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Investigation of compressible electromagnetic flute mode instability in finite beta plasma in support of Z-pinch and laboratory astrophysics experiments

Communications in Computational Physics

Sotnikov, V.I.; Ivanov, V.V.; Presura, R.; Leboeuf, J.N.; Onishchenko, O.G.; Oliver, Bryan V.; Jones, Brent M.; Mehlhorn, Thomas A.; Deeney, Chris

Flute mode turbulence plays an important role in numerous applications, such as tokamak, Z-pinch, space and astrophysical plasmas. In a low beta plasma flute oscillations are electrostatic and in the nonlinear stage they produce large scale density structures co-mingling with short scale oscillations. Large scale structures are responsible for the enhanced transport across the magnetic field and appearance of short scales leads to ion heating, associated with the ion viscosity. In the present paper nonlinear equations which describe the nonlinear evolution of the flute modes treated as compressible electromagnetic oscillations in a finite beta inhomogeneous plasma with nonuniform magnetic field are derived and solved numerically. For this purpose the 2D numerical code FLUTE was developed. Numerical results show that even in a finite beta plasma flute mode instability can develop along with formation of large scale structures co-existing with short scale perturbations in the nonlinear stage. © 2008 Global-Science Press.

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TYPE Conference YEAR 2008
ScopusOSTI

Monochromatic soft x-ray self-emission imaging in dense z pinches

AIP Conference Proceedings

Jones, Brent M.; Deeney, C.; Meyer, C.J.; Coverdale, Christine A.; LePell, P.D.; Apruzese, J.P.; Clark, R.W.; Davis, J.; Peterson, K.J.

The Z machine at Sandia National Laboratories drives 20 MA in 100 ns through a cylindrical array of fine wires which implodes due to the strong j × B force, generating up to 250 TW of soft x-ray radiation when the z-pinch plasma stagnates on axis. The copious broadband self-emission makes the dynamics of the implosion well suited to diagnosis with soft x-ray imaging and spectroscopy. A monochromatic self-emission imaging instrument has recently been developed on Z which reflects pinhole images from a multilayer mirror onto a 1 ns gated microchannel plate detector. The multilayer can be designed to provide narrowband (∼10 eV) reflection in the 100-700 eV photon energy range, allowing observation of the soft emission from accreting mass as it assembles into a hot, dense plasma column on the array axis. In the present instrument configuration, data at 277 eV photon energy have been obtained for plasmas ranging from Al to W, and the z-pinch implosion and stagnation will be discussed along with > 1 keV self-emission imaging and spectroscopy. Collisional-radiative simulations are currently being pursued in order to link the imaged emissivity to plasma temperature and density profiles and address the role of opacity in interpreting the data. © 2007 American Institute of Physics.

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TYPE Conference YEAR 2007
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Planar wire array performance scaling at multi-MA levels on the Saturn generator

Jones, Brent M.

A series of twelve shots were performed on the Saturn generator in order to conduct an initial evaluation of the planar wire array z-pinch concept at multi-MA current levels. Planar wire arrays, in which all wires lie in a single plane, could offer advantages over standard cylindrical wire arrays for driving hohlraums for inertial confinement fusion studies as the surface area of the electrodes in the load region (which serve as hohlraum walls) may be substantially reduced. In these experiments, mass and array width scans were performed using tungsten wires. A maximum total radiated x-ray power of 10 {+-} 2 TW was observed with 20 mm wide arrays imploding in {approx}100 ns at a load current of {approx}3 MA, limited by the high inductance. Decreased power in the 4-6 TW range was observed at the smallest width studied (8 mm). 10 kJ of Al K-shell x-rays were obtained in one Al planar array fielded. This report will discuss the zero-dimensional calculations used to design the loads, the results of the experiments, and potential future research to determine if planar wire arrays will continue to scale favorably at current levels typical of the Z machine. Implosion dynamics will be discussed, including x-ray self-emission imaging used to infer the velocity of the implosion front and the potential role of trailing mass. Resistive heating has been previously cited as the cause for enhanced yields observed in excess of jxB-coupled energy. The analysis presented in this report suggests that jxB-coupled energy may explain as much as the energy in the first x-ray pulse but not the total yield, which is similar to our present understanding of cylindrical wire array behavior.

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TYPE SAND Report YEAR 2007
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Towards a predictive MHD simulation capability for designing hypervelocity magnetically-driven flyer plates and PWclass z-pinch x-ray sources on Z and ZR

Mehlhorn, Thomas A.; Yu, Edmund Y.; Vesey, Roger A.; Cuneo, M.E.; Jones, Brent M.; Knudson, Marcus D.; Sinars, Daniel S.; Robinson, Allen C.; Trucano, Timothy G.; Brunner, Thomas A.; Desjarlais, Michael P.; Garasi, Christopher J.; Haill, Thomas A.; Hanshaw, Heath L.; Lemke, Raymond W.; Oliver, Bryan V.; Peterson, Kyle J.

Abstract not provided.

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TYPE Conference YEAR 2007
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K-shell radiation physics in low- to moderate-atomic-number z-pinch plasmas on the Z accelerator

Journal of Quantitative Spectroscopy and Radiative Transfer

Jones, Brent M.; Deeney, C.; Coverdale, Christine A.; LePell, P.D.; Mckenney, John M.; Apruzese, J.P.; Thornhill, J.W.; Whitney, K.G.; Clark, R.W.; Velikovich, A.L.; Davis, J.; Maron, Y.; Kantsyrev, V.; Safronova, A.; Oreshkin, V.I.

Dense z-pinches produced by 100 ns implosions of wire arrays or gas puffs produce substantial soft X-ray power. One class of z-pinch radiation sources includes low- to moderate-atomic-number K-shell radiators, such as aluminum and iron. These loads are designed for 1-10 keV K-shell X-ray generation, and offer opportunities for crystal spectroscopy that can reveal fundamental properties of the plasma when studied using plasma spectroscopic modeling. Typically these plasmas are characterized by ion densities of ∼1020 cm-3, diameters of 1-5 mm, electron temperatures up to several keV, and a range of opacities of the K-shell lines. Measurements from wire arrays on Sandia's 20 MA Z accelerator are presented along with collisional radiative and hydrodynamic simulations. The impact of opacity and 3D structure on non-LTE, non-diffusive radiation transport and X-ray production is discussed. © 2005 Elsevier Ltd. All rights reserved.

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TYPE Journal Article YEAR 2006
ScopusOSTI

Implosion dynamics and spectroscopy of X-pinches and wire arrays with doped Al wires on the UNR 1MA Z-pinch generator

AIP Conference Proceedings

Safronova, Alla; Kantsyrev, Victor; Esaulov, Audrey; Fedin, Dmitry; Ouart, Nicholas; Yilmaz, Fatih; Osborne, Glenn; Nalajala, Vidya; Pokala, Shivaji; Shrestha, Ishor; Astanovitsky, Alexey; Batie, Steve; Legalloudec, Bruno; Cowan, Tom; Jones, Brent M.; Coverdale, Christine A.; Deeney, Christopher D.; Lepell, David; Gradel, Josh

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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TYPE Conference YEAR 2006
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Investigation of regimes of wire array implosion on the 1 MA Zebra accelerator

Physics of Plasmas

Ivanov, V.V.; Kantsyrev, V.L.; Sotnikov, V.I.; Fedin, D.A.; Astanovitskiy, A.L.; Le Galloudec, B.; Nalajala, V.; Shrestha, I.; Cowan, T.E.; Jones, Brent M.; Coverdale, Christine A.; Deeney, C.; LePell, P.D.

Implosion of wire arrays was investigated at the 1 MA Zebra accelerator by multiframe laser probing and gated x-ray self-emission diagnostics. Different regimes of implosion were observed in Al and Cu wire arrays. Implosion of Al loads with masses of 33-37 μgcm produces a dense pinch 1-1.5 mm in diameter. Strong instabilities are observed in the Z pinch at the time of stagnation. Implosion of "overmassed" loads produces a plasma column 3-4 mm in diameter with a core. The plasma column does not collapse during the x-ray pulse. The core of the plasma column is not subjected to the kink instability and transforms to a chain of dense spots in the later stage. Different regimes of implosion were observed in Al 8×15 μm loads presumably due to variations in the current pulse and load conditions. Observed regimes are compared to three-dimensional hybrid simulation of ideal and nonideal magnetohydrodynamics modes of implosion. © 2006 American Institute of Physics.

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TYPE Journal Article YEAR 2006
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Radiative properties of asymmetric and symmetric X-pinches with two and four wires recently produced on the UNR 1 MA Zebra generator

Journal of Quantitative Spectroscopy and Radiative Transfer

Kantsyrev, Victor; Safronova, A.; Ivanov, V.; Fedin, D.; Mancini, R.; Astanovitsky, A.; LeGalloudec, B.; Batie, S.; Brown, D.; Nalajala, V.; Shrestha, I.; Pokala, S.; Ouart, N.; Yilmaz, F.; Clinton, A.; Johnson, M.; Cowan, T.; Jones, Brent M.; Coverdale, Christine A.; Deeney, C.; LePell, P.D.; Jobe, D.; Nielson, D.

Experimental results of studies of the 1 MA X-pinch X-ray source in a wide spectral region are overviewed. Implosion dynamics and radiative properties of various X-pinches were studied by spatially and time-resolved X-ray and optical diagnostics. In particular, dynamics of spatial and temporal developments of the structure of X-ray emitting regions (1-5 keV), temporal characteristics of X-ray pulses, X-ray radiation outputs and electron beam characteristics from symmetric and asymmetric Mo, Cu, and combined asymmetric Mo/W X-pinches with two or four wires were studied. The mechanisms of X-ray multiburst generation are discussed. The future applications of the high-current X-pinch as a 5-10 kJ sub-keV-10 keV radiation driver are considered. © 2005 Elsevier Ltd. All rights reserved.

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TYPE Conference YEAR 2006
ScopusOSTI

Seeded perturbations in wire array Z-Pinches

Jones, Brent M.; Deeney, Christopher D.; Mckenney, John M.; Garasi, Christopher J.; Mehlhorn, Thomas A.; Robinson, Allen C.; Coverdale, Christine A.

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.

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TYPE Conference YEAR 2005
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Dynamics of copper wire arrays at 1 MA and 20 MA

Coverdale, Christine A.; Jones, Brent M.; Deeney, Christopher D.

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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TYPE Conference YEAR 2005
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Hybrid simulations of Z-Pinches in support of wire array implosion experiments at NTF

Coverdale, Christine A.; Jones, Brent M.; Deeney, Christopher D.

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.

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TYPE Conference YEAR 2005
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Spectroscopic modeling and comparison of radiation from X-pinches and wire arrays produced on the 1 MA pulsed power generator at UNR

Coverdale, Christine A.; Jones, Brent M.; Deeney, Christopher D.

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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TYPE Conference YEAR 2005
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Ion viscous heating in a magnetohydrodynamically unstable Z-pinch at over two billion Kelvin

Proposed for publication in Nature.

Jones, Brent M.; Coverdale, Christine A.; Deeney, Christopher D.

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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TYPE Journal Article YEAR 2005
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Seeded perturbations in wire array z-pinches

Jones, Brent M.; Deeney, Christopher D.; Mckenney, John M.; Garasi, Christopher J.; Mehlhorn, Thomas A.; Robinson, Allen C.; Wunsch, Scott E.

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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TYPE Conference YEAR 2004
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Ideal and non-ideal MHD regimes of wire array implosion obtained in 3D hybrid simulations and observed during experiments at NTF

Mehlhorn, Thomas A.; Coverdale, Christine A.; Jones, Brent M.; Deeney, Christopher D.

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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TYPE Conference YEAR 2004
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Nested stainless steel wire array diameter variations on the Z accelerator

Jones, Brent M.; Deeney, Christopher D.

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.

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TYPE Conference YEAR 2004
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Chemically etched modulation in wire radius for wire array Z-pinch perturbation studies

Proposed for publication in Review of Scientific Instruments.

Deeney, Christopher D.; Jones, Brent M.; Mckenney, John M.

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.

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TYPE Journal Article YEAR 2004
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Results 201–293 of 293
Results 201–293 of 293