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We present data from experiments using nested wire-array Z -pinches where the wires of one of the arrays (the outer or inner array) are inclined to produce a cone. The use of these nested conical arrays can potentially provide a valuable tool in X-ray radiation pulse shaping for Z-pinch-driven high-yield inertial-confinement-fusion schemes. Conical nested arrays can produce a zippered implosion which broadens the main radiation pulse and, possibly, the pulse associated with the interaction of the two arrays. Results from experiments at current levels of 1 MA (240 ns) and 18 MA (100 ns) are presented and compared. Experiments at 1 MA with a conical outer array indicate broadening of the full-width at half-maximum of the main stagnation radiation pulse, with results at 20 MA showing a similar result. Conical inner arrays do not broaden the main radiation pulse because a longer inner array ablation time offsets the earlier interaction time of the outer array with the inner array. Although array dynamics data suggest that a conical inner or outer array can potentially provide control of the interaction radiation pulse, this was not observed. Observations are consistent with a wire-array trajectory model incorporating outer and inner array ablation, snowplow physics, and a simplified array interaction model.

We present the preliminary results from the first ablation studies of wire-array z-pinches using a short-pulse linear transformer driver (LTD) providing a current of 200 kA in 150 ns. Laser imaging is used to recover the ablation-flare wavelength for comparison with previous studies and to provide quantitative information on the axial-density variation of this structure. The results are used to determine the general form of the variation of the ablation velocity in the axial direction, and fits to data can infer the range of values required to describe observed modulation. In addition, conical arrays demonstrate the formation and propagation of moderate atomic-number plasma jets comparable with those observed in mega-ampere experiments.

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.

We report on the first investigations of the use of carbon fibers in an X-pinch load using a 250-kA linear transformer driver (LTD). Multiframe laser shadowgraphy is used to examine the evolution of the pinch and shows that carbon loads demonstrate wire expansion and cross-point pinch and gap formation as observed in X-pinches constructed from high Z materials. Radiographs taken using the carbon X-pinch as the source demonstrate both that sufficient flux is emitted to provide a good contrast image at source-to-image distances of > 10 cm and that the cross point produces a relatively small hot spot. Radiographs of a series of fine wires (5-30 mum) using X-rays > 500 eV demonstrated that 25-mum wires can be resolved in this energy range. Time-resolved X-ray emission measurements showed that, while emission in the hnu > 500 eV range shows long (> 100 ns) timescales, emission in the hnu > 1 keV range shows a multiple-peaked structure with durations as short as 20 ns.

Experiments were performed on an x-pinch using a pulsed power current generator capable of producing an 80 kA current with a rise time of 50 ns. Molybdenum wires with and without gold coating were employed to study the effect of high z coating on the low-density (

We present experimental data on the steady state deflection of a highly supersonic jet by a side-wind in the laboratory. The use of a long interaction region enables internal shocks to fully cross the jet, leading to the development of significantly more structure in the jet than in previous work with a similar setup (Lebedev et al., 2004). The ability to control the length of the interaction region in the laboratory allows the switch between a regime representing a clumpy jet or wind and a regime similar to a slowly varying mass loss rate. The results indicate that multiple internal oblique shocks develop in the jet and the possible formation of a second working surface as the jet attempts to tunnel through the ambient medium.

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

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.

This special issue on Z-pinch plasmas in the IEEE Transactions on Plasma Science is the seventh to be published [1] – [6] , starting with the first one with Guest Editors J. Davis and C. Deeney in 1998. The publication of so many special issues in just 20 years is a testament to the continued vitality and variety of research in the area of pulsed-power-driven loads. The present Special Issue grew out of interest to record the 10th Dense Z-Pinch Conference held in Stateline, NV, USA, during August 2017. In addition to papers from attendees at this conference, the submission was open to other scientists who wanted to archive their research. As a consequence, this Special Issue has the largest number of published papers compared to previous ones. In addition to authors from the USA, there are articles from the U.K., Russia, China, Hungary, the Czech Republic, and Chile. The topics in this collection of Z-pinch research cover a broad range of load designs and applications: liners and foils; wire arrays; X-pinches; gas puffs; dense plasma foci (DPF); diagnostics; and devices and pulsed-power facilities—comprising many theoretical and simulation studies together with experimental findings. Most of the papers arise from work or study with university class drivers, ~1 MA and 100–200-ns implosion times, and there are a few reports of pinches on multi-mega-ampere (MA) generators.

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 (

In this work we demonstrate a design for obtaining laser backlighting (e.g., interferometry) and time-resolved extreme ultraviolet self-emission images along the same line-of-sight. This is achieved by modifying a single optical component in the laser collection optics with apertures and pinhole arrangements suitable for single or multiple frame imaging onto a gated detector, such as a microchannel plate. Test results for exploding wire experiments show that machining of the optic does not affect the overall quality of the recovered laser images, and that, even with a multiple frame system, the area sacrificed to achieve collinear imaging is relatively small. The diagnostics can therefore allow direct correlation of laser and self-emission images and their derived quantities, such as electron density in the case of interferometry. Simple methods of image correlation are also demonstrated.

We report experimental evidence for a Rayleigh-Taylor-like instability driven by radiation pressure of an ultraintense (${10}^{21}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) laser pulse. The instability is witnessed by the highly modulated profile of the accelerated proton beam produced when the laser irradiates a 5 nm diamondlike carbon (90% C, 10% H) target. Clear anticorrelation between bubblelike modulations of the proton beam and transmitted laser profile further demonstrate the role of the radiation pressure in modulating the foil. Measurements of the modulation wavelength, and of the acceleration from Doppler-broadening of back-reflected light, agree quantitatively with particle-in-cell simulations performed for our experimental parameters and which confirm the existence of this instability.

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.

Accurately determining magnetic diffusion parameters in plasmas is important for benchmarking 3D MHD codes used for the design and interpretation of Z pinch and other plasma experiments. In this work, two planar arrays consisting of four wires each are at a fixed inter-plane spacing. The inter-wire spacing is then varied to alter the ratio of local to global magnetic field. This in turn determines the location of local precursor column structures, the current carrying regions, and the rate at which plasma may travel across the magnetic field toward the axis.

Summary form only given. Whilst the dynamical evolution of wire arrays is well understood, and multi-dimensional Magneto-Hydrodynamic (MHD) modeling has demonstrated significant progress, a comprehensive predictive capability has not been realized to date. Recent experimental investigations have highlighted the need to more closely examine the ablation structure and its dependence on the initial parameters of the array. In particular, the range over which the ablated plasma is accelerated, and hence extent to which magnetic flux is convected into the array, is often a disputed point in the comparison simulation and analytical work. Recent work at UC San Diego [ 1 ] uses a modification of the Lebedev rocket model of wire ablation to fit the range of ablation velocities which are observed in experiments. This analysis can be extended to infer the 2D distribution of effective velocities which redistribute the mass as a function of time. From this data it may be possible to direct observe the acceleration region in a wire array. An analysis of the effect of array geometry on the determined acceleration region will be attempted using interferographic and radiographic data from experiments at 200 kA to 1 MA. Conclusions and possible extensions to this work will be presented and discussed.

Previous experiments on the 1-MA “Zebra” generator at UNR studied precursor plasmas with Ni-60 alloy (96% Cu) wire arrays1. Those precursor plasmas were shown to consistently have electron temperatures > 400 eV 1. New recent experiments performed on Zebra using cylindrical wire arrays (CWA) continue to emphasize stronger radiating precursor plasma but from other mid-atomic number wire arrays. Time-integrated spatially-resolved (TISR) and time-gated spatially-integrated (TGSI) x-ray spectral data, time-integrated and time-gated pinhole x-ray images, shadowgraphy, as well as optical streak camera images were obtained during the recent mid-atomic number (Cu, brass, and alumel) CWA experiments. Preliminary results on these CWA are consistent with previous Ni-60 results. Non-LTE kinetic models of Cu, Zn, and Ni have been applied to account for the L-shell radiation from precursor and main x-ray burst plasmas. The resulting plasma parameters from modeling of TGSI and TISR spectra together with analysis of corresponding images allow for the study of precursor plasma formation in time and in space, respectively.

We present the application of a short rise ($\ensuremath{\sim}150\text{ }\text{ }\mathrm{ns}$) 250 kA linear transformer driver (LTD) to wire array $z$-pinch loads for the first time. The generator is a modification of a previous driver in which a new conical power feed provides a low inductance coupling to wire loads. Performance of the new design using both short circuit and plasma loads is presented and discussed. The final design delivers $\ensuremath{\sim}200\text{ }\text{ }\mathrm{kA}$ to a wire array load which is in good agreement with SCREAMER calculations using a simplified representative circuit. Example results demonstrate successful experiments using cylindrical, conical, and inverse wire arrays as well as previously published work on $x$-pinch loads.

The aim of this work is to model the jets produced by conical wire arrays on the MAGPIE generator, and to design and test new setups to strengthen the link between laboratory and astrophysical jets. We performed the modelling with direct three-dimensional magneto-hydro-dynamic numerical simulations using the code GORGON. We applied our code to the typical MAGPIE setup and we successfully reproduced the experiments. We found that a minimum resolution of approximately 100 is required to retrieve the unstable character of the jet. We investigated the effect of changing the number of wires and found that arrays with less wires produce more unstable jets, and that this effect has magnetic origin. Finally, we studied the behaviour of the conical array together with a conical shield on top of it to reduce the presence of unwanted low density plasma flows. The resulting jet is shorter and less dense., Accepted for publication in Astrophysics & Space Science. HEDLA 2010 conference procedings. Final pubblication will be available on Springer

Summary form only given. The ablation phase of exploding multi-wire experiments driven by fast-rising currents in which both global and local global magnetic are dynamically significant is poorly understood at present. In particular, a quasi-periodic modulation in the plasma flow accelerated from the wire cores following initiation appears at all current levels, and is not fully explained. The lack of a complete description of the physical process which drive this ablation structure leads to uncertainties in the scaling of the plasma parameters with drive current, and performance at very high current levels cannot be predicted with a high degree of confidence. Such scaling is particularly important for wire array Z-pinches which show promise as a driver for high yield Inertial Confinement Fusion (ICF), as well as the generation of novel High Energy Density Physics (HEDP) states which may be achievable using exploding wire systems. We present a quantitative investigation of the ablated plasma from a wire system in which magnetic field local to the wire is fixed and the global field varies, namely the X-pinch. The variation of Bglobal/Blocal with spatial position offers an opportunity to examine the modulation of the plasma as a function of this parameter and therefore infer likely mechanisms. Laser interferometry is used to examine 2 wire x-pinches formed from W, Al, Cu and Ni wires at 80 kA, along with gated XUV imaging and time integrated soft X-ray imaging. Two-dimensional electron density maps of the ablation structure are recovered as a function of both space and time with spatial resolution ~50 mum. Results are compared to parallel wire loads on the same pulser, as well as analytical models.

Summary form only given. The development of shocks in plasma flows occurs in a wide range of environments, including fusion schemes and astrophysical objects. In wire array experiments, the plasma accelerated from the wire via the Lorentz force rapidly exceeds both the local sound and Alfven speeds, providing an interesting source for shock studies. Recently, the plasma flow in a 1 MA wire array z-pinch demonstrated both the formation of bow shocks around an obstacle in the plasma, and the feasibility of testing the effect of magnetic fields and radiation cooling on the shock formation in these systems.In this work we present examination of bow shock formation in wire array plasma flows driven by the 250 kA GenASIS Linear Transformer Driver (LTD). The plasma densities produced are lower than on the 1 MA device used in Ref 1, and this allows continuous 2-dimensional quantitative measurements of the electron density, which was not previously possible. A closer examination of the shock structure is therefore possible for comparison for both analytical theory and simulation work. Shock formation is examined as a function of obstacle geometry and size, and preliminary results from both standard and inverse wire arrays will be presented.

International audience; We present experimental results of the formation of magnetically driven plasma jets, showing for the first time a way of producing episodic jet/ouflows in the laboratory. The jets are produced using a 6.5 mum thick aluminum disk (a radial foil), which is subjected to the 1 MA, 250 ns current pulse from the MAGPIE generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)]. The early time motion of the foil is characterized by the bulk motion of the mass due to the magnetic pressure, together with the formation of a surface plasma following the direction of the J×B force. A low density plasma fills the region above the foil preceding the formation of subsequent magnetically driven jets on the axis of expanding magnetic bubbles. The outflows emerge in timescales of ~30-40 ns and their episodic nature is the result of current reconnection in the foil, aided by the formation of current-driven instabilities in the jet and the distribution of mass available from the foil. The additional inductance due to the new current path inside the cavities was measured using an inductive probe, allowing to estimate the energy balance associated with the episodes. The measured temperature of the compressed jet resulted in Te~300 eV and a magnetic Reynolds number of ReM~200-1000, allowing the experiments to be in the regime relevant for scaled representations of astrophysical outflows.

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

International audience; The operation of radial wire array Z-pinches driven by a 1-MA 250-ns current pulse was studied. Variation in the cathode diameter and wire diameter does not affect the overall plasma dynamics but controls the time of wire breakage and the time of pinch formation. The measured times of full wire ablation at the cathode were used to determine the ablation velocity (V abl), and the results give a scaling V abl ~ (wire diameter)-0.46. Experiments with added axial magnetic field show an increase in the pinched plasma diameter, possibly due to the compression of the axial magnetic flux by the imploding plasma.

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.

The ablation phase of wire array z-pinch wire experiments driven by fast-rising currents is poorly understood at present. In particular, the of a quasi-periodic modulation in the plasma flow accelerated from the wire cores appears in systems in which both global and local global magnetic are dynamically significant. This structure is observed at all current levels, and is not fully explained. This lack of a complete description of the physical process which drive the ablation structure leads to uncertainties in the scaling of the plasma parameters with drive current. In order to increase confidence in projected performance, data from a range of experiments can be used to benchmark computational codes. Small scale generators have played a vital role in this area during the recent renaissance of wire arrays as a high power x-ray source and possible driver for Inertial Confinement Fusion (ICF) and Inertial Fusion Energy (IFE).

Summary form only given. The Dense Plasma Focus (DPF) is a versatile source of soft x-ray and neutron emission, for calibration purposes. This paper describes results from a 500J/130kA and a 2kJ/300kA DPF, operated at 0.2-2Hz rep-rates. Using the five-phase, Sing Lee1 model of the DPF, optimized head designs were tested with Deuterium, Neon and Argon gas fills. The 20- 40ns FWHM neutron output was measured using plastic scintillators and fast photomultiplier tubes. The soft x-ray outputs at -IkeV and ~3.2keV from Ne and Ar K-shell emissions were measured using standard x-ray diodes and AASC's diamond radiation detectors, filtered appropriately. The DPF sources may be fired in bursts of -1000-5000 pulses at a time, allowing for very accurate (

We report on experiments in which magnetically driven radiatively cooled plasma jets were produced by a 1 MA, 250 ns current pulse on the MAGPIE pulsed power facility. The jets were driven by the pressure of a toroidal magnetic field in a ''magnetic tower'' jet configuration. This scenario is characterized by the formation of a magnetically collimated plasma jet on the axis of a magnetic ''bubble'', confined by the ambient medium. The use of a radial metallic foil instead of the radial wire arrays employed in our previous work allows for the generation of episodic magnetic tower outflows which emerge periodically on timescales of ~30 ns. The subsequent magnetic bubbles propagate with velocities reaching ~300 km/s and interact with previous eruptions leading to the formation of shocks., 6 pages, 5 figures. Accepted for publication in Astrophysics & Space Science

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,θ 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 J×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 ablation streamers occur, large wavelength coils were constructed such that the breaks that form in the wires had sufficient axial separ...

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...

We report on experiments in which magnetically driven radiatively cooled plasma jets were produced by a 1 MA, 250 ns current pulse on the MAGPIE pulsed power facility. The jets were driven by the pressure of a toroidal magnetic field in a “magnetic tower” jet configuration. This scenario is characterized by the formation of a magnetically collimated plasma jet on the axis of a magnetic cavity, confined by the ambient medium. The use of a radial metallic foil instead of the radial wire arrays employed in our previous work allows for the generation of episodic magnetic tower outflows which emerge periodically on timescales of ∼ 30 ns. The subsequent magnetic bubbles propagate with velocities reaching ∼ 300 km/s and interact with previous eruptions. This setup also allowed for the addition of a neutral gas above the foil in order to study the effect of the ambient density on the dynamics of both the early time hydrodynamic jet formed from plasma ablated from the foil and of the subsequent magnetic tower outflows.

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.

Summary form only given. Experiments investigating the long time scale X-ray emission from X-pinches have been performed on a compact laboratory-scale pulser. The device has a maximum current of 80 kA with a rise time of ~40 ns, and wire materials including aluminum, nickel, stainless steel, molybdenum and tungsten were studied. Time resolved X-ray signal data have been recorded and analyzed, which included a set of Si diodes filtered using Ross pair filter sets. Simultaneous optical probing techniques (Schlieren and interferometry) were used to correlate X-ray emission with pinching of the plasma and gap formation. A scan of wire number and wire material was carried out whilst maintaining constant mass in the X-pinch. Various features including peak timing, number of peaks, FWHM, rise time, and time varying spectrum are reviewed. Results show that typically low Z metals have single long rise time peaks, while high Z materials have multiple sharp peaks. This analysis shows that the material and configurations play a profound role in the X-ray emission characteristics of the X-pinch. In addition, the variation of emission from bare Mo wires and Mo wires coated with a thin layer (0.04 mum) of gold was investigated. In cases with the gold coating, 3 distinct, well spaced peaks are observed in the X-ray emission, while uncoated wire signals generally comprise one primary and anywhere from 1-3 secondary peaks. Within a single shot, one can observe peaks with different profiles, and bare Mo X-pinches give peaks with rise times from 2-8 ns. This variable emission suggests that X-rays are produced by various mechanisms, which will be presented and discussed.

Summary form only given. Understanding the ablation phase of wire arrays is a crucial part of their continued development in the drive towards inertial confinement fusion (ICF). In particular, accurate recovery of the ablated mass density profile evolution from experiments will provide invaluable data for comparison and validation of MHD codes, which in turn provides greater confidence in the scaling to higher current drivers. Two drivers (80 kA, 40ns risetime and 250 kA, 150ns risetime) have been used at UCSD to drive fine wires in several different configurations. These include x-pinches, wire-'pusher' experiments and various wire array designs. The ablation rate of the wire cores can be inferred from measurements of the density profile at several times, and give information regarding the scaling of the ablation rate with current as well as the ratio of local and global magnetic fields in comparable set-ups. In addition, the results obtained from the above study can be compared to those from cylindrical wire arrays at IMA on the MAGPIF generator at Imperial College. Quantitative measurement of precursor plasma structures can again infer the ablation rate history of the array for comparison to modeling and lower current scalings. Diagnostics used in these experiments include XUV framing cameras, XUV spectroscopy, 2-frame interferometry, and 2 frame, quantitative x-pinch radiography. Results comparing the experimentally observed density profiles will be presented along with comparisons to MHD simulations and analytical models.

Summary form only given. We present detailed measurements of the implosion and X-ray emission mechanisms of radial wire array z-pinches - array configurations where the wires stretch radially outwards from a central cathode. Initially the wires ablate, then an implosion begins, starting close to the cathode where the global magnetic field is highest. Implosion continues as bubbles of plasma being accelerated above the cathode where they collide forming a highly emitting column. In the radial configuration emission from the snow-ploughing of ablated plasma can be more easily separated from that of the stagnating plasma column than in standard cylindrical wire arrays, allowing the different physical processes that determine emission to be examined. The scaling of power and yield with array configuration is explored in order to test whether radial arrays can provide a compact source for the energisation of small scale hohlraums on larger facilities. In addition to fine metallic wires, solid metal foils have also been imploded. The gross dynamics of foil experiments are similar to those with wires - with plasma first being observed to be ablated from the foil and then an implosion and stagnation occurring close to the cathode. Differences in the use of foils and arrays will be highlighted, and current stratification explored.

Summary form only given. We present data from experiments using nested wire array Z-pinches where the wires of one of the arrays (outer or inner array) are inclined to produce a cone. The use of conical arrays can potentially provide a valuable tool in pulse-shaping for Z-pinch driven high yield Inertial Confinement Fusion schemes. Results from both Z and MAGPIE (18MA, 100ns and IMA, 240ns respectively) are presented and compared. Diagnostics employed to study the effect of the conical arrays on the nested implosion included laser shadowgraphy, time resolved XUV and soft x-ray emission imaging, radial optical streak photography simultaneously at multiple axial positions in the array, PCD and XRD detectors, bolometers and an axially resolved XRD (zipper) array. Results indicate that the use of a conical outer array with cylindrical inner array causes an axially-non-uniform implosion of the outer, with the region near the smaller diameter cathode leading ahead of the larger diameter anode. A uniform variation between the two electrodes produces an axial zipper. This leads to an axial dependence of the time that the imploding outer array reaches the inner array, hence elongating the interaction pulse emitted. This axial dependence seeds a zippered implosion and stagnation of the inner array, causing a temporal lengthening of the main X-ray pulse. Separate experiments show that while a conical inner array has a strong effect on the length of the interaction pulse, it has a less significant effect on the main X-ray pulse. The possible pulse-shaping capability of combining two nested conical arrays allowing independent control of the length of the two pulses was also described.

Summary form given only. We report on detailed density measurements of the ablating plasma in wire array z-pinch experiments. Previously such measurements have been restricted to relatively early times in an experiment, where laser probing could penetrate the ablated plasma without being refracted by steep gradients. Here we show measurements made by 2 new diagnostics that compliment the earlier laser based data. In one system XUV point projection backlighting is used to probe both the ablated plasma streams and the precursor plasma column formed on the axis of the array by collision of the streams. Radiation for the back lighter is produced by a 5.1 laser pulse focused to a Cu target within a few cm of the array. Recording is via a large format MCP camera. In the second system, the KeV radiation from an X-pinch is projected along the axis of an array. Using this probing direction the relatively long lengths of plasma provide significant absorption of the probing radiation even at the multi KeV energies produced by the X-pinch, allowing good estimates of the plasma density. Results of experiments utilizing the diagnostics are compared to the rocket ablation model and 2 and 3-D MHD simulations. Attempts are now being made to improve both the XUV and hard X-ray systems by using spherical multilayer mirrors (for XUV) and spherical crystals (for X-pinch) to project parallel beams of monochromatic radiation through the array. This will allow absolute density measurements of the ablated plasma.

Summary form only given. Evidence for non-uniform mass ablation has been observed on various wire array experiments. Non-uniform wire ablation leads to non-uniform wire breakup, providing the means for wire array material to be left behind the implosion of the bulk array mass. Mass ablation variation along the length of the wire vary periodically, with a wavelength of /spl sim/250 /spl mu/m (Al) and /spl sim/100 /spl mu/m (W). The mechanism which seeds this perturbation is currently not well understood however attempts have been made to chemically etch wires (axially modulating the wire radius) after they are manufactured to seed a perturbation that might dominate the subsequent wire array dynamics. We present three-dimensional simulations of modulated wire experiments performed on the MAGPIE accelerator. The simulations are used to provide insight into the time evolution of low wire number modulated wire arrays, examining how wire-to-wire modulation correlation and anti-correlation impacts non-uniform wire ablation and array implosion. Comparisons will be made with data obtained from experiments with varying wire modulation wavelength.

Radiative high-Z plasma bow shocks driven by a 200-kA current are investigated by using high-resolution dark-field laser Schlieren imaging and a Mach-Zehnder interferometer. Results demonstrate stationary high-density compressible bow shocks and provide data on the plasma Mach number and electron density.

International audience; We present experimental results on the formation of supersonic, radiatively cooled jets driven by 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 the jet on the axis of a magnetic cavity expanding into an ambient medium is collimated by a toroidal magnetic field. The jets in our experiments have similar Mach number, plasma beta and cooling parameter to those in protostellar jets and 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 generates episodic magnetic cavities, suggesting that periodic formation of jets in astrophysical situations could be responsible for some of the variability observed in astrophysical jets. The dynamics of the formation of laboratory jets are presented, together with new results including preliminary measurements of magnetic, kinetic and Poynting energy of the outflows. In addition first estimates of jet temperature and trapped toroidal magnetic field are presented and discussed.

X-pinches driven by an 80-kA current are investigated by using high-resolution dark-field laser Schlieren imaging. Results demonstrate the evolution of the low-density coronal plasma and the formation of an axial jet. Quantitative measurements of plasma expansion rates and the jet velocity are carried out for several materials.

X-pinch radiography data are presented for wire array Z-pinches in which the wires initially have an imposed modulation. Calibrated data indicate a sharp contrast between the initially thicker regions of the wire, which remain intact, and the thinner regions which become fully depleted of material.

Bow shock structures are observed in a nested wire array z-pinch as ablation streams from the outer array pass the inner array. The jump in plasma conditions across these shocks results in an enhancement of snowplow emission from the imploding plasma piston. Results from a snowplow model modified to account for the shock jumps are discussed and compared to experimental data from MAGPIE. Magnetohydrodynamic simulations indicate that this is the primary heating mechanism responsible for the interaction pulse recorded on the Z generator, which is required for pulse shaping for inertial confinement fusion.

We report the results of experiments where a quantitative analysis of the behavior of ablation modulations in X-pinches (including wavelength and contrast between streams) was carried out as a function of distance from the cross-point, and consequently, as a function of the global to local magnetic field ratio. Experiments were performed using two tungsten wires of 10 μm diameter in an X-pinch configuration on a pulsed power generator capable of producing an 80 kA current over a 50 ns rise time. We compare these findings directly to three-dimensional magneto-hydrodynamic (MHD) simulations using the MHD resistive code GORGON. Here, we demonstrate a dependence between the magnetic field ratio and ablation properties, suggesting a transition to a classical m = 0 instability far from the cross-point. Additionally, we examine the ablation properties of similar Z number nickel and copper wire X-pinches.

The early stages of tungsten (W) wire-array Z-pinch implosions have been studied using two-frame point projection x-ray backlighting on the 1MA COBRA pulsed power generator [J. D. Douglass, J. B. Greenly, D. A. Hammer, and B. R. Kusse, in Proceedings of the 15th IEEE International Pulsed Power Conference, Monterey, 2005 (to be published)]. X-pinch backlighter images with subnanosecond time resolution and 4–10μm spatial resolution have been obtained of individual W exploding wires in 8-wire arrays that show evolution of wire-core and coronal plasma structures. The timing of the X-pinch x-ray bursts relative to the Z-pinch initiation time was adjusted over a 50ns time interval by varying the X-pinch mass per unit length. Wire-cores seen in two images separated in view by 120° show that the expansion is remarkably azimuthally symmetric. A strong correlation is observed between the structure on the dense exploding wire-cores and the structure of the ⩾1018∕cm3 ablation plasma being drawn from radial prominence...

In order to study wire array Z-pinch instabilities, perturbations have been seeded by etching 15μm diameter aluminum wires to introduce 20% modulations in radius with a controlled axial wavelength. These perturbations seed additional imploding structures that are studied experimentally on the 1MA, 250ns MAGPIE generator [S. V. Lebedev et al., Plasma Phys. Control. Fusion 47, A91 (2005)] and with three-dimensional magnetohydrodynamic calculations using the ALEGRA-HEDP [A. C. Robinson and C. J. Garasi, Comput. Phys. Commun. 164, 408 (2004)] and GORGON [J. P. Chittenden et al., Plasma Phys. Control. Fusion 46, B457 (2004)] codes. Simulations indicate that current path nonuniformity at discontinuities in the wire radius result in perturbation-induced magnetic bubble formation. Imploding bubbles originating from discontinuities are observed experimentally, and their collision on axis determines the start of the main x-ray pulse rise. These mechanisms likely govern dynamics of standard wire array Z pinches, and...

Observations of supersonic jet propagation in low-current x-pinches are reported. X-pinches comprising of four 7.5 μm diameter tungsten wires were driven by an 80 kA, 50 ns current pulse from a compact pulser. Coronal plasma surrounding the wire cores was accelerated perpendicular to their surface due to the global J×B force, and traveled toward the axis of the x-pinch to form an axially propagating jet. These jets moved towards the electrodes and, late in time (∼150 ns), were observed to propagate well above the anode with a velocity of 3.3±0.6×104 m/s. Tungsten jets remained collimated at distances of up to 16 mm from the cross point, and an estimate of the local sound speed gives a Mach number of ∼6. This is the first demonstration that supersonic plasma jets can be produced using x-pinches with such a small, low current pulser. Experimental data compares well to three-dimensional simulations using the GORGON resistive MHD code, and possible scaling to astrophysical jets is discussed.

Summary form only given. We describe experiments on the MAGPIE facility (1 MA, 250ns) with radial wire array Z-pinches, in which the wires form a wheel-spoke configuration. Variation of the global magnetic field and the corresponding JxB force along the wires in this configuration allows studying an effect of the global magnetic field on the ablation rate of the wires. The knowledge of the scaling of the ablation rate is important for interpretation of the dynamics of wire array Z-pinch plasma radiation sources. The parameters of the coronal plasma, which is pushed away from the wire cores by the JxB force, were measured using laser interferometry, X-ray and XUV imaging. The complete ablation of the wires near the inner electrode, where the ablation rate is the highest, triggers an axial acceleration of the plasma. This leads to formation of plasma configuration similar to a plasma focus and generation of the X-ray pulse. The results of the experiments with tungsten and aluminium radial wire arrays are presented.