E. P. Yu

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (56)83.61 Total impact

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    ABSTRACT: Recent experiments on Sandia's Z facility have confirmed simulation predictions of dramatically reduced instability growth in solid metallic rods when thick dielectric coatings are used to mitigate density perturbations arising from an electrothermal instability. These results provide further evidence that the inherent surface roughness as a result of target fabrication is not the dominant seed for the growth of magneto-Rayleigh-Taylor instabilities in liners with carefully machined smooth surfaces, but rather electrothermal instabilities that form early in the electrical current pulse as Joule heating melts and vaporizes the liner surface. These results suggest a new technique for substantially reducing the integral magneto-Rayleigh-Taylor instability growth in magnetically driven implosions, such as cylindrical dynamic material experiments and inertial confinement fusion concepts.
    Physical Review Letters 04/2014; 112(13):135002. · 7.73 Impact Factor
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    ABSTRACT: Detailed spectroscopic diagnostics of the stagnating plasma in two disparate z pinches allow, for the first time, the examination of the plasma properties within a 1D shock wave picture, demonstrating a good agreement with this picture. The conclusion is that for a wide range of imploding-plasma masses and current amplitudes, in experiments optimizing non-Planckian hard radiation yields, contrary to previous descriptions the stagnating plasma pressure is balanced by the implosion pressure, and the radiation energy is provided by the imploding-plasma kinetic energy, rather than by the magnetic-field pressure and magnetic-field-energy dissipation, respectively.
    Physical Review Letters 07/2013; 111(3):035001. · 7.73 Impact Factor
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    ABSTRACT: form only given. Sandia is investigating a new magnetized liner inertial fusion (MagLIF) concept that uses cylindrical Be or Al liners to compress magnetized and preheated fusion fuel. As part of this work, we have been studying the growth of instabilities in initially solid liners driven with 20-24 MA, 100-ns current pulses on the Z pulsed power facility. The magneto-Rayleigh-Taylor instability in particular can disrupt the plasma liner during its implosion. A remarkable degree of azimuthal symmetry is observed near stagnation in beryllium liner implosions. This symmetry is captured in 3D calculations only when some azimuthally correlated perturbations are seeded initially. One possibility is that the MRT instability is directly seeded by azimuthally correlated, fine-scale structures on the surface that result from diamond turning the liner on a lathe. A second possibility is that the electro-thermal instability is seeding the MRT instability. Simulations suggest that the level of instability growth seeded by the ET instability is not strongly dependent on the surface roughness. Understanding the surface finish requirements for liner implosions is an important practical question. We will discuss the results of experiments in which the surface of Be liners was altered by polishing the liners along the axial direction after machining. This removes the azimuthally correlated structure, leaving only axially correlated grooves. The latter should noticeably affect the growth of MRT if the surface structure is directly seeding the instability, but may not have any impact if the ET instability is the dominant seed.
    Plasma Science (ICOPS), 2013 Abstracts IEEE International Conference on; 01/2013
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    ABSTRACT: A recent publication [K. J. Peterson et al., Phys. Plasmas 19, 092701 (2012)] describes simulations and experiments of electrothermal instability growth on well characterized initially solid aluminum and copper rods driven with a 20 MA, 100 ns rise time current pulse on Sandia National Laboratories Z accelerator. Quantitative analysis of the high precision radiography data obtained in the experiments showed excellent agreement with simulations and demonstrated levels of instability growth in dense matter that could not be explained by magneto-Rayleigh-Taylor instabilities alone. This paper extends the previous one by examining the nature of the instability growth in 2D simulations in much greater detail. The initial instability growth in the simulations is shown via several considerations to be predominantly electrothermal in nature and provides a seed for subsequent magneto-Rayleigh-Taylor growth.
    Physics of Plasmas 11/2012; 20(5). · 2.38 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: This paper explores the role of electro-thermal instabilities on the dynamics of magnetically accelerated implosion systems. Electro-thermal instabilities result from non-uniform heating due to temperature dependence in the conductivity of a material. Comparatively little is known about these types of instabilities compared to the well known Magneto-Rayleigh-Taylor (MRT) instability. We present simulations that show electrothermal instabilities form immediately after the surface material of a conductor melts and can act as a significant seed to subsequent MRT instability growth. We also present the results of several experiments performed on Sandia National Laboratories Z accelerator to investigate signatures of electrothermal instability growth on well characterized initially solid aluminum and copper rods driven with a 20 MA, 100 ns risetime current pulse. These experiments show excellent agreement with electrothermal instability simulations and exhibit larger instability growth than can be explained by MRT theory alone.
    Physics of Plasmas 09/2012; 19(9). · 2.38 Impact Factor
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    ABSTRACT: The observation of Doppler splitting in K-shell x-ray lines emitted from optically thin dopants is used to infer implosion velocities of up to 70 cm/μs in wire-array and gas-puff Z pinches at drive currents of 15-20 MA. These data can benchmark numerical implosion models, which produce reasonable agreement with the measured velocity in the emitting region. Doppler splitting is obscured in lines with strong opacity, but red-shifted absorption produced by the cooler halo of material backlit by the hot core assembling on axis can be used to diagnose velocity in the trailing mass.
    Physical Review E 11/2011; 84(5 Pt 2):056408. · 2.31 Impact Factor
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    ABSTRACT: Two-dimensional 3-ns-gated visible images, recorded at different times during the implosion of plasma under azimuthal magnetic field (Z-pinch), revealed ringlike instabilities followed by the development of axially and azimuthally nonuniform structures in the imploding plasma. Remarkably, the evolution in time of all structures was found to be highly repeatable in different shots, which should allow, through 3-D magnetohydrodynamics model- ing, for systematically studying the development in time of these complex phenomena and correlating them with the initial plasma parameters. The data are also used to infer the time-dependent outer plasma radius and plasma radial velocities. Index Terms—Plasma implosion, visible imaging, Z-pinch.
    IEEE Transactions on Plasma Science 01/2011; 39(11):2392-2393. · 0.87 Impact Factor
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    ABSTRACT: form only given. Numerous plasma pinch devices have reported the presence of brief, intense, soft x-ray bursts from tiny regions of plasma that are variously called “bright spots,” “hot spots,” or “micropinches.” The X-pinch load geometry was originally conceived as a method to study the formation and properties of bright spots in z-pinch plasmas. X-pinch plasmas driven by 0.2 MA can form bright spots that are 0.8-1.5 microns in diameter, have temperatures of about 1 keV, densities >;10% of solid density, and last for 10-100 ps. These conditions are believed to result from the direct magnetic compression of radiating matter. Physical models that capture the behavior of 0.2 MA X pinches predict more extreme parameters at currents >;1 MA. We will present X-pinch research from the 6 MA SATURN facility. A large portion of this work was devoted to finding load designs that would scale well to higher currents and included experiments at 1 MA. Source sizes of 5-8 microns were observed in SATURN experiments along with evidence for high temperatures (several keV) and short time durations (
    IEEE International Conference on Plasma Science 01/2011;
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    ABSTRACT: Recent 3D RMHD simulations at Sandia and experiments at Weizmann Institute of Science have demonstrated that axially and azimuthally averaged dynamics of a strongly radiating stagnated Z-pinch column resembles a self-similar, cylindrically symmetric motion. The cold, rapidly imploding plasma transforms into the hot stagnated plasma heated and compressed in the diverging shock wave that propagates from the pinch axis. The simplest analytical solution describing such flow was given by Noh. Here we discuss generalizations of the classical Noh's solution, which take into account the non-uniform density and velocity profiles in the incident plasma, as well as the presence of azimuthal magnetic field in it. These new solutions are found to be surprisingly close to the observations and simulation results. They have also been used for verification tests of MHD codes.
    IEEE International Conference on Plasma Science 01/2011;
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    ABSTRACT: Existing monochromatic X-ray backlighting diagnos- tics at 1.865 and 6.151 keV have been combined to create a two-color monochromatic X-ray backlighting diagnostic. The use of different photon energies can allow a much broader range of areal densities to be observed in a single experiment. Here, we apply the two-color backlighter to the study of instability growth on the outside edge of an initially solid copper rod target driven by a 100-ns rise-time current pulse with a peak value of 20 MA. The different opacity of Cu at these two photon energies allows a dynamic range of ∼1600x to be surveyed instead of ∼60x (assuming a useful transmission range of 5%-95%). Index Terms—Inertial confinement, plasma diagnostics, radiography.
    IEEE Transactions on Plasma Science 01/2011; 39(11):2408-2409. · 0.87 Impact Factor
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    ABSTRACT: form only given. Large diameter nested wire array z-pinches imploded on the Z-generator at Sandia National Laboratories have been used to generate high intensity K-shell radiation. Previous experiments have used different materials between the inner and outer array to allow radiation emitted during the implosion and stagnation to provide some insight into the structure of the imploding plasma and the way in which the different arrays participate in the implosion. We present 3D-resistive MHD calculations of the implosion and stagnation of mixed material nested wire arrays, one array being Al and the other being Ni-clad Ti, imploded on the Z generator pre-refurbishment. These implosions are compared and contrasted with recent single material Al array implosions. Calculation results are compared to pinhole images, spectroscopic measurements, and radiated powers and yields to constrain and validate our 3D MHD calculations. Through reproducing synthetic diagnostics from calculation results we investigate how plasma structure and implosion dynamics effect diagnostic interpretation. Utilizing the different radiative properties of the different materials, and contrasting the behavior of different loads we can gain some insight into the stagnation mechanisms that potentially drive the high intensity, high photon energy radiation. This allows us to further our understanding of how these sources may be better optimized.
    Plasma Science (ICOPS), 2011 Abstracts IEEE International Conference on; 01/2011
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    ABSTRACT: Detailed understanding of the stagnation process of an imploding wire-array Z pinch has been hampered by its complicated 3D nature, which would seem to preclude use of the usual 1D models of stagnation. However, the 3D connected nature of the Z pinch's trailing mass network also endows the wire array with certain properties which might allow a 1D description. In this work we explore the applicability of simple 1D pictures of stagnation, such as the Noh shock stagnation and isentropic compression, to 3D wire array simulations. We also discuss the role of the magnetic field, ram pressure, and angular momentum during the stagnation process. Finally, we comment on implications for measuring the ion temperature.
    11/2010;
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    ABSTRACT: Fast z-pinches provide intense 1-10 keV photon energy radiation sources. Here, we analyze time-, space-, and spectrally- resolved ˜2 keV K-shell emissions from Al (5% Mg) wire array implosions on Sandia's Z machine pulsed power driver. The stagnating plasma is modeled as three separate radial zones, and collisional-radiative modeling with radiation transport calculations are used to constrain the temperatures and densities in these regions, accounting for K-shell line opacity and Doppler effects. We discuss plasma conditions and dynamics at the onset of stagnation, and compare inferences from the atomic modeling to three-dimensional magneto-hydrodynamic simulations.
    11/2010;
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    ABSTRACT: Large diameter nested wire array z-pinches imploded on the Z-generator at Sandia National Laboratories have been used extensively to generate high intensity K-shell radiation. Large initial radii are required to obtain the high implosion velocities needed to efficiently radiate in the K-shell. This necessitates low wire numbers and large inter-wire gaps which introduce large azimuthal non-uniformities. Furthermore, the development of magneto-Rayleigh-Taylor instabilities during the implosion are known to generate large axial non-uniformity These effects motivate the complete, full circumference 3-dimensional modeling of these systems. Such high velocity implosions also generate large voltages, which increase current losses in the power feed and limit the current delivery to these loads. Accurate representation of the generator coupling is therefore required to reliably represent the energy delivered to, and the power radiated from these sources. We present 3D-resistive MHD calculations of the implosion and stagnation of a variety of large diameter stainless steel wire arrays (hv ~ 6.7keV), imploded on the Z-generator both before and after its refurbishment. Use of a tabulated Kshell emission model allows us to compare total and K-shell radiated powers to available experimental measurements. Further comparison to electrical voltage and current measurements allows us to accurately assess the power delivered to these loads. These data allow us to begin to constrain and validate our 3D MHD calculations, providing insight into ways in which these sources may be further optimized.
    IEEE International Conference on Plasma Science 01/2010;
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    ABSTRACT: Azimuthally correlated wire core ablation was compared for closely spaced versus widely spaced wires in a 1 MA Z-pinch. X-ray point-projection diagnostics revealed that 240 μm spaced wires exhibited a correlation coefficient approaching unity in both real space and in k-space. This correlated ablation between wires at a fixed axial location is believed to occur due to an enhanced, localized Joule heating. Wires separated by 2.47 mm or greater were uncorrelated in real space, but correlated in k-space, indicating the ablation structure between wires was shifted in phase.
    Physics of Plasmas 10/2009; 16(10):102702-102702-6. · 2.38 Impact Factor
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    ABSTRACT: X-ray production by imploding wire-array Z pinches is studied using radiation magnetohydrodynamics simulation. It is found that the density distribution created by ablating wire material influences both x-ray power production, and how the peak power scales with applied current. For a given array there is an optimum ablation rate that maximizes the peak x-ray power, and produces the strongest scaling of peak power with peak current. This work is consistent with trends in wire-array Z pinch x-ray power scaling experiments on the Z accelerator.
    Physical Review Letters 02/2009; 102(2):025005. · 7.73 Impact Factor
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    ABSTRACT: Short-implosion-time 20-mm diameter, 300-wire tungsten arrays maintain high peak x-ray powers despite a reduction in peak current from 19 to 13 MA. The main radiation pulse on tests with a 1-mm on-axis rod may be explained by the observable j x B work done during the implosion, but bare-axis tests require sub-mm convergence of the magnetic field not seen except perhaps in >1 keV emission. The data include the first measurement of the imploding mass density profile of a wire-array Z pinch that further constrains simulation models.
    Physical Review Letters 04/2008; 100(14):145002. · 7.73 Impact Factor
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    ABSTRACT: The main physical processes responsible for plasma ablation in multiwire Z pinches are considered via eigensolutions to one-dimensional steady state magnetohydrodynamics. A double scale-length structure of the plasma accelerating layer is demonstrated. The width of the resistive scale-length that defines the current layer structure is significantly larger than the thermal scale-length, where transport of energy toward the cores and plasma pressure play important roles. The transport of energy is provided mainly by radiation, though electron thermal conduction is also important very close to the plasma-core interface. Another type of solution of the steady state problem is revealed, when local Ohmic heating is important down to the interface. Selection between these two types of solutions is considered from multiple points of view. Although the one-dimensional problem is mainly considered in this paper, it is shown how the one-dimensional results may help to understand results of two-dimensional models.
    Physics of Plasmas 02/2008; 15(2):022702-022702-13. · 2.38 Impact Factor
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    ABSTRACT: The implosion phase of a wire-array Z pinch is investigated using three-dimensional (3D) simulations, which model the mass ablation phase and its associated axial instability using a mass injection boundary condition. The physical mechanisms driving the trailing mass network are explored, and it is found that in 3D the current paths though the trailing mass can reduce bubble growth on the imploding plasma sheath, relative to the 2D (r,z) equivalent. Comparison between the simulations and a high quality set of experimental radiographs is presented.
    Physics of Plasmas 02/2008; 15(5):056301-056301-9. · 2.38 Impact Factor