Publications

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The study of implosion dynamics and spectroscopy of X-pinches and wire arrays with Al wires alloyed or coated with other near-Z or higher-Z materials is discussed. In particular, X-pinches from two combined Al 5056 and Mo wires and composed from four identical Al 5056 (5%Mg) wires and Cu clad Al (90% Al and 10%Cu) are studied. In addition, wire arrays with Alumel wires (95% Ni and 5% Al) and with Al 5056 wires (uncoated) and coated with 5% NaF are investigated. Spatially-resolved and integrated x-ray spectral data and time integrated and time-gated pinhole x-ray images accumulated in these X-pinch and wire array experiments on the UNR 1MA Zebra generator are analyzed. Modeling of K-shell radiation from Mg provides K-shell plasma parameters for all Al 5056 wire experiments, whereas modeling of L-shell radiation from Ni, Cu, and Mo provide parameters for L-shell plasmas. The importance of using different materials or dopants for understanding of implosion dynamics of different wire materials is illustrated. © 2006 American Institute of Physics.

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Controlled seeding of perturbations is employed to study the evolution of wire array z-pinch implosion instabilities which strongly impact x-ray production when the 3D plasma stagnates on axis. Wires modulated in radius exhibit locally enhanced magnetic field and imploding bubble formation at discontinuities in wire radius due to the perturbed current path. Wires coated with localized spectroscopic dopants are used to track turbulent material flow. Experiments and MHD modeling offer insight into the behavior of z-pinch instabilities.

Experiments to study the implosion dynamics and radiation characteristics of copper z-pinches have been fielded at the 1 MA Zebra facility and the 20 MA Z facility. The impact of initial load mass, initial load diameter, and nesting of wire arrays on the precursor and the stagnated plasma has been evaluated through spectroscopy, shadowgraphy, and fluence measurements. Plasma parameters extracted from modeling of the time-integrated L-shell spectra indicate the presence of more than one plasma source contributing to the radiation, likely due to non-uniform hot spot x-ray emission or temporal gradients.

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Three-dimensional hybrid simulation of a plasma current-carrying column reveal two different regimes of sausage and kink instability development. In the first regime, with small Hall parameter, development of instabilities leads to the appearance of large-scale axial perturbations and eventually to bending of the plasma column. In the second regime, with a four-times-larger Hall parameter, small-scale perturbations dominate and no bending of the plasma column is observed. Simulation results are compared with laser probing experimental data obtained during wire array implosions on the Zebra pulse power generator at the Nevada Terawatt Facility.

The conclusions of this report are: (1) 1D and 2D RMHD simulations indicate feasibility of producing high thermonuclear neutron yields in deuterium and DT gas-puff Z-pinches -- (a) Z 1.7 x 10{sup 13} DD neutrons at 70 kV, 13 MA (Z1384); (b) (3 to 6) x 10{sup 14} at 90 kV, 17 MA (Z1422); (c) Predicted for ZR 2 x 10{sup 15} DD and 6 x 10{sup 16} DT neutrons; (2) Theory and modeling issues -- collisionless ions, nonthermal ions; (3) Experimental data on the origin of the neutrons not yet conclusive, need more shots; and (4) Applications -- (a) Fusion 2.5 and 14 MeV neutron source; (b) Pulsed subcritical neutron source with uranium blanket for {approx}10x neutron and {approx}1000x energy multiplication (Smirnov, Feoktistov and Klimov); and (c) Fusion-assisted keV x-ray plasma radiation source.

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X-ray spectra and images from Al (with 5% of Mg and some with 5% of NaF dopants) and Cu (pure and with 4% of Ni) wire arrays and X-pinches were accumulated in experiments on the 1 MA pulsed power generator at UNR. In particular, axially and radially resolved K-shell X-ray spectra of Al, Mg, and Na and L-shell X-ray spectra of Cu and Ni were recorded by a KAP crystal (in a spectral region from 6 to 15 Aring) through different slits from 50 mum to 3 mm. In addition, spatially integrated harder X-ray spectra were monitored by a LiF crystal. Non-LTE kinetic models of Al, Mg, and Na, and of Cu and Ni provided spatially resolved electron temperatures and densities for experiments with Al and Cu loads, respectively. Advantages of using alloys and dopants with small concentrations for spectroscopic plasma diagnostics will be presented. Dependence of the plasma's spatial structures, temperatures, and densities from wire material and load configurations, sizes, and masses will be discussed .

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Three nested molybdenum wire arrays with initial outer diameters of 45, 50, and 55 mm were imploded by the - 20 MA, 90 ns rise-time current pulse of Sandia's Z accelerator. The implosions generated Mo plasmas with {approx} 10% of the array's initial mass reaching Ne-like and nearby ionization stages. These ions emitted 2-4 keV L-shell x rays with radiative powers approaching 10 TW. Mo L-shell spectra with axial and temporal resolution were captured and have been analyzed using a collisional-radiative model. The measured spectra indicate significant axial variation in the electron density, which increases from a few times 10{sup 20} cm{sup -3} at the cathode up to - 3 x 10{sup 21} cm{sup -3} near the middle of the 20 mm plasma column (8 mm from the anode). Time-resolved spectra indicate that the peak electron density is reached before the peak of the L-shell emission and decreases with time, while the electron temperature remains within 10% of 1.7 keV over the 20-30 ns L-shell radiation pulse. Finally, while the total yield, peak total power, and peak L-shell power all tended to decrease with increasing initial wire array diameters, the L-shell yield and the average plasma conditions varied little with the initial wire array diameter.

Experimental evidence suggests that the energy balance between processes in play during wire array implosions is not well understood. In fact the radiative yields can exceed by several times the implosion kinetic energy. A possible explanation is that the coupling from magnetic energy to kinetic energy as magnetohydrodynamic plasma instabilities develop provides additional energy. It is thus important to model the instabilities produced in the after implosion stage of the wire array in order to determine how the stored magnetic energy can be connected with the radiative yields. To this aim three-dimensional hybrid simulations have been performed. They are initialized with plasma radial density profiles, deduced in recent experiments [C. Deeney et al., Phys. Plasmas 6, 3576 (1999)] that exhibited large x-ray yields, together with the corresponding magnetic field profiles. Unlike previous work, these profiles do not satisfy pressure balance and differ substantially from those of a Bennett equilibrium. They result in faster growth with an associated transfer of magnetic energy to plasma motion and hence kinetic energy.

Pulsed power driven metallic wire-array Z pinches are the most powerful and efficient laboratory x-ray sources. Furthermore, under certain conditions the soft x-ray energy radiated in a 5 ns pulse at stagnation can exceed the estimated kinetic energy of the radial implosion phase by a factor of 3 to 4. A theoretical model is developed here to explain this, allowing the rapid conversion of magnetic energy to a very high ion temperature plasma through the generation of fine scale, fast-growing m=0 interchange MHD instabilities at stagnation. These saturate nonlinearly and provide associated ion viscous heating. Next the ion energy is transferred by equipartition to the electrons and thus to soft x-ray radiation. Recent time-resolved iron spectra at Sandia confirm an ion temperature T{sub i} of over 200 keV (2 x 10{sup 9} degrees), as predicted by theory. These are believed to be record temperatures for a magnetically confined plasma.

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The impact of 3D structure on wire array z-pinch dynamics is a topic of current interest, and has been studied by the controlled seeding of wire perturbations. First, Al wires were etched at Sandia, creating 20% radial perturbations with variable axial wavelength. Observations of magnetic bubble formation in the etched regions during experiments on the MAGPIE accelerator are discussed and compared to 3D MHD modeling. Second, thin NaF coatings of 1 mm axial extent were deposited on Al wires and fielded on the Zebra accelerator. Little or no axial transport of the NaF spectroscopic dopant was observed in spatially resolved K-shell spectra, which places constraints on particle diffusivity in dense z-pinch plasmas. Finally, technology development for seeding perturbations is discussed.

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Recent 3D hybrid simulation of a plasma current-carrying column revealed two regimes of sausage and kink instability development. In the first regime, with small Hall parameter, development of instabilities leads to appearance of large-scale axial perturbations and eventually to the bending of the plasma column. In the second regime, with five times larger Hall parameter, small-scale perturbations dominated and no bending of the plasma column was observed. Simulation results are compared to recent experimental data, including laser probing, x-ray spectroscopy and time-gated x-ray imaging during wire array implosions at NTF.

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A quasi-spherical z-pinch may directly compress foam or deuterium and tritium in three dimensions as opposed to a cylindrical z-pinch, which compresses an internal load in two dimensions only. Because of compression in three dimensions the quasi-spherical z-pinch is more efficient at doing pdV work on an internal fluid than a cylindrical pinch. Designs of quasi-spherical z-pinch loads for the 28 MA 100 ns driver ZR, results from zero-dimensional (0D) circuit models of quasi-spherical implosions, and results from 1D hydrodynamic simulations of quasi-spherical implosions heating internal fluids will be presented. Applications of the quasi-spherical z-pinch implosions include a high radiation temperature source for radiation driven experiments, a source of neutrons for treating radioactive waste, and a source of fusion energy for a power generator.

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Over the last few years, a variety of experiments studying higher photon energy (>4 keV) radiators have been performed, primarily at the Z accelerator. In this paper, the results of experiments designed to study the effects of initial load diameter on the radiated output of stainless steel wire arrays are presented. Stainless steel is primarily iron, which radiates in the K-shell at 6.7 keV. Nested wire arrays from 45 mm initial outer diameter to 80 mm outer diameter were fielded at the Z accelerator. A nested array consists of two wire arrays, with the inner concentric to an outer. All of the arrays fielded for this work had a 2:1 mass and diameter ratio (outer:inner), and the arrays were designed to have the same implosion time. A degradation of K-shell output was observed (pulse shape and power) for the smallest and largest diameter arrays, suggesting a region in which optimal conditions exist for K-shell output. The degradation at small diameters results from the reduced eta value, due to low implosion velocity. Eta is defined as the kinetic energy per ion divided by the energy required to get to the K-shell. At large diameters, a dramatic degradation of output is observed not just for the K-shell, but also for the lower energy X-rays. This may be the result of the low mass required to maintain an appropriate implosion time - there simply aren't many radiators available to participate. One other possibility is that the higher acceleration necessary at large diameters to achieve the same implosion time results in additional instability growth. Also necessary to consider are the effects of interwire gap: due to the limited wire sizes available, the interwire gap on the large diameter loads is large, in one case more than 3 mm. Comparisons of the trends observed in the experiments (radiated yield, pulse shape, and spectra) will be made to calculations previously benchmarked to K-shell data obtained at Z. The reproducibility of the arrays, advanced imaging diagnostics fielded, current diagnostics, and sensitivities of the calculations are also discussed.

We present the first comprehensive study of high wire-number, wire-array Z-pinch dynamics at 14-18 MA using x-ray backlighting and optical shadowgraphy diagnostics. The cylindrical arrays retain slowly expanding, dense wire cores at the initial position up to 60% of the total implosion time. Azimuthally correlated instabilities at the array edge appear during this stage which continue to grow in amplitude and wavelength after the start of bulk motion, resulting in measurable trailing mass that does not arrive on axis before peak x-ray emission.

A technique for manufacturing wires with imposed modulation in radius with axial wavelengths as short as 1 mm is presented. Extruded aluminum 5056 with 15 {micro}m diameter was masked and chemically etched to reduce the radius by {approx}20% in selected regions. Characterized by scanning electron microscopy, the modulation in radius is a step function with a {approx}10 {micro}m wide conical transition between thick and thin segments, with some pitting in etched regions. Techniques for mounting and aligning these wires in arrays for fast z-pinch experiments will be discussed. Axially mass-modulated wire arrays of this type will allow the study of seeded Rayleigh-Taylor instabilities in z pinches, corona formation, wire initiation with varying current density in the wire core, and correlation of perturbations between adjacent wires. This tool will support magnetohydrodynamics code validation in complex three-dimensional geometries, and perhaps x-ray pulse shaping.

The experimental and computational investigations of nanosecond electrical explosion of thin Al wire in vacuum are presented. We have demonstrated that increasing the current rate leads to increased energy deposited before voltage collapse. Laser shadowgrams of the overheated Al core exhibit axial stratification with a {approx}100 {micro}m period. The experimental evidence for synchronization of the wire expansion and light emission with voltage collapse is presented. Two-wavelength interferometry shows an expanding Al core in a low-ionized gas condition with increasing ionization toward the periphery. Hydrocarbons are indicated in optical spectra and their influence on breakdown physics is discussed. The radial velocity of low-density plasma reaches a value of {approx}100 km/s. The possibility of an overcritical phase transition due to high pressure is discussed. 1D MHD simulation shows good agreement with experimental data. MHD simulation demonstrates separation of the exploding wire into a high-density cold core and a low-density hot corona as well as fast rejection of the current from the wire core to the corona during voltage collapse. Important features of the dynamics for wire core and corona follow from the MHD simulation and are discussed.

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The high-pressure response of cryogenic liquid deuterium (LD{sub 2}) has been studied to pressures of {approx}400GPa and densities of {approx}1.5g/cm{sup 3}. Using intense magnetic pressure produced by the Sandia National Laboratories Z accelerator, macroscopic aluminum or titanium flyer plates, several mm in lateral dimensions and a few hundred microns in thickness, have been launched to velocities in excess of 22 km/s, producing constant pressure drive times of approximately 30 ns in plate impact, shock wave experiments. This flyer plate technique was used to perform shock wave experiments on LD{sub 2} to examine its high-pressure equation of state. Using an impedance matching method, Hugoniot measurements of LD{sub 2} were obtained in the pressure range of {approx}22-100GPa. Results of these experiments indicate a peak compression ratio of approximately 4.3 on the Hugoniot. In contrast, previously reported Hugoniot states inferred from laser-driven experiments indicate a peak compression ratio of approximately 5.5-6 in this same pressure range. The stiff Hugoniot response observed in the present impedance matching experiments was confirmed in simultaneous, independent measurements of the relative transit times of shock waves reverberating within the sample cell, between the front aluminum drive plate and the rear sapphire window. The relative timing was found to be sensitive to the density compression along the principal Hugoniot. Finally, mechanical reshock measurements of LD{sub 2} using sapphire, aluminum, and {alpha}-quartz anvils were made. These results also indicate a stiff response, in agreement with the Hugoniot and reverberating wave measurements. Using simple model-independent arguments based on wave propagation, the principal Hugoniot, reverberating wave, and sapphire anvil reshock measurements are shown to be internally self-consistent, making a strong case for a Hugoniot response with a maximum compression ratio of {approx}4.3-4.5. The trends observed in the present data are in very good agreement with several ab initio models and a recent chemical picture model for LD{sub 2}, but in disagreement with previously reported laser-driven shock results. Due to this disagreement, significant emphasis is placed on the discussion of uncertainties, and the potential systematic errors associated with each measurement.

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When several kA pulses are passed through single, fine 25 {micro}m diameter wires, the wire material heats, melts, vaporizes and expands. Initially the voltage across--and current through--a wire increases until an abrupt voltage collapse occurs. After this collapse the voltage remains at a relative small value while the current continues to increase. In order to understand how this early time behavior may affect the subsequent implosion, small-scale experiments at Cornell University's Laboratory of Plasma Studies concentrated on diagnosing expanding single wire dynamics. X-ray backlighting, interferometry and Schlieren imaging as well as current and voltage measurements have been employed. The voltage collapse has been attributed to the formation of plasma around the wire and a transfer of current to this highly conducting coronal plasma. Interferometry has confirmed the plasma formation, but the current transfer has only been postulated. Subsequent experiments on the Z-Facility at Sandia National Laboratories have produced impressive x-ray yields etc.

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Using intense magnetic pressure, a method was developed to launch flyer plates to velocities in excess of 20 km s{sup -1}. This technique was used to perform plate-impact, shock wave experiments on cryogenic liquid deuterium (LD{sub 2}) to examine its high-pressure equation of state (EOS). Using an impedance matching method, Hugoniot measurements were obtained in the pressure range of 22--100 GPa. The results of these experiments disagree with the previously reported Hugoniot measurements of LD2 in the pressure range above {approx}40 GPa, but are in good agreement with first principles, ab initio models for hydrogen and its isotopes.

Recently an innovative technique known as the Isentropic Compression Experiment (ICE) was developed that allows the dynamic compressibility curve of a material to be measured in a single experiment. Hence, ICE significantly reduces the cost and time required for generating and validating theoretical models of dynamic material response. ICE has been successfully demonstrated on several materials using the 20 MA Z accelerator, resulting in a large demand for its use. The present project has demonstrated its use on another accelerator, Saturn. In the course of this study, Saturn was tailored to produce a satisfactory drive time structure, and instrumented to produce velocity data. Pressure limits are observed to be approximately 10-15 GPa (''LP'' configuration) or 40-50 GPa (''HP'' configuration), depending on sample material. Drive reproducibility (panel to panel within a shot and between shots) is adequate for useful experimentation, but alignment fixturing problems make it difficult to achieve the same precision as is possible at Z. Other highlights included the useful comparison of slightly different PZT and ALOX compositions (neutron generator materials), temperature measurement using optical pyrometry, and the development of a new technique for preheating samples. 28 ICE tests have been conducted at Saturn to date, including the experiments described herein.

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Numerical calculations have been performed to investigate the role that load thickness may play in the performance of fast annular z pinch implosions. In particular, the effects of load thickness on the mitigation of the magnetically-driven Rayleigh-Taylor (RT) instability and energy coupling between the load and generator are addressed. using parameters representative of the Z accelerator [R.B.Spielman et al., Phys.Plasmas, 5, 2105 (1998)] at Sandia National Laboratories, two dimensional magnetohydrodynamic (MHD) simulations show that increased shell thickness results in lower amplitude, slightly longer wavelength RT modes. In addition, there appears to be an optimum in load velocity which is directly associated with the thickness of the sheath and subsequent RT growth. Thin, annular loads, which should couple efficiently to the accelerator, show a large reduction in implosion velocity due to extreme RT development and increased load inductance. As a consequence, thicker loads on the order of 5 mm, couple almost as efficiently to the generator since the RT growth is reduced. This suggests that z-pinch loads can be tailored for different applications, depending on the need for uniformity or high powers.