D. S. Nielsen

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (32)49.74 Total impact

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    ABSTRACT: The pinned optically aligned diagnostic dock (PODD) is a multi-configuration diagnostic platform designed to measure x-ray emission on the Z facility. The PODD houses two plasma emission acquisition (PEA) systems, which are aligned with a set of precision machined pins. The PEA systems are modular, allowing a single diagnostic housing to support several different diagnostics. The PEA configurations fielded to date include both time-resolved and time-integrated, 1D spatially resolving, elliptical crystal spectrometers, and time-integrated, 1D spatially resolving, convex crystal spectrometers. Additional proposed configurations include time-resolved, monochromatic mirrored pinhole imagers and arrays of filtered x-ray diodes, diamond photo-conducting diode detectors, and bolometers. The versatility of the PODD system will allow the diagnostic configuration of the Z facility to be changed without significantly adding to the turn-around time of the machine. Additionally, the PODD has been designed to allow instrument setup to be completed entirely off-line, leaving only a refined alignment process to be performed just prior to a shot, which is a significant improvement over the instrument the PODD replaces. Example data collected with the PODD are presented.
    The Review of scientific instruments 10/2012; 83(10):10D714. · 1.52 Impact Factor
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    ABSTRACT: Magnetic implosions provide extremely intense soft X-ray radiation on the Z accelerator. Shock heating at stagnation provides temperatures that are capable of producing K-shell radiation from stainless steel plasma. Time-gated multicolor X-ray pinhole imaging is used to study stagnation and disruption in fast Z pinches. Magnetohydrodynamic instabilities are observed to grow, following peak X-ray power until the Z-pinch column disrupts well after the main power pulse.
    IEEE Transactions on Plasma Science 12/2011; · 0.87 Impact Factor
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    ABSTRACT: form only given. Magnetically driven implosions of wire array and gas puff loads on the 20 MA Z and 8 MA Saturn pulsed power drivers provide extremely intense x-ray sources. Each facility fields multi-frame, multi-color, x-ray pinhole cameras in order to assess source uniformity and study implosion dynamics. The instruments feature cameras filtered for >;1 keV photons, as well as pinhole cameras in which
    IEEE International Conference on Plasma Science 01/2011;
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    ABSTRACT: Iron plasma opacity influences the internal structure of the sun. However, opacity models have never been experimentally tested at stellar interior conditions. Initial experiments at the Sandia Z facility reached temperatures high enough to investigate the iron charge states that exist near the convection/radiation zone (CZ) boundary. In these experiments the density was an order of magnitude lower than at the CZ boundary, preventing studies of important effects such as line broadening. New experiments have reached higher densities and temperatures. Progress to solidify these results and use them to examine opacity models will be described.++Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.
    11/2010;
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    ABSTRACT: The internal structure of stars depends on the radiative opacity of the stellar matter. However, opacity models have never been experimentally tested at the conditions that exist inside stars. Experiments at the Sandia Z facility are underway to measure the x-ray transmission of iron, an important stellar constituent, at temperature and density high enough to evaluate the physical underpinnings of stellar opacity models. Initial experiments provided information on the charge state distribution and the energy level structure for the iron ions that exist at the solar radiation/convection boundary. Data analysis and new experiments at higher densities and temperatures will be described.
    11/2009;
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    ABSTRACT: Recent experiments extended iron opacity model tests to temperatures above 150 eV for the first time. The experiments use the Z Facility to volumetrically heat a CH-tamped Fe/Mg plasma using x-rays. The frequency dependent sample transmission is measured by viewing a backlight through the sample. The plasma conditions are inferred from the Mg K-shell absorption. The strategy for this research is to examine the underlying physics within Fe opacity models by comparisons with the measured transmission. Physics topics of interest include charge state distribution, energy level structure, and line broadening. In this talk we discuss methods to exploit the data and advance understanding for these topics. In addition, we review new experiments under way to further improve the data and to achieve higher energy density conditions.
    AIP Conference Proceedings 09/2009; 1161(1).
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    ABSTRACT: Understanding stellar interiors, inertial confinement fusion, and Z pinches depends on opacity models for mid-Z plasmas in the 100-300 eV temperature range. These models are complex and experimental validation is crucial. In this paper we describe the diagnosis of the first experiments to measure iron plasma opacity at a temperature high enough to produce the charge states and electron configurations that exist in the solar interior. The dynamic Hohlraum x-ray source at Sandia National Laboratories' Z facility was used to both heat and backlight Mg/Fe CH tamped foils. The backlighter equivalent brightness temperature was estimated to be T(r) approximately 314 eV+/-8% using time-resolved x-ray power and imaging diagnostics. This high brightness is significant because it overwhelms the sample self-emission. The sample transmission in the 7-15.5 A range was measured using two convex potassium acid phthalate crystal spectrometers that view the backlighter through the sample. The average spectral resolution over this range was estimated to be lambda/deltalambda approximately 700 by comparing theoretical crystal resolution calculations with measurements at 7.126, 8.340, and 12.254 A. The electron density was determined to be n(e)=6.9+/-1.7 x 10(21) cm(-3) using the Stark-broadened Mg Hebeta, Hegamma, and Hedelta lines. The temperature inferred from the H-like to He-like Mg line ratios was T(e)=156+/-6 eV. Comparisons with three different spectral synthesis models all have normalized chi(2) that is close to unity, indicating quantitative consistency in the inferred plasma conditions. This supports the reliability of the results and implies the experiments are suitable for testing iron opacity models.
    The Review of scientific instruments 12/2008; 79(11):113104. · 1.52 Impact Factor
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    ABSTRACT: A multicolor, time-gated, soft x-ray pinhole imaging instrument is fielded as part of the core diagnostic set on the 25 MA Z machine [M. E. Savage et al., in Proceedings of the Pulsed Power Plasma Sciences Conference (IEEE, New York, 2007), p. 979] for studying intense wire array and gas puff Z-pinch soft x-ray sources. Pinhole images are reflected from a planar multilayer mirror, passing 277 eV photons with <10 eV bandwidth. An adjacent pinhole camera uses filtration alone to view 1-10 keV photons simultaneously. Overlaying these data provides composite images that contain both spectral as well as spatial information, allowing for the study of radiation production in dense Z-pinch plasmas. Cu wire arrays at 20 MA on Z show the implosion of a colder cloud of material onto a hot dense core where K-shell photons are excited. A 528 eV imaging configuration has been developed on the 8 MA Saturn generator [R. B. Spielman et al., and A. I. P. Conf, Proc. 195, 3 (1989)] for imaging a bright Li-like Ar L-shell line. Ar gas puff Z pinches show an intense K-shell emission from a zippering stagnation front with L-shell emission dominating as the plasma cools.
    The Review of scientific instruments 11/2008; 79(10):10E906. · 1.52 Impact Factor
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    ABSTRACT: Microchannel plates (MCPs) are a standard detector for fast-framing x-ray imaging and spectroscopy of high-temperature plasmas. The MCP is coated with conductive striplines that carry short duration voltage pulses to control the timing and amplitude of the signal gain. This gain depends on the voltage to a large exponent so that small reflections or impedance losses along the striplines can have a significant impact on the position-dependent amplitude and pulse width of the gain. Understanding the pulsed gain response therefore requires careful measurements of the position- and time-dependent surface voltage coupled with detailed modeling of the resulting electron cascade. We present measurements and modeling of the time- and space-dependent gain response of MCP detectors designed for use at Sandia National Laboratories' Z facility. The pulsed gain response is understood through measurements using a high impedence probe to determine the voltage pulse propagating along the stripline surface. Coupling the surface voltage measurements with Monte Carlo calculations of the electron cascade in the MCP provides a prediction of the time- and position-dependent gain that agrees with measurements made on a subpicosecond UV laser source to within the 25% uncertainty in the simulations.
    The Review of scientific instruments 11/2008; 79(10):10E902. · 1.52 Impact Factor
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    ABSTRACT: Understanding of microchannel plate (MCP) detectors with x-ray energy is important for applications in high energy density research such as broadband imaging and x-ray spectroscopy. The relative sensitivity with photon energy for Cu/Au coated MCPs in the range of 250 eV<hν<5000 eV has been measured at the National Synchrotron Light Source. A model of this response that includes contributions from secondary photoelectron yield and interactions with multiple channels is presented. This model is shown to agree with the measured MCP response to <20% over the majority of the spectral range using cross sections determined from an independent analysis of the MCP glass composition.
    Review of Scientific Instruments 10/2006; 77(10):10E323-10E323-4. · 1.60 Impact Factor
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    ABSTRACT: An on-axis time-resolved x-ray pinhole camera has been used on the 20 MA 100 ns driver Z to image the converging shock wave in dynamic Hohlraum experiments and to image pellet hot spots in inertial confinement fusion implosions. This instrument is susceptible to detecting significant amounts of pinch bremsstrahlung radiation with energies at hundreds of keV and yields of roughly 1 kJ. Quite often the bremsstrahlung noise signals have overwhelmed the desired x-ray images. In an effort to eliminate this large source of noise we have incorporated a 6° gold-coated grazing incidence mirror into the time-resolved x-ray pinhole camera system. The mirror reflects soft x rays at energies under 2 keV but does not reflect bremsstrahlung radiation at hundreds of keV. We will present data from the instrument without the mirror showing large amounts of bremsstrahlung noise contamination and data with the mirror in the system showing greatly reduced noise levels.
    Review of Scientific Instruments 10/2006; 77(10):10E319-10E319-3. · 1.60 Impact Factor
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    ABSTRACT: Z-pinch dynamic hohlraums are a promising indirect-drive inertial confinement fusion approach. Comparison of multiple experimental methods with integrated Z-pinch∕hohlraum∕capsule computer simulations builds understanding of the hohlraum interior conditions. Time-resolved x-ray images determine the motion of the radiating shock that heats the hohlraum as it propagates toward the hohlraum axis. The images also measure the radius of radiation-driven capsules as they implode. Dynamic hohlraum LASNEX [ G. Zimmerman and W. Kruer, Comments Plasma Phys. Control. Fusion 2, 85 (1975) ] simulations are found to overpredict the shock velocity by ∼ 20–40%, but simulated capsule implosion trajectories agree reasonably well with the data. Measurements of the capsule implosion core conditions using time- and space-resolved Ar tracer x-ray spectroscopy and the fusion neutron yield provide additional tests of the integrated hohlraum-implosion system understanding. The neutron yield in the highest performing CH capsule implosions is ∼ 20–30% of the yield calculated with unperturbed 2D LASNEX simulations. The emissivity-averaged electron temperature and density peak at approximately 900 eV and 4×1023 cm−3, respectively. Synthetic spectra produced by postprocessing 1D LASNEX capsule implosion simulations possess spectral features from H-like and He-like Ar that are similar in duration to the measured spectra. However, the simulation emissivity-averaged density peaks at a value that is four times lower than the experiment, while the temperature is approximately 1.6 times higher. The agreement with the capsule trajectory measurements and the ability to design capsule implosions that routinely produce implosion cores hot and dense enough to emit fusion neutrons and Ar spectra are evidence for a respectable degree of dynamic hohlraum understanding. The hohlraum shock velocity and implosion core discrepancies imply that calculations of the hohlraum radiation driving capsule implosions require further refinement.
    Physics of Plasmas 05/2006; 13(5). · 2.38 Impact Factor
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    ABSTRACT: The z-pinch dynamic hohlraum is used as a high-power x-ray source for a variety of HEDP applications including radiation physics, opacity measurements, and inertial confinement fusion (ICF). In each of these applications, the usefulness of the source depends on the reproducibility of the pulsed power performance and the resulting x-ray energy emission and pulse shape. A statistical analysis of a number of different performance metrics has been completed for > 10 experiments with nearly identical z-pinch target geometry and diagnostic viewing access. It is found that the 1-sigma reproducibility of the x-ray energy emission and pulse-shape is < 13% and < 4% respectively. A discussion of this analysis and the impact on the various HEDP applications is included.
    01/2006;
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    ABSTRACT: Experimental results of studies of the 1MA 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 subkeV–10 keV radiation driver are considered.
    Journal of Quantitative Spectroscopy and Radiative Transfer 01/2006; 99(1-3):349-362. · 2.38 Impact Factor
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    ABSTRACT: We present observations for 20-MA wire-array z pinches of an extended wire ablation period of 57%+/-3% of the stagnation time of the array and non-thin-shell implosion trajectories. These experiments were performed with 20-mm-diam wire arrays used for the double- z -pinch inertial confinement fusion experiments [M. E. Cuneo, Phys. Rev. Lett. 88, 215004 (2002)] on the Z accelerator [R. B. Spielman, Phys. Plasmas 5, 2105 (1998)]. This array has the smallest wire-wire gaps typically used at 20 MA (209 microm ). The extended ablation period for this array indicates that two-dimensional (r-z) thin-shell implosion models that implicitly assume wire ablation and wire-to-wire merger into a shell on a rapid time scale compared to wire acceleration are fundamentally incorrect or incomplete for high-wire-number, massive (>2 mg/cm) , single, tungsten wire arrays. In contrast to earlier work where the wire array accelerated from its initial position at approximately 80% of the stagnation time, our results show that very late acceleration is not a universal aspect of wire array implosions. We also varied the ablation period between 46%+/-2% and 71%+/-3% of the stagnation time, for the first time, by scaling the array diameter between 40 mm (at a wire-wire gap of 524 mum ) and 12 mm (at a wire-wire gap of 209 microm ), at a constant stagnation time of 100+/-6 ns . The deviation of the wire-array trajectory from that of a thin shell scales inversely with the ablation rate per unit mass: f(m) proportional[dm(ablate)/dt]/m(array). The convergence ratio of the effective position of the current at peak x-ray power is approximately 3.6+/-0.6:1 , much less than the > or = 10:1 typically inferred from x-ray pinhole camera measurements of the brightest emitting regions on axis, at peak x-ray power. The trailing mass at the array edge early in the implosion appears to produce wings on the pinch mass profile at stagnation that reduces the rate of compression of the pinch. The observation of precursor pinch formation, trailing mass, and trailing current indicates that all the mass and current do not assemble simultaneously on axis. Precursor and trailing implosions appear to impact the efficiency of the conversion of current (driver energy) to x rays. An instability with the character of an m = 0 sausage grows rapidly on axis at stagnation, during the rise time of pinch power. Just after peak power, a mild m = 1 kink instability of the pinch occurs which is correlated with the higher compression ratio of the pinch after peak power and the decrease of the power pulse. Understanding these three-dimensional, discrete-wire implosion characteristics is critical in order to efficiently scale wire arrays to higher currents and powers for fusion applications.
    Physical Review E 04/2005; 71(4 Pt 2):046406. · 2.31 Impact Factor
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    ABSTRACT: Summary form only given. X-ray spectrometers used in modern high energy density (HED) plasma experiments must provide high time, space, and spectral resolution while overcoming the difficulties imposed by the environment, including x-ray background, debris, and mechanical shocks. At the Z facility these problems are addressed using a suite of elliptical crystal spectrometers. The elliptical geometry isolates the detector from the line of sight with a slit placed at the elliptical focus, while the sensitivity enables locating the crystal 2-4 meters from the plasma source. Space and time resolution are obtained by using an array of slits to project a ID plasma image onto the crystal and recording the spectrally-dispersed image with a gated MCP detector. A spectrometer equipped with two crystals is used to double the number of time frames, enabling longer time durations to be spanned by a single instrument. Alternatively, the double crystal design allows simultaneous measurements with different crystals to investigate different spectral regimes. A spectrometer observing the plasma from a given direction suffers from a problem common to many HED plasma diagnostics: the data are integrated along the instrument line of sight, possibly leading to complications if the emitting plasma is non-uniform. This problem is being addressed at Z by deploying three separate spectrometers that observe the plasma from three almost-orthogonal directions. Multi-view measurements are common in relatively-large magnetic fusion plasmas, but have not been previously acquired in HED plasmas because of the need for simultaneous high spatial and temporal resolution. Spectra acquired from multiple views can provide information about the plasma symmetry from diverse experiments, including inertial confinement fusion capsule implosions and Z-pinch plasmas.
    IEEE International Conference on Plasma Science 01/2005;
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    ABSTRACT: Implosions of various wire arrays and x-pinches were studied by spatially- and time-resolved x-ray and optical diagnostics, spectroscopic modeling and hybrid simulation. In particular, data were obtained on investigation of particle transport in Al and Ti wire arrays with thin coating, Cu wire-array initiation, and Ti x-pinch properties with Mo coating wires. X-ray radiation and electron beam characteristics from symmetric and asymmetric Mo, Cu, and combined Mo/W x-pinches with two or four wires were studied. The work was supported by DOE/NNSA under UNR grant DE-FC52-01NV14050, and by Sandia National Laboratories under DOE contract 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: Hot dense capsule implosions driven by Z-pinch x rays have been measured using a approximately 220 eV dynamic Hohlraum to implode 1.7-2.1 mm diameter gas-filled CH capsules. The capsules absorbed up to approximately 20 kJ of x rays. Argon tracer atom spectra were used to measure the T(e) approximately 1 keV electron temperature and the n(e) approximately 1-4 x 10(23) cm(-3) electron density. Spectra from multiple directions provide core symmetry estimates. Computer simulations agree well with the peak emission values of T(e), n(e), and symmetry, indicating reasonable understanding of the Hohlraum and implosion physics.
    Physical Review Letters 03/2004; 92(8):085002. · 7.94 Impact Factor
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    ABSTRACT: We present a technique for experimentally measuring two-dimensional radiation temperatures in dynamic Hohlraums on Z. In principle the technique can be applied to any radiation source. Total radiated power from the source is measured by normalizing the area under an x-ray diode signal to energy yield measured by a bolometer. The radiated power as a function of time, which is just the normalized x-ray diode signal, can then be used to normalize gated microchannel plate x-ray pinhole camera images. The procedure is most accurate when the gated x-ray pinhole camera has the same filter as the x-ray diode and when the filter is transmissive near the peak of the Planckian radiation temperature being measured. We present results for two-dimensional radiation temperatures as a function of time for dynamic Hohlraum experiments on Z. In these experiments a z pinch consisting of nested tungsten wire arrays driven by the 20 MA, 100 ns Z accelerator implodes onto cylindrical foam located on axis. X-ray diodes and bolometers located along the axis measure the power radiated along the pinch axis. Pinhole-imaged time-resolved microchannel plate framing cameras located on axis measure the spatial distribution of this radiation. Results from the analysis of many shots taken on Z show that a symmetrical strongly radiating shock wave is launched in the foam. The shock wave stagnates to less than 1 mm diameter with radiation temperatures exceeding 300 eV. Applications for this source include driving inertial confinement fusion capsules within the dynamic Hohlraum and weapons physics experiments that use the dynamic Hohlraum as a radiation source. © 2003 American Institute of Physics.
    Review of Scientific Instruments 02/2003; 74(3):2211-2214. · 1.60 Impact Factor