D. J. Ampleford

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (198)185.86 Total impact

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    ABSTRACT: Presented are voltage measurements taken near the load region on the Z pulsed-power accelerator using an inductive voltage monitor (IVM). Specifically, the IVM was connected to, and thus monitored the voltage at, the bottom level of the accelerator's vacuum double post-hole convolute. Additional voltage and current measurements were taken at the accelerator's vacuum-insulator stack (at a radius of 1.6 m) by using standard D -dot and B -dot probes, respectively. During postprocessing, the measurements taken at the stack were translated to the location of the IVM measurements by using a lossless propagation model of the Z accelerator's magnetically insulated transmission lines (MITLs) and a lumped inductor model of the vacuum post-hole convolute. Across a wide variety of experiments conducted on the Z accelerator, the voltage histories obtained from the IVM and the lossless propagation technique agree well in overall shape and magnitude. However, large-amplitude, high-frequency oscillations are more pronounced in the IVM records. It is unclear whether these larger oscillations represent true voltage oscillations at the convolute or if they are due to noise pickup and/or transit-time effects and other resonant modes in the IVM. Results using a transit-time-correction technique and Fourier analysis support the latter. Regardless of which interpretation is correct, both true voltage oscillations and the excitement of resonant modes could be the result of transient electrical breakdowns in the post-hole convolute, though more information is required to determine definitively if such breakdowns occurred. Despite the larger oscillations in the IVM records, the general agreement found between the lossless propagation results and the results of the IVM shows that large voltages are transmitted efficiently through the MITLs on Z . These results are complementary to previous studies [R. D. McBride et al., Phys. Rev. ST Accel. Beams 13, 120401 (2010)] that showed efficient transmission of large currents through the MITLs on Z . Taken together, the two studies demonstrate the overall efficient delivery of very large electrical powers through the MITLs on Z .
    11/2014; 17(12).
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    ABSTRACT: Advancements have been made in the diagnostic techniques to measure accurately the total radiated x-ray yield and power from z-pinch implosion experiments at the Z machine with high accuracy. The Z machine is capable of outputting 2 MJ and 330 TW of x-ray yield and power, and accurately measuring these quantities is imperative. We will describe work over the past several years which include the development of new diagnostics, improvements to existing diagnostics, and implementation of automated data analysis routines. A set of experiments on the Z machine were conducted in which the load and machine configuration were held constant. During this shot series, it was observed that the total z-pinch x-ray emission power determined from the two common techniques for inferring the x-ray power, a Kimfol filtered x-ray diode diagnostic and the total power and energy diagnostic, gave 449 TW and 323 TW, respectively. Our analysis shows the latter to be the more accurate interpretation. More broadly, the comparison demonstrates the necessity to consider spectral response and field of view when inferring x-ray powers from z-pinch sources.
    Review of Scientific Instruments 08/2014; 85(8):083501-083501-11. · 1.58 Impact Factor
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    ABSTRACT: Aluminum wire array z pinches imploded on the Z generator are an extremely bright source of 1–2 keV radiation, with close to 400 kJ radiated at photon energies >1 keV and more than 50 kJ radiated in a single line (Al Ly-α). Opacity plays a critical role in the dynamics and K-shell radiation efficiency of these pinches. Where significant structure is present in the stagnated pinch this acts to reduce the effective opacity of the system as demonstrated by direct analysis of spectra. Analysis of time-integrated broadband spectra (0.8–25 keV) indicates electron temperatures ranging from a few 100 eV to a few keV are present, indicative of substantial temperature gradients.
    Physics of Plasmas 03/2014; 21(3):031201. · 2.25 Impact Factor
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    ABSTRACT: Recent experiments on the Z accelerator have produced high-energy (17 keV) inner-shell K-alpha emission from molybdenum wire array z-pinches. Extensive absolute power and spectroscopic diagnostics along with collisional-radiative modeling enable detailed investigation into the roles of thermal, hot electron, and fluorescence processes in the production of high-energy x-rays. We show that changing the dimensions of the arrays can impact the proportion of thermal and non-thermal K-shell x-rays.
    Physics of Plasmas 02/2014; 21(3). · 2.25 Impact Factor
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    ABSTRACT: A comprehensive gas puff capability is being developed on the Z pulsed power generator. We describe the methodology employed for developing a gas puff load on Z, which combines characterization and modeling of the neutral gas mass flow from a supersonic nozzle, numerical modeling of the implosion of this mass profile, and experimental evaluation of these magnetic implosions on Z. We are beginning a multiyear science program to study gas puff z-pinch physics at high current, starting with an 8-cm diameter double-shell nozzle, which delivers a column of Ar gas that is imploded by the machine's fast current pulse. The initial shots have been designed using numerical simulation with two radiation-magnetohydrodynamic codes. These calculations indicate that 1 mg/cm should provide optimal coupling to the driver and 1.6:1 middle:outer shell mass ratio will best balance the need for high implosion velocity against the need to mitigate the magnetic Rayleigh–Taylor instability. The models suggest 300–500-kJ Ar K-shell yield should be achievable on Z, and we report an initial commissioning shot at lower voltage in which 250 kJ was measured. Future experiments will pursue optimization of Ar and Kr K-shell X-ray sources, study fusion in deuterium gas puffs, and investigate the physics of gas puff implosions including energy coupling, instability growth, and radiation generation.
    IEEE Transactions on Plasma Science 01/2014; 42(5):1145-1152. · 0.95 Impact Factor
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    ABSTRACT: Recent experiments at the 20 MA Z Accelerator have demonstrated, for the first time, implosion velocities up to 110-130 cm/μs in imploding stainless steel wire arrays. These velocities, the largest inferred in a magnetically driven implosion, lead to ion densities of 2 × 1020 cm-3 with electron temperatures of ~5 keV. These plasma conditions have resulted in significant increases in the K-shell radiated output of 5-10 keV photons, radiating powers of >30 TW and yields >80 kJ, making it the brightest laboratory x-ray source in this spectral region. These values represent a doubling of the peak power and a 30% increase in the yield relative to previous studies. The experiments also included wire arrays with slower implosions, which were observed to have lower temperatures and reduced K-shell output. These colder pinches, however, radiated 260 TW in the soft x-ray region, making them one of the brightest soft x-ray sources available.
    Physics of Plasmas 10/2013; 20(10):3116-. · 2.25 Impact Factor
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    ABSTRACT: Tests are ongoing to conduct ∼20 MA z-pinch implosions on the Z accelerator at Sandia National Laboratory using Ar, Kr, and D2 gas puffs as the imploding loads. The relatively high cost of operations on a machine of this scale imposes stringent requirements on the functionality, reliability, and safety of gas puff hardware. Here we describe the development of a prototype gas puff system including the multiple-shell nozzles, electromagnetic drivers for each nozzle's valve, a UV pre-ionizer, and an inductive isolator to isolate the ∼2.4 MV machine voltage pulse present at the gas load from the necessary electrical and fluid connections made to the puff system from outside the Z vacuum chamber. This paper shows how the assembly couples to the overall Z system and presents data taken to validate the functionality of the overall system.
    The Review of scientific instruments 06/2013; 84(6):063504. · 1.58 Impact Factor
  • Physics of Plasmas 05/2013; · 2.25 Impact Factor
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    ABSTRACT: Localized, intensely radiating regions are often observed in Z pinches. High resolution images of such areas have been recorded at least as far back as the 1970s. However, there is as yet no widely accepted consensus on the nature of these “bright spots” or how they are formed. This phenomenon has also been referred to “hot spots” or “micropinches.” To shed further light on this issue, we have analyzed axially resolved K-shell spectra from 4 Z pinches driven by the refurbished Z generator (“ZR”) at Sandia National Laboratories, and the previous version of the Z machine (“Z”). The atomic numbers of the loads varied from 13 to 29. We find that higher spatial K-shell intensity in the Al pinch correlates with density. The K-shell intensity within a copper shot taken on ZR correlates strongly with increased electron temperature, but another, somewhat less well-diagnosed copper shot from Z shows correlation with density. The bright spots in a Ti pinch correlate with neither density nor temperature, but do correlate with the product of density and diameter (proportional to opacity). This opacity correlation is also observed in the other 3 pinches.
    Physics of Plasmas 02/2013; 20(2). · 2.25 Impact Factor
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    ABSTRACT: Advances in diagnostic techniques at the Sandia Z-facility have facilitated the production of very detailed spectral data. In particular, data from the copper nested wire-array shot Z1975 provides a wealth of information about the implosion dynamics and ionization history of the pinch. Besides the dominant valence K- and L-shell lines in Z1975 spectra, K-α lines from various ionization stages were also observed. K-shell vacancies can be created from inner-shell excitation and ionization by hot electrons and from photo-ionization by high-energy photons; these vacancies are subsequently filled by Auger decay or resonance fluorescence. The latter process produces the K-α emission. For plasmas in collisional equilibrium, K-α emission usually occurs from highly charged ions due to the high electron temperatures required for appreciable excitation of the K-α transitions. Our simulation of Z1975 was carried out with the NRL 1-D DZAPP non-LTE radiation-hydrodynamics model, and the resulting K- and L-shell synthetic spectra are compared with measured radiation data. Our investigation will focus on K-α generation by both impacting electrons and photons. Synthetic K-α spectra will be generated either by self-consistently calculating the K-shell vacancy production in a full Z-pinch simulation, or by post-processing data from a simulation. The analysis of these K-α lines as well as K- and L-shell emission from valence electrons should provide quantitative information about the dynamics of the pinch plasma.
    High Energy Density Physics 02/2013; 9(2). · 1.52 Impact Factor
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    ABSTRACT: form only given. We present a study of the formation of bow shocks in radiatively cooled plasma flows. This work uses the XP generator (260kA, 145ns) at Cornell University to drive an inverse wire array. This generates a quasi-uniform, large scale hydrodynamic flow accelerated by Lorentz forces to Ma > 1. This flow impacts a stationary object placed in its path, forming a well-defined Mach cone. Collinear interferogram and gated-self emission diagnostics demonstrate that the cone angle with distance from the wire decreases (increasing Mach number) and is indicative of a strongly cooling flow. Rapid density increase from the background flow into the object is indicative of a strong density jump at the shock. High resolution self-emission imaging shows the formation of a thin (
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: form only given. Wire array z-pinches on the Z generator are bright sources of radiation from 200eV to 9keV. Typically, wire materials are varied to provide emission in a specific spectral band, and the array setup is varied in order to provide appropriate energy per ion to heat the stagnated plasma to an electron temperature conducive to efficient emission within that band. Here we discuss a series of experiments where the wire material is varied (Al, Stainless Steel, Cu, W), however the array setup is fixed at 65mm diameter and 2.5mg mass. We discuss similarities and differences in the implosion dynamics and the differences in the plasma conditions achieved at stagnation. We will explore the changes in total, thermal K-shell and non-thermal K-shell emission between the different materials.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: form only given. We have reestablished gas puff z-pinch capability on Sandia's 20 MA Z machine, including a Sandia-operated driver system and an imaging interferometer to characterize nozzle mass flow [1]. Initial experiments have focused on developing a 3 keV Ar K-shell x-ray source. We have pursued a design-driven approach to planning these experiments, utilizing numerical simulation to predict Ar K-shell yield for various nozzle mass profile configurations. In particular, we study coupling to the generator and how the distribution of mass between the two shells impacts magnetic Rayleigh-Taylor instability evolution. Two-dimensional radiation-magneto-hydrodynamic (MHD) simulations at NRL for a number of density profiles produced by the nozzle have predicted yields in excess of 300 kJ, and indicated that a 1:1.6 outer-to innershell mass ratio would produce the most stable implosion with high enough temperature to optimize Ar K-shell output [2]. This result was also consistent with 3D MHD modeling using the Gorgon code [3] at Sandia. Both models used tabulated non-LTE atomic models for Ar K-shell photon emission. We will present Z experimental data from the first gas puff shots on the accelerator since 2006, and compare these to the numerical models. Spectral output is measured from 1-20 keV. Electrical current measurements at different positions along the power flow section provide information on current coupling to the load. Time-gated pinhole imaging and radially-resolved spectroscopy indicate ~60 cm/μs implosion velocities and >1 keV electron temperatures.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: Bow shock structures are observed in a nested wire array z-pinch as ablation streams from the outer array pass the inner array. The jump in plasma conditions across these shocks results in an enhancement of snowplow emission from the imploding plasma piston. Results from a snowplow model modified to account for the shock jumps are discussed and compared to experimental data from MAGPIE. Magnetohydrodynamic simulations indicate that this is the primary heating mechanism responsible for the interaction pulse recorded on the Z generator, which is required for pulse shaping for inertial confinement fusion.
    Physics of Plasmas 12/2012; 19(12). · 2.25 Impact Factor
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    ABSTRACT: The Z Facility at Sandia National Laboratories can produce >2 MJ of X-ray energy with peak powers > 280 TW. Achieving optimal load performance requires the control of the implosion time of the Z-pinch with respect to the current pulse shape from the pulsed-power driver. Additionally, accurate estimates of the implosion time are required for the precise timing of diagnostics used to study the implosion. A key aspect for assessing this load–driver match, both computationally and experimentally, is the mass of the wire array. In this paper, we present new methods to characterize wire arrays. Accurate mass measurements were made of individual wires. Individual wire diameters were measured from a sampling of wire sections, and a statistical analysis was performed. Finally, the wire arrays were imaged to ensure and document the quality of the final wire array. Data provide an estimate of the load mass that can be used to track experimental trends, provide accurate implosion time predictions for diagnostic timing, and provide input for computational models.
    IEEE Transactions on Plasma Science 12/2012; 40(12):3372. · 0.95 Impact Factor
  • IEEE Transactions on Plasma Science 12/2012; 40(12):3334-3346. · 0.95 Impact Factor
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    ABSTRACT: The 24 papers in this special issue focus on Z-pinch plasmas. There are three invited papers and 25 contributed papers.
    IEEE Transactions on Plasma Science 12/2012; 40(12):3186-3188. · 0.95 Impact Factor
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    ABSTRACT: Using solid, machined X-pinch targets driven by currents rising from 0 to 5-6 MA in 60 ns, we observed bright spots of 5-9-keV continuum radiation from 5±2-μm diameter regions. The >6-keV radiation is emitted in about 0.4 ns, and the bright spots are roughly 75 times brighter than the bright spots measured at 1 MA. A total x-ray power of 10 TW peak and yields of 165±20  kJ were emitted from a 3-mm height. The 3-5-keV continuum radiation had a 50-90-GW peak power and 0.15-0.35-kJ yield. The continuum is plausibly from a 1275±75-eV blackbody or alternatively from a 3500±500-eV bremsstrahlung source.
    Physical Review Letters 10/2012; 109(15):155002. · 7.73 Impact Factor
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    ABSTRACT: K-shell x-ray sources have been studied extensively at the Z Accelerator both pre- and post-facility refurbishment. Radiation and spectra from sources such as Al (5% Mg), stainless steel, and Cu (4% Ni) have been analyzed to understand the dominant mechanisms for the K-shell emission. Recent work (J.P. Apruzese et al., ICOPS 2012) has shown that for some experiments, the K-shell emission is dominated by initial mass and plasma density, and in other cases the emission is driven by electron temperature. In this work, the K-shell emission from the primary materials (Al, Fe, Cu) and the dopant materials (Mg, Cr, Ni) are compared to evaluate opacity effects for these z-pinch plasmas. Experimental data from pre-refurbished Z illustrating that opacity limits the Al K-shell output, but does not significantly limit the Cu K-shell output will be presented, along with observations from the same sources on post-refurbished Z. Comparisons will also be made with the dominant plasma emission properties (mass, temperature, density) to understand the correlation with opacity.
    10/2012;
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    ABSTRACT: Wire array Z-pinches consisting of Al wires are a bright source of >1 keV emission. We discuss data from recent experiments with large diameter Al wire array Z-pinches on the Z generator. Wire arrays at 40mm, 50mm and 65mm were imploded on the Z generator. Soft x-ray self-emission imaging is used to study the implosion dynamics. Spectroscopy is used to diagnose plasma conditions as material stagnates on the axis. Data indicates significant free-bound continuum emission is present. We will discuss the variation in plasma conditions and emission characteristics as the initial wire array setup is varied.
    10/2012;

Publication Stats

961 Citations
185.86 Total Impact Points

Institutions

  • 2004–2014
    • Sandia National Laboratories
      • Advanced Materials Laboratory
      Albuquerque, New Mexico, United States
  • 2011
    • Cornell University
      • Department of Electrical and Computer Engineering
      Итак, New York, United States
  • 2001–2010
    • Imperial College London
      • Department of Physics
      Londinium, England, United Kingdom
  • 2006
    • University of Rochester
      • Department of Physics and Astronomy
      Rochester, NY, United States