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We report results of the experimental campaign, which studied the initiation, implosion dynamics, and radiation yield of tungsten wire arrays as a function of the wire number. The wire array dimensions and mass were those of interest for the Z-pinch driven Inertial Confinement Fusion (ICF) program. An optimization study of the x-ray emitted peak power, rise time, and full width at half maximum was effectuated by varying the wire number while keeping the total array mass constant and equal to ∼5.8 mg. The driver utilized was the ∼20-MA Z accelerator before refurbishment in its usual short pulse mode of 100 ns. We studied single arrays of 20-mm diameter and 1-cm height. The smaller wire number studied was 30 and the largest 600. It appears that 600 is the highest achievable wire number with present day's technology. Radial and axial diagnostics were utilized including crystal monochromatic x-ray backlighter. An optimum wire number of ∼375 was observed which was very close to the routinely utilized 300 for the ICF program in Sandia. [ABSTRACT FROM AUTHOR]

Z-pinch experiments at pulsed-power facilities generally field a wide range of diagnostics, designed to make measurements and study the relevant physics during all phases of the Z-pinch. Optical and X-ray images are collected to study ablation, implosion, and the stagnated pinch; yield and power measurements are made at various photon energies; and time-integrated and time-resolved spectroscopies are recorded to infer plasma conditions before, during, and after stagnation. Typically, diagnostics fielded are dispersed azimuthally around the Z-pinch and at varying distances relative to the Z-pinch. The data are then analyzed and interpreted as a single entity to provide detailed information about a particular process or physics effect. Correlation of the results of the individual diagnostics in this fashion assumes that each diagnostic would measure the same result regardless of its azimuthal or radial location. Assessments of diagnostics to measure K-shell yield at the Z accelerator, which are fielded at varying azimuthal locations and radial distances, are presented. These measurements illustrate that the K-shell emission is azimuthally uniform for titanium, stainless-steel, and copper wire arrays and that the standard correction for radial distance is valid. The importance of view-factor corrections is also discussed. [ABSTRACT FROM AUTHOR]

Nested stainless steel wire array variations were investigated on the 20 MA Z machine [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)]. In order to reach experimentally observed electron temperatures near 3.8 keV and excite the K shell, these ∼6.7 keV photon energy x-ray sources must be of large initial diameter (45–80 mm) which poses a concern for magnetic Rayleigh–Taylor instability growth. We discuss the implosion dynamics in these large diameter wire arrays, including an analysis of the ablation phase indicating that the prefill material is snowplowed at large radius. Nested array configurations with various mass and radius ratios are compared for instability mitigation and K-shell scaling. Degradation of the K-shell x-ray power and yield was observed for shots that did not have simultaneous implosion of the outer and inner wire arrays. Shots that were designed per this constraint exhibited K-shell yield scaling consistent with the model of J. W. Thornhill et al. [IEEE Trans. Plasma Sci. 34, 2377 (2006)] which had been benchmarked to single array results. This lends confidence to K-shell yield predictions using this model for future shots on the refurbished Z machine. Initial results employing a triple nested wire array to stabilize the large diameter implosion are also reported. [ABSTRACT FROM AUTHOR]

The dynamics of large- and small-scale plasma structures is investigated in the precursor of 1-MA wire array Z-pinches by laser probing diagnostics. It is found that plasma streams from the wires induce density perturbations in the pre- cursor. Small-scale perturbations and large-scale cells arise in the nonlinear stage before implosion. The spatial and temporal scales of the observed structures are in agreement with the theoretical investigation for current-driven excitation of electromagnetic flute modes. [ABSTRACT FROM AUTHOR]

Experiments in the last few years at the 20-MA Z Accelerator have produced significant K-shell X-ray output from a variety of imtial load materials, including aluminum (1.7-keV photons, > 400-kJ yield), argon (3.1-keV photons, > 300-kJ yield), titanium (4.8-keV photons, 100-kJ yield), stainless steel (6.7-keV photons, > 50-kJ yield), and copper (8.4-keV photons, 20-kJ yield). K-shell scaling theories developed at the Naval Research Laboratory [K. G. Whitney et al., Phys. Rev. E 50, 2166 (1994)] in the 1990s were benchmarked against the Al K-shell emission data from < 10-MA facilities. The experiments at Z have not only led to a heuristic validation of this original theory but have also provided the data to fine tune the models for application to higher photon energies and for extension to higher current generators. The upgrade of the Z Accelerator to ZR, which will provide 26 MA to a Z-pinch load, should increase the radiated K-shell output for sources previously fielded at Z and will extend the range of photon energies where measurable radiation can be observed, which is likely up to 13 keV. A summary of the K-shell experiments at Z is presented, as well as an overview of the modified empirical-scaling theory. Proposed load configurations for ZR are discussed, as well as predictions for K-shell output. [ABSTRACT FROM AUTHOR]

This paper presents theoretical extrapolations for the multi-keV X-ray radiation production capability of the 26-MA ZR accelerator at Sandia National Laboratories, which is scheduled to become available for experiments in 2007. These extrapolations are based on scaling models and ideas that have been developed over the years. These models and ideas have evolved and been refilled through the process of benchmarking one-dimensional nonlocal-thermodynamic equilibrium magnetohydrodynamic model results to experimental K-shell yields and powers as well as inferred temperatures and densities. For this ZR assessment, the models are first benchmarked to K-shell yields obtained from argon, titanium, stainless-steel, and .copper Z experiments and then they are applied to extrapolate yield predictions to the ZR machine. Extrapolations are based on 2-cm-length loads and similar wire configurations and nozzle designs as those employed in Z experiments. Projected K-shell yields for Ar (photon energy ∼3 keV), Ti (∼5 keV), stainless steel (∼7 keV), and Cu (∼8.6 keV) are 520, 300, 200, and 80 k J, respectively. In addition, the high-energy free-bound continuum emission above 10 keV is calculated to be 40 kJ on ZR. [ABSTRACT FROM AUTHOR]

A Faraday rotation diagnostic was applied for the investigation of magnetic fields in plasma of 1-MA wire arrays and X-pinches. Laser-probing diagnostics at the Zebra generator include a four-channel polarointerferometer and a four-frame shadowgraphy. The Faraday rotation diagnostic consists of shadow and Faraday channels, shearing air-wedge interferometer, and an additional schlieren channel. The implosion dynamics of the wire arrays were studied. A current in the plasma column of AI low-wire number arrays was found by the Faraday rotation diagnostic. Optical diagnostics showed a turbulent plasma and bubblelike objects in the plasma column of AI wire arrays. The Faraday rotation diagnostic demonstrated a complicated structure of magnetic fields in X-pinch plasma. [ABSTRACT FROM AUTHOR]

Recent experiments (laser multiframe shadowgraphy and Faraday rotation diagnostics) with AI imploding wire arrays in the 1-MA ‘Zebra’ accelerator at the Nevada Terawatt Facility have demonstrated that a significant amount of current is going through the central region of the high-beta precursor plasma. With time, the perturbations develop both long scale structures and short-wavelength oscillations in the directions perpendicular to the magnetic field. The most likely candidate for the observed phenomena is the flutelike electromagnetic modes. These are excited in the current-carrying high-beta inhomogeneous precursor plasma due to the curvature of the magnetic field produced by the current. These modes may also play a role in the stagnation phase of Z-pinches. In this paper, nonlinear equations describing electromagnetic-flute-type oscillations in the high-beta precursor plasma ale derived, and the linear stage of instability of these waves is examined. A comparison with the experimental results is performed as well. [ABSTRACT FROM AUTHOR]

In order to study wire array Z-pinch instabilities, perturbations have been seeded by etching 15 μm 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 1 MA, 250 ns 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 tailoring the profile of imploding mass may allow x-ray pulse shaping for inertial confinement fusion applications. [ABSTRACT FROM AUTHOR]

In the following experiments, we studied implosions of different wire arrays and X-pinches produced on the 1-MA Zebra generator at the University of Nevada, Reno. Diagnostics included both spatially-resolved and time-gated X-ray imaging and spectroscopy, and laser probing. In particular, we compared planar wire arrays, to which little energy could be coupled via the conventional magnetic-to-kinetic conversion mechanism, to cylindrical wire arrays of comparable dimensions and mass. The planar wire arrays were shown to radiate much higher peak power and more energy in subkiloelectronvolt and kiloelectronvolt spectral ranges than cylindrical wire arrays. We tested the theoretical conjecture that enhanced resistivity due to the small-scale inhomogeneity of wire-array plasmas has a major effect on dynamics, energy coupling and radiation performance of wire-array Z-pinches. The study of Al, Alumel, and W cylindrical wire arrays shows a wide variety of characteristic behaviors in plasma implosions discussed hereinafter. Additional experimental results for symmetric and asymmetric, uniform stainless steel, Cu, Mo, combined Al/Mo, Mo/Al, Al/W, W/Al, and Mo/W X-pinches are also presented. New data for the total radiation yield are obtained. The planar structures of X-pinch plasma and the corresponding electron beam was observed for most of X-pinches. The generation of hot spots along original wires positions—cooler than those from the cross-wire region—and arc structures with hot spots between wires were found for X-pinches composed from Al, Cu, and W wires. [ABSTRACT FROM AUTHOR]

A monochromatic X-ray self-emission imaging diagnostic has been developed for the Z accelerator, which drives 20 MA in 100 ns to implode wire array Z-pinches, generating up to 250 TW of soft X-ray radiation. This instrument reflects eight pinhole images from a flat Cr/C multilayer mirror (MLM) onto a 1-ns time-resolved microchannel plate detector. The MLM reflects 277-eV photons with ∼5-eV bandwidth and 20% peak reflectivity, and an aluminized parylene filter shields the detector from visible light. High-energy bremsstrahlung X-rays do not follow the reflected beam path, and so the background on the shielded detector is reduced compared to a standard pinhole camera. The MLM-reflected images offer low-photon-energy spectral resolution that filtration alone cannot, yielding high-quality images of the final stages of the Z-pinch implosion. Initial data on Z from a Cu wire array will be presented. Observed phenomena include implosion instabilities, zippered implosion of a piston onto a precursor column during the onset of stagnation, accretion of trailing colder mass during the X-ray pulse, and cathode reemission. The inferred implosion velocity is significantly less than thin-shell implosion model calculations, and well below what is required for efficient Cu K-shell radiation. Instability-dominated, bright-spot Cu K-shell emission is seen on a second adjacent eight-frame filtered pinhole camera that is in the same beamline. [ABSTRACT FROM AUTHOR]

A technique has previously been developed on the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] to generate ramped compression waves in condensed matter for equation-of-state studies [C. A. Hall, J. R. Asay, M. D. Knudson, W. A. Stygar, R. B. Spielman, T. D. Pointon, D. B. Reisman, A. Toor, and R. C. Cauble, Rev. Sci. Instrum. 72, 3587 (2001)] by using the Lorentz force to push on solid electrodes rather than to drive a Z pinch. This technique has now been extended to multimegabar pressures by shaping the current pulse on Z to significantly increase the sample thickness through which the compression wave can propagate without forming a shock. Shockless, free-surface velocity measurements from multiple sample thicknesses on a single experiment can be analyzed using a backward integration technique [D. B. Hayes, C. A. Hall, J. R. Asay, and M. D. Knudson, J. Appl. Phys. 94, 2331 (2003)] to extract an isentropic loading curve. At very high pressures, the accuracy of this method is dominated by relative uncertainty in the transit time between two thicknesses. This paper discusses in some detail the issues involved with accurate measurement of a multimegabar isentrope, including experiment design trade-offs and mechanics of pulse shaping on Z. [ABSTRACT FROM AUTHOR]

In this note, a novel, but simple system is described which gives both continuous time resolution and limited two-dimensional spatial resolution. This system uses multiple, displaced pinholes in conjunction with a streak camera. As an example of this technique, the thermalization of a plasma focus has been studied. [ABSTRACT FROM AUTHOR]