T. W. L. Sanford

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (188)181.92 Total impact

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    T.W.L. Sanford
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    ABSTRACT: Progress in understanding the physics of dynamic hohlraums is reviewed for a system that is capable of generating 10 TW of axial radiation for high-temperature ( 200 eV) radiation-flow experiments and inertial confinement fusion capsule implosions. Two-dimensional magneto-hydrodynamic simulation comparisons with data show the need to include wire initiation physics and subsequent discrete-wire dynamics in the simulations if a predictive capability is to be achieved.
    IEEE Transactions on Plasma Science 03/2008; · 0.87 Impact Factor
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    ABSTRACT: The mass of the outer and inner wire array used to drive the baseline dynamic hohlraum (DH) with pedestal target [ Sanford et al., Phys. Plasmas 13, 012701 (2006) ] is reversed in order to determine if the nested wire array is operating in a hydrodynamic, or transparent-like mode [ J. Davis et al., Appl. Phys. Lett. 70, 170 (1997) ], when the outer array arrives at the radius of the inner array. In contrast to the baseline, mass reversal allows the modes to be distinguished by the difference in the timing of characteristic features of the x-ray radiation pulses in the two modes. For the reversed-mass DH, all parameters such as wire number, array radii, and target remained the same, except the diameters of the individual wires were adjusted to reverse the array masses. Measurements show unambiguously that the reversed-mass DH operates in a transparent-like mode, the outer array passing through the inner array with limited collisional interaction. Numerical simulations in the r-θ plane suggest that the underlying physics of the outer array collision with the inner between the two DHs (baseline and reversed-mass), remains similar, implying that the baseline also operates with transparency. Inflection in the rate of change of the current is measured 4–7 ns after the radiation signal and is associated with the outer-inner array interaction, indicating that the rear portion of the resulting plasma shell of the outer array carries the current prior to the collision. Numerical simulations together with analytic theory describe probable dynamics of the current switching from the outer to inner array.
    Physics of Plasmas 05/2007; 14(5):052703-052703-17. · 2.38 Impact Factor
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    ABSTRACT: A Dynamic Hohlraum (DH) is formed when arrays of tungsten wires driven by a high-current pulse implode and compress a cylindrical foam target. The resulting radiation is confined by the wire plasma and forms an intense, ∼ 200–250 eV Planckian x-ray source. The internal radiation can be used for indirect drive inertial confinement fusion. The radiation emitted from the ends can be employed for radiation flow and material interaction studies. This external radiation is accompanied by an expanding blowoff plasma. We have diagnosed this blowoff plasma using K-shell spectra of Mg tracer layers placed at the ends of some of the Dynamic Hohlraum targets. A similar diagnosis of the interior hohlraum has been carried out using Al and Mg tracers placed at 2 mm depth from the ends. It is found that the blowoff plasma is about 20–25% as dense as that of the interior hohlraum, and that its presence does not significantly affect the outward flow of the nearly Planckian radiation field generated in the hohlraum interior. However, the electron temperature of the blowoff region, at ∼ 120 eV, is only about half that of the interior hohlraum plasma.
    Physics of Plasmas 04/2007; 14(4):042702-042702-5. · 2.38 Impact Factor
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    ABSTRACT: Axial symmetry in x-ray radiation of wire-array z pinches is important for the creation of dynamic hohlraums used to compress inertial-confinement-fusion capsules. We present the first evidence that this symmetry is directly correlated with the magnitude of the negative radial electric field along the wire surface. This field (in turn) is inferred to control the initial energy deposition into the wire cores, as well as any current shorting to the return conductor.
    Physical Review Letters 03/2007; 98(6):065003. · 7.73 Impact Factor
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    ABSTRACT: Dynamic hohlraum (DH) z-pinches on the Z machine at Sandia National Laboratories have proven to be an excellent radiation source for driving high energy density experiments. The Z DH source produces as much as 100 kJ with powers up to 17 TW into a 4-mm diameter, axially-located radiation exit hole above the source. The Sandia Laboratories¿ ZR upgrade of Z may be able to similarly produce 200 kJ. The complex Z DH radiation source varies in spectral power and timing from shot-to-shot. We are characterizing the source physics through simulation and experiment in order to minimize this variability. We are studying how the complex spatially and temporally-dependent behavior of the DH source affects the performance of a number of axially-located experiments. Using the Z DH as a source, we developed the new blast wave diagnostic, and applied it in a number of novel experiments. A large number of physics issues can be addressed by the Z DH and blast wave diagnostic, including experiments to study opacity, radiation transport and hydrodynamics. We will show how some experiments are quite sensitive to the source details while others are much less so. A synopsis and analysis of recently completed experiments as well as proposals for future work are presented.
    Magagauss Magnetic Field Generation and Related Topics, 2006 IEEE International Conference on; 12/2006
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    ABSTRACT: A dynamic Hohlraum (DH) is formed when arrays of tungsten wires driven by a high current pulse implode upon a cylindrical foam target. At impact, the wire plasma launches a radiating shock in the foam and confines the radiation. This sequence of events forms an intense, ∼ 200–250 eV Planckian x-ray source which is a prime candidate for indirect drive inertial confinement fusion. In recent DH experiments on the 20 MA Z facility, Al and MgF2 tracer layers were embedded in the cylindrical foam targets to provide K-shell lines in the keV spectral region for diagnosing the conditions of the interior Hohlraum plasma. Time-resolved K-shell spectra of both Al and Mg show mostly absorption lines. These data can be understood and quantitatively analyzed with detailed atomic and radiation transport models. The analyses show no evidence of intrinsic differences in the properties of the tops and bottoms of the Hohlraums. The interiors of the cylindrical Hohlraums are found to be hotter than the ends.
    Review of Scientific Instruments 09/2006; 77(10):10F303-10F303-4. · 1.60 Impact Factor
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    ABSTRACT: Measurements and analyses [ J. P. Apruzese et al., Phys. Plasmas 12, 012705 (2005) ] of Al and Mg K-shell lines from tracer layers symmetrically embedded in cylindrical dynamic-Hohlraum (DH) targets, driven by two nested tungsten-wire arrays in a z pinch, suggest that the radiation temperatures near either end of top or bottom radiation exit holes (REHs) of the DHs are similar. Moreover, the measured radii inferred for the shock developed within the targets converge towards the z axis symmetrically when viewed simultaneously through either end of the Hohlraums. These two results support the earlier observation [ T. W. L. Sanford et al., Phys. Plasmas 12, 022701 (2005) ] that the anticorrelation of the axial power with the magnitude of tungsten-wire material flowing near (or across) the given REH is due to increased tungsten opacity at the REH. This mechanism appears to be dominant in affecting the top-bottom (anode-cathode) symmetry in axial power, rather than there being any significant up-down difference in Hohlraum temperature or shock development. Additionally, we show that the insertion of two thin annular pedestals extending into the anode-cathode gap from either electrode, just radially outside of the REHs, improves the up-down power symmetry, decreases the rise time of the axial radiation, and decreases the shot-to-shot variation in the radiation pulse shape, and shock velocity. These improvements suggest that the quality of the plasma shell, which forms within the central region of the implosion, is superior to that adjacent to either electrode. Finally, enhanced emission on axis is observed, prior to the arrival of the main mass-driven shock from the impact of the wire arrays on the target. This phenomenon is consistent with the existence of a radiation-driven shock in the foam target which calculations indicate forms from radiation generated when the outer wire-array plasma impacts the inner array of the nest.
    Physics of Plasmas 01/2006; 13(1):012701-012701-21. · 2.38 Impact Factor
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    ABSTRACT: Diagnostic tracer layers of Al and/or Mg have been embedded in Dynamic Hohlraum targets which are imploded on Sandia National Laboratories' Z generator by surrounding them with nested arrays of tungsten wires. The K-shell lines of these elements are observed, usually in absorption, in both time-resolved and time-integrated spectra. The radiation physics of line formation in this environment is well understood and captured with a detailed model. A {chi}2 fit to the measured line intensities is used in conjunction with the model to determine the hohlraums' intrinsic properties. Among other features, our analyses find no evidence of intrinsic top-bottom asymmetry in the Dynamic Hohlraums.
    AIP Conference Proceedings. 01/2006; 808(1).
  • M G Haines, T W L Sanford, V P Smirnov
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    ABSTRACT: The wire-array z-pinch has in a very short time achieved remarkable performance as a powerful (>200 TW), pulsed soft x-ray source of high efficiency (~15%) and of great relevance to inertial confinement fusion. The underlying physics involves the transformation of wire cores to a plasma corona, the occurrence of uncorrelated axial instabilities, inward flowing low magnetic Reynolds number jets, sometimes an accumulated stable and dynamically confined precursor column, an almost constant velocity implosion when gaps occur in the wire cores and finally at stagnation a fast-rising soft x-ray pulse of typically 5 ns FWHM. Nested arrays improve the performance and can operate in several modes. Three hohlraum designs have been tested; one of these, the dynamic hohlraum, has achieved a radiation temperature of ~230 eV and has compressed a capsule from 2 to ~0.8 mm diameter with a neutron yield of > 1010 thermal DD neutrons. Lower mass stainless steel wire arrays are used for Kα radiation sources. Generally implosions lead to more energy radiated than the implosive kinetic energy, and this is hypothesized as being due to ion viscous heating, as fast-growing short wavelength nonlinear MHD instabilities are dissipated; record ion temperatures of 200–300 keV are predicted and have been measured for the stainless steel array on Z at Sandia.
    Plasma Physics and Controlled Fusion 11/2005; 47(12B):B1. · 2.37 Impact Factor
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    ABSTRACT: The Z accelerator [ R. B. Spielman, W. A. Stygar, J. F. Seamen et al., Proceedings of the 11th International Pulsed Power Conference, Baltimore, MD, 1997, edited by G. Cooperstein and I. Vitkovitsky (IEEE, Piscataway, NJ, 1997), Vol. 1, p. 709 ] at Sandia National Laboratories delivers ∼ 20 MA load currents to create high magnetic fields (>1000 T) and high pressures (megabar to gigabar). In a z-pinch configuration, the magnetic pressure (the Lorentz force) supersonically implodes a plasma created from a cylindrical wire array, which at stagnation typically generates a plasma with energy densities of about 10 MJ/cm3 and temperatures >1 keV at 0.1% of solid density. These plasmas produce x-ray energies approaching 2 MJ at powers >200 TW for inertial confinement fusion (ICF) and high energy density physics (HEDP) experiments. In an alternative configuration, the large magnetic pressure directly drives isentropic compression experiments to pressures >3 Mbar and accelerates flyer plates to >30 km/s for equation of state (EOS) experiments at pressures up to 10 Mbar in aluminum. Development of multidimensional radiation-magnetohydrodynamic codes, coupled with more accurate material models (e.g., quantum molecular dynamics calculations with density functional theory), has produced synergy between validating the simulations and guiding the experiments. Z is now routinely used to drive ICF capsule implosions (focusing on implosion symmetry and neutron production) and to perform HEDP experiments (including radiation-driven hydrodynamic jets, EOS, phase transitions, strength of materials, and detailed behavior of z-pinch wire-array initiation and implosion). This research is performed in collaboration with many other groups from around the world. A five year project to enhance the capability and precision of Z, to be completed in 2007, will result in x-ray energies of nearly 3 MJ at x-ray powers >300 TW.
    Physics of Plasmas 05/2005; 12(5):055503-055503-16. · 2.38 Impact Factor
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    ABSTRACT: 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.
    Physics of Plasmas 05/2005; 12(5):052705-052705-9. · 2.38 Impact Factor
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    ABSTRACT: Dynamic hohlraums driven by arrays consisting of large numbers of tungsten wires in Z pinches exhibit differences in radiation emitted from REHs (radiation exit holes) symmetrically located at either end of the hohlraum [Sanford et al., Phys. Plasmas 10, 1187 (2003)]. Significantly greater peak power is radiated from the top (anode) REH relative to the bottom (cathode) REH. Spectral measurements of tungsten M-shell emission (2–2.4 keV) indicate the peak radiated power from either REH anticorrelates with the fraction of wire-array tungsten plasma inferred to sweep across (or into the field of view of) the REH near the time of peak axial emission. In all cases, greater M-shell emission relative to the total emission in the band 1.4–4 keV is measured at the bottom REH in comparison to the top REH. The decrease in peak power radiated from the bottom REH relative to the top appears to be due, in part, to an increase in localized opacity arising from the presence of increased wire-array tungsten plasma near the bottom REH. The asymmetry in both peak axial power and pulse shape is largely removed by adding two thin annular pedestals extending 3 mm into the anode-cathode gap from either electrode, just radially outboard of the REHs. The pedestals are designed to prevent the radial flow of tungsten plasma from prematurely crossing the REHs. A polarity effect [Sarkisov et al., Phys. Rev. E 66, 046413(6) (2002)] during wire initiation may offer one possible explanation for the underlying cause of such a tungsten-related axial power asymmetry.
    Physics of Plasmas 01/2005; 12(2):022701-022701-15. · 2.38 Impact Factor
  • T. W. L. Sanford, R. G. Watt
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    ABSTRACT: Dynamic Hohlraums (DH) [1] driven by W wire-array Z pinches are being developed and used as intense black-body x-ray sources for high temperature radiation flow and ICF experiments on Z. They are currently the most energetic and intense pulsed-power driven radiation sources in the laboratory for these applications. Three methods for positioning and holding the wires in place, within these loads, have been developed: the ``flop-over'' [2], the ``hang-down'' [2], and the ``weightless''. The shot-to-shot variation in magnitude and shape of the radial and top-bottom axial powers and spectra are used to establish the efficacy of each wire-fixturing method. Comparisons among the 3 fixturing techniques illustrate the importance of good wire-cathode contact. In general, poor wire-electrode contact leads to a less coherent implosion, and to excessive W-wire plasma flowing across the REHs (radiation exit holes) located at either end of the DH, increasing the opacity of the REH, with subsequent lowering of axial power. [1] T. W. L. Sanford. et al, Phys. Plasmas 9, 3573 (2002). [2] T. W. L. Sanford. et al, Digest Tech. Papers, IEEE Inter. Conf. On Pulsed Power (Dallas, TX, 2003), pp 733-6. ^*Sandia is a multiprogram laboratory operated by the Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under Contract No. DE-AC04-94AL85000.
    01/2005;
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    ABSTRACT: Summary form only given. Critical parameters required to perform a radiation-flow experiment on a Z-pinch are the drive radiation spectrum, intensity, and time history. With the advent of the dynamic hohlraum source at the Sandia National Laboratories Z-machine the diagnostic access to monitor the drive radiation was limited to one end of a cylindrical source. Initial comparisons showed large asymmetries in intensity and time history between the two ends of a cylindrical TPX foam implosion target. Spectral analysis showed tungsten from the imploding wire array was blocking the radiation exit hole (REH) on the cathode end of the pinch. A 3 mm tall pedestal placed on the cathode end of the TPX cylinder has restored the time and intensity profile to match the output from the anode end REH. Profiles measured from some 30 shots are presented which illustrate the evolution of the diagnostic technique into a reliable power monitoring method. We have installed computer software at the Z-machine which allows spectral deconvolution and power calculations to be performed within 30 minutes after firing the machine. This rapid data analysis identifies diagnostic problems and allows adjustments to be made in experiment parameters for the following day's shot. We also present initial results from a small, 1 mm diameter, hole placed in the coupling funnel that connects the anode side REH with a radiation flow experiment. Expectations of correlating the reliable cathode end 4 mm diameter REH measurement with the small anode end REH monitor hole have proven to be both encouraging and frustrating
    IEEE International Conference on Plasma Science 01/2005;
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    ABSTRACT: The quality of high wire-number z-pinch implosions on Z using a dynamic hohlraum (DH) configuration [Sanford, et al., Phys. Plasmas 9, 3573 (2002)] is significantly affected by the method of holding the wires. The three arrangements discussed here have led to differences in radial and axial x-ray powers of factors of 1.6+/-0.2 and 1.5+/-0.2, respectively. An increase in power is accompanied by reductions in rise time and pulse width, and improvements in shot-to-shot reproducibility. Higher powers are produced by fixtures that enable the wires to be maintained taut, which also produce superior current contacts at the electrodes (and in particular at the cathode) prior to implosion. The increased axial power, and decreased variation in power and pulse shape, correlate with decreased wire-plasma material observed at the axial radiation exit holes of the DH.
    Physics of Plasmas 01/2005; 12. · 2.38 Impact Factor
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    ABSTRACT: In recent dynamic hohlraum experiments on the Z facility, Al and MgF2 tracer layers were embedded in cylindrical CH2 foam targets to provide K-shell lines in the keV spectral region for diagnosing the conditions of the interior hohlraum plasma. The position of the tracers was varied: sometimes they were placed 2 mm from the ends of the foam cylinder and sometimes at the ends of the cylinder. Also varied was the composition of the tracers in the sense that pure Al layers, pure MgF2 layers, or mixtures of the elements were employed on various shots. Time-resolved K-shell spectra of both Al and Mg show mostly absorption lines. These data can be analyzed with detailed configuration atomic models of carbon, aluminum, and magnesium in which spectra are calculated by solving the radiation transport equation for as many as 4100 frequencies. We report results from shot Z1022 to illustrate the basic radiation physics and the capabilities as well as limitations of this diagnostic method.
    Physics of Plasmas 12/2004; 12(1):012705-012705-8. · 2.38 Impact Factor
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    ABSTRACT: Progress in understanding the physics of dynamic-hohlraums is reviewed for a system capable of generating 13 TW of axial radiation for high temperature (>200 eV) radiation-flow experiments and ICF capsule implosions.
    Plasma Physics and Controlled Fusion 11/2004; 46(12B):B423. · 2.37 Impact Factor
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    ABSTRACT: Dynamic-hohlraums (DH) driven by a Z pinch are being developed and used as intense blackbody x-ray sources for ICF and radiation-transport experiments [1]. Evidence from other experimental geometries indicates that for high wire-number tungsten arrays [2], the collision of the outer array with the inner array is more transparent than hydrodynamic. For our DH, with an on-axis foam target, computational differences in implosion characteristics (such as implosion time) are minimal between the two collision mechanisms: hydrodynamic versus transparent. To help resolve this ambiguity, the 2-to-1 mass ratio between the outer and inner arrays used in the DH driver was reversed keeping all other parameters of the array, such as wire number and array radii, the same. For the reversed mass ratio experiments, preliminary analysis supports the likelihood of a greater transparency in the collision of the arrays than previously assumed in the radiation-hydrodynamic simulations. [1] T. W. L. Sanford, et al, Phys.Plasmas 9, 3573 (2002); [2] M. E. Cuneo et al, this conference. *Sandia is a multiprogram laboratory operated by the Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy under Contract No. DE-AC04-94AL85000.
    11/2004;
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    ABSTRACT: A bottom axial diagnostic package has recently been developed and fielded on the 100 ns, 20 MA pinch-driver Z. The bottom package was developed to measure the power radiated to the bottom of Z and compare it to the power radiated to the top of Z on dynamic hohlraum pinch loads. When an up∕down power asymmetry was measured, the bottom package was expanded in an effort to determine the source of the asymmetry. The bottom package contains one port directly on axis, six ports at 3.4° to the axis, and four ports at 9° to the axis. Typical diagnostics fielded on the bottom package are a time-resolved pinhole camera, time-integrated spatially resolved convex crystal spectrometers, a time-resolved crystal spectrometer, x-ray diodes, bolometers, and photoconducting detectors. We will present some typical data from these bottom diagnostics on dynamic hohlraum shots on Z and briefly discuss their relevance to the up∕down power asymmetry.
    Review of Scientific Instruments 10/2004; 75(10):3684-3686. · 1.60 Impact Factor
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    ABSTRACT: Summary form only given. We present laser shadowgraph "movies" of plasma structures associated with wire array implosion dynamics for several different load geometries and wire materials fielded on the Z accelerator. A sub-nanosecond visible framing camera is used to record 6 sequential frames of laser illumination or plasma self-emission. The strength of these observations is the ability to correlate early ablation features with later stagnation structures on the same shot. Simultaneously, a monochromatic X-ray backlighter was used to probe the wire array plasma at a single moment in time but at a greater density and spatial resolution. This set of complimentary diagnostics provides new possibilities for the direct observation of wire array ablation and stagnation dynamics. We observe a strong disagreement between experimental results and a 0-D compression trajectory. The compression starts later in time and has a steeper trajectory than the 0-D prediction due to prolonged plasma generation. Periodic structures, or flares, form initially at the cathode end of the array with a spatial period of ∼250 μm and angle up towards the anode at 9°. Correlated with the formation of these flares is the observation, later in time, of lower plasma densities in this region near the cathode. This implosion asymmetry is thought to be responsible for the observed top/bottom asymmentry of the X-ray power. Also occuring during initiation are small explosions which are randomly distributed along the lengths of the wires. These explosions are strongly correlated to regions where mass remains behind which does not participate in the implosion. Even after the main implosion front has passed, this mass remains at the original position of the wires drifting up towards the anode faster than it drifts in towards the axis.
    Plasma Science, 2004. ICOPS 2004. IEEE Conference Record - Abstracts. The 31st IEEE International Conference on; 08/2004