Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session QI3: Z-pinch, HED Magnetic Fields |
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Chair: Mingsheng Wei, General Atomics Room: Ballroom BC |
Wednesday, October 31, 2012 3:00PM - 3:30PM |
QI3.00001: Beryllium liner implosion experiments on the Z accelerator in preparation for Magnetized Liner Inertial Fusion (MagLIF)* Invited Speaker: Ryan D. McBride Magnetized Liner Inertial Fusion (MagLIF) [1] is a concept that involves using a pulsed electrical current to implode an initially-solid, cylindrical metal tube (liner) filled with preheated and magnetized fusion fuel. One- and two-dimensional simulations predict that if sufficient liner integrity can be maintained throughout the implosion, then significant fusion yield ($>$100 kJ) is possible on the 25-MA, 100-ns Z accelerator. The greatest threat to the liner integrity is the Magneto-Rayleigh-Taylor (MRT) instability, which first develops on the outer liner surface, and then works its way inward toward the inner surface throughout the implosion. Two-dimensional simulations predict that a thick liner, with R$_{outer}$/$\Delta $R=6, should be robust enough to keep the MRT instability from overly disrupting the fusion burn at stagnation. This talk will present the first experiments designed to study a thick, MagLIF-relevant liner implosion through to stagnation on Z [2]. The use of beryllium for the liner material enabled us to obtain penetrating monochromatic (6151$\pm $0.5 eV) radiographs that reveal information about the entire volume of the imploding liner. This talk will also discuss experiments that investigated Z's pulse-shaping capabilities to either shock- or shocklessly-compress the imploding liners [3], as well as our most recent experiments that used 2-micron-thick aluminum sleeves to provide high-contrast tracers for the positions and states of the inner surfaces of the imploding beryllium liners. The radiography data to be presented provide stringent constraints on the simulation tools used by the broader high energy density physics and inertial confinement fusion communities, where quantitative areal density measurements, particularly of convergent fusion targets, are relatively scarce. We will also present power-flow tests of the MagLIF load hardware as well as new micro-B-dot measurements of the azimuthal drive magnetic field that penetrates the initially vacuum filled interior of the liner during the implosion.\\[4pt] *This work was conducted in collaboration with S. A. Slutz, C. A. Jennings, D. B. Sinars, M. E. Cuneo, M. C. Herrmann, R. W. Lemke, M. R. Martin, R. A. Vesey, K. J. Peterson, A. B. Sefkow, C. Nakhleh, et al., B. E. Blue {\&} General Atomics, J. B. Greenly {\&} Cornell University, and the Z {\&} ZBL operations, diagnostics, engineering, load hardware, and target teams. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. \\[4pt] [1] S.A. Slutz, et al., Phys. Plasmas \textbf{17}, 056303 (2010).\\[0pt] [2] R.D. McBride et al., ``Penetrating radiography of imploding and stagnating beryllium liners on the Z accelerator,'' submitted to Phys. Rev. Lett. (May 2012).\\[0pt] [3] M. R. Martin, R. W. Lemke, R. D. McBride, et al., Phys. Plasmas \textbf{19}, 056310 (2012). [Preview Abstract] |
Wednesday, October 31, 2012 3:30PM - 4:00PM |
QI3.00002: Experiments and simulations studying electrothermal instabilities in magnetically accelerated implosion systems Invited Speaker: Kyle Peterson Electrothermal instabilities in electrical conductors can occur whenever the electrical conductivity depends on temperature. Of particular interest are electrothermal instabilities that occur when the electrical conductivity decreases with temperature, which is the case in most metals until they are heated into a Spitzer-like conductivity regime. These instabilities form stratified structures perpendicular to the current flow that are similar in appearance to m=0 sausage type instabilities and can geometrically couple to magneto-Rayleigh-Taylor (MRT) instabilities as the system is accelerated. Several experiments were performed using the 100-ns Z accelerator that drove up to 20 MA through well-characterized, initially solid and smooth ($<$50 nm RMS) Al and Cu rods. The experiments used 2-frame (6151 eV) or 2-color (1865/6151 eV) monochromatic x-ray backlighting to image instability growth on the surface of the rods. Excellent agreement is obtained between measurements and simulations that show the majority of the instability growth occurs immediately after the surface of the rod melts and is in regions that are stable to MRT instabilities and unstable to electrothermal instabilities. We will also show how MRT instability theory alone cannot explain the levels of instability growth observed in experiments. [Preview Abstract] |
Wednesday, October 31, 2012 4:00PM - 4:30PM |
QI3.00003: Measurements of Rayleigh-Taylor-Induced Magnetic Fields in the Linear and Non-linear Regimes Invited Speaker: Mario Manuel Magnetic fields are generated in plasmas by the Biermann-battery, or thermoelectric, source driven by non-collinear temperature and density gradients. The ablation front in laser-irradiated targets is susceptible to Rayleigh-Taylor (RT) growth that produces gradients capable of generating magnetic fields. Measurements of these RT-induced magnetic fields in planar foils have been made using a combination of x-ray and monoenergetic-proton radiography techniques. At a perturbation wavelength of 120 $\mu $m, proton radiographs indicate an increase of the magnetic-field strength from $\sim $1 to $\sim $10 Tesla during the linear growth phase. A characteristic change in field structure was observed later in time for irradiated foils of different initial surface perturbations. Proton radiographs show a regular cellular configuration initiated at the same time during the drive, independent of the initial foil conditions. This non-linear behavior has been experimentally investigated and the source of these characteristic features will be discussed. [Preview Abstract] |
Wednesday, October 31, 2012 4:30PM - 5:00PM |
QI3.00004: The mitigating effect of self-generated magnetic field on Rayleigh-Taylor unstable inertial confinement fusion plasmas Invited Speaker: Bhuvana Srinivasan It has long been expected that Rayleigh-Taylor instabilities (RTI) in ICF can generate magnetic fields at the gas-ice interface and at the ice-ablator interface during the deceleration phase of target implosion. The focus here is on the gas-ice interface where the temperature gradient is the largest. Nonlinear evolution of RTI leads to undesirable mixing of hot and cold plasmas and enhances target energy loss. RTI is also expected to generate magnetic fields via the Biermann battery effect, which is related to fluid vorticity generation by RTI. The magnetic field wraps around the bubbles and spikes and concentrates in flux bundles at the perturbed gas-ice interface where fluid vorticity is large. The generated magnetic field can then be further amplified via the MHD dynamo effect. While the planar $2$-D simulations only generate out-of-plane magnetic fields, $3$-D simulations will result in further amplification of the complex magnetic field structures via the MHD dynamo. This is studied by including a seed in-plane magnetic field in $2$-D and examining the resulting magnetic field structure and magnitude. The self-generated out-of-plane magnetic fields depend on ICF parameters via the scaling law, $m_i \sqrt{A g/\lambda}$ where $m_i$ is the ion mass, $A$ is the Atwood number, $g$ is the acceleration, and $\lambda$ is the wavelength. These magnetic fields grow to magnitudes of $10^2$-$10^3$ T for ICF relevant parameter regimes. While this is dynamically insignificant due to the plasma pressure far exceeding the magnetic pressure, it can significantly reduce perpendicular electron thermal conductivity by a factor of $2$-$10$. Such a reduction in thermal conductivity perpendicular to the magnetic field contributes to lowering of radial energy transport in the implosion target. [Preview Abstract] |
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