' class machine can produce a mm sized slug of a few ns duration containing several hundred kJ of kinetic energy. This object therefore has the potential to deliver /spl sim/100 TW of kinetic flux (similar to a singly charged heavy ion beam of /spl sim/100 MA current) into a hohlraum of small dimension. Simulations of the interaction of such a plasma slug with a converter foil show efficient conversion of kinetic energy into radiation. 3D view-factor calculations of a roughly NIF-scale cylindrical hohlraum with twin converter foils are then used to establish the drive temperature and symmetry for capsule implosion experiments. The potential advantages of the system for the ignition of relatively small capsules on modest sized facilities with the potential for limited stand-off between the driver and the target are discussed.

Summary form only given. We present the results of two dimensional magnetohydrodynamic simulations of nested array collisions. Results for nested tungsten arrays on the "Z" generator at Sandia National Laboratories, show that elements of all three modes are at work. The fraction of the circumference occupied by the exploded inner array wires determines the fraction of momentum transferred between arrays at collision. For large numbers of wires in the inner array, the momentum transfer is large and a hydrodynamic collision mode results. For much lower numbers of wires in the inner, the outer array material is free to pass through the gaps and a transparent inner mode results. In all cases, flux compression is found to enhance the level of momentum transfer. A zero dimensional model was modified to mock-up the different effects of nested array interactions and used to provide scaling of X-ray power with array diameters and masses. Maximum powers were obtained for the transparent inner mode with light inner arrays. However some artificial mechanism for limiting current flow in the inner array was required since flux compression appears to excite the Rayleigh-Taylor instability in the inner array.

We present a new approach to indirect-drive inertial confinement fusion which makes use of highly supersonic, radiatively cooled, slugs of plasma to energize a hohlraum. 2D resistive magnetohydrodynamic simulations of slug formation in shaped liner Z-pinch implosions are presented along with 2D-radiation-hydrodynamic simulations of the slug impacting a converter foil and 3D-view-factor simulations of a double-ended hohlraum. Results for the Z facility at Sandia National Laboratory indicate that two synchronous slugs of 250 kJ kinetic energy could be produced, resulting in a capsule surface temperature of approximately 225 eV.

The results of experiments that changed the current distribution inside the radius of an array, or between the wires of an array, are presented. Use of a low number of wires (4), Ni wires, or the introduction of a pre-pulse current to the array all resulted in the pre-cursor plasma on axis displaying MHD like instabilities. Implosion dynamics were affected in a consistent manner, with the arrays imploding later than expected. In arrays made from alternating Al and W wires, current was initially concentrated around the Al wires. When the Al wires started to implode, current was switched to the W wires, producing a 2 stage implosion.

A radiation source has been developed on the 20-MA Z facility that produces a high power x-ray pulse (9.7±1.8 TW, 52±10 kJ), generated primarily from the interior of an imploding dynamic-hohlraum target centered along the z-axis. The radiation pulse, characterized here together with its underlying dynamics, is used for performing radiation-flow and ICF experiments with drive temperatures in excess of 200 eV.

An effect of the global magnetic field on the ablation rate of wire cores in wire array Z-pinch experiments was studied using wire arrays with fixed wire number but different array diameter. The data suggest that the ablation rate is higher for the arrays with smaller diameter, when the global magnetic field is larger. A model for the scaling of the ablation rate with array parameters is also presented.

During the early stages of a wire-array Z-pinch implosion, low density plasma streams toward the axis by virtue of the Lorentz force. This streaming precursor plasma is initially highly collisionless and therefore cannot be modeled using standard fluid theory. A hybrid method (fluid electrons and particle ions) capable of modeling collisionless behaviour is presented. We show that the axial stagnation of the plasma flow occurs once the density becomes sufficiently high to initiate a non-linear rise in collisionality. Radiation and electron-ion energy exchange effects then result in the attainment of a dense, long-lived precursor column on axis, as observed experimentally. The column is held in place by the kinetic pressure of the streaming precursor plasma, which is balanced by the thermal pressure of the plasma in the column at the column's edge.

A review of recent experiments at the MAGPIE facility (1MA, 250ns) at Imperial College is presented. The experiments show that the core-corona structure of the plasma, combined with the 3-D topology of the magnetic field in wire array z-pinches, results in the implosion dynamics being significantly different from that of a thin plasma shell. During the first ∼80% of the implosion time the interior of the array is gradually filled by the plasma ablated from the stationary wire cores. This phase ends with the formation of gaps in the wire cores, which occurs due to non-uniformity of the ablation rate along the wires. The final phase of the implosion, starting at this time, occurs as a rapid snowplough-like implosion of the radially distributed plasma, previously injected into 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 wire array implosions.