Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session BO7: Magnetized Plasmas and Z-Pinch Physics |
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Chair: Daniel Sinars, Sandia National Laboratories Room: Galerie 6 |
Monday, October 27, 2014 9:30AM - 9:42AM |
BO7.00001: Pulsed Magnetic Field System for Magnetized Target Experiments at the National Ignition Facility M.A. Rhodes, J.M. Solberg, B.G. Logan, L.J. Perkins High-magnitude magnetic fields applied to inertially confined targets may improve fusion yield and enable basic science applications. We discuss the development of a pulsed magnetic field system for NIF with the goal of applying 10-70T to various NIF targets. While the driver may be little more than a spark-gap switched capacitor, numerous complex challenges exist in fielding such a system on NIF. The coil surrounding the metallic hohlraum drives induced current in the hohlraum wall. Both the coil and hohlraum wall must survive ohmic heating and JxB forces for several microseconds. Pulsed power must couple to the coil in the NIF environment. The system must not cause late-time optics damage due to debris. There is very limited volume for the driver in a NIF Diagnostic Instrument Manipulator (DIM). We are modeling the coil and hohlraum MHD effects with the LLNL code, ALE3D. However, the simulations lack complete and accurate data for all the required thermo-physical material properties over the expected range of temperatures (below vaporization) and pressures. Therefore, substantial experimental development is planned in the coming year. We present coil and hohlraum simulations results, overall system design, and progress towards an operational prototype test-stand. [Preview Abstract] |
Monday, October 27, 2014 9:42AM - 9:54AM |
BO7.00002: Magnetized HDC ignition capsules for yield enhancement and implosion magnetohydrodynamics G. Zimmerman, D. Ho, J. Perkins, G. Logan, S. Hawkins, M. Rhodes Imposing a magnetic field on capsules can turn capsules that fail, because of low 1-D margin, into igniting capsules that give yield in the MegaJoule range. The imposed magnetic field can be amplified by up to O(10$^{\mathrm{3}})$ as it is being compressed by the imploding shell, e.g. if the initial field is 50T, then the field in the hot spot of the assembled configuration can reach \textgreater 10$^{\mathrm{4}}$ T. (We are currently designing hardware that can provide a field in the 50T range inside NIF hohlraums.) With this highly compressed field strength, the gyro radius of alpha particles becomes smaller than the hot spot size. Consequently, the heating of the hot spot becomes more efficient. The imposed field can also prevent hot electrons in the holhraum from reaching the capsule. We choose capsules with high-density carbon (HDC) ablators for this study. HDC capsules have good 1-D performance and also have short pulses (10 ns or less), allowing the use of low gas-filled or near-vacuum hohlraums which provide high coupling efficiency. We describe a 2-D simulation of a 3-shock HDC capsule. We will show detailed magnetohydrodynamic evolution of the implosion. HDC capsules with 2-shock pulses have low margin because of their high adiabat, and it is difficult to achieve ignition in realistic 2-D simulations. The improvement in performance for 2-shock magnetized capsules will be presented. [Preview Abstract] |
Monday, October 27, 2014 9:54AM - 10:06AM |
BO7.00003: Use of External Magnetic Fields in Hohlraum Plasmas to Improve Laser-Coupling D.S. Montgomery, B.J. Albright, J.L. Kline, L. Yin, P.Y. Chang, J.R. Davies, G. Fiksel, D.H. Froula, R. Betti, M.J. MacDonald Controlling laser plasma instabilities and beam propagation in hohlraum plasmas is important for achieving high-gain inertial fusion using indirect drive. Experiments at the National Ignition Facility (NIF) suggest that coronal electron temperatures in NIF hohlraums may be cooler than initially thought due to efficient thermal conduction from the under dense low-Z plasma to the dense high-Z hohlraum wall [1]. This leads to weaker Landau damping and stronger growth of stimulated Raman scatter, and poorer laser transmission due to absorption in the cooler plasma. Magnetic insulation of the heat conducting electrons can occur when the Hall parameter $\omega_{ce}\tau_{ei} \gg 1$, where $\omega_{ce}$ is the electron-cyclotron frequency, and $\tau_{ei}$ is the electron-ion collision time. For NIF laser-plasma conditions, it is shown that a 10-T external magnetic field may substantially reduce cross-field transport and may increase coronal plasma temperatures, thus increasing linear Landau damping and mitigating SRS. We will present calculations and simulations supporting this concept, and will present initial results from Omega experiments using gas-filled hohlraums with external B-fields up to 10-T.\\[4pt] [1] M.D. Rosen et al., High Energy Density Phys. \textbf{7}, 180 (2011). [Preview Abstract] |
Monday, October 27, 2014 10:06AM - 10:18AM |
BO7.00004: Study of Strong Magnetic Fields Using Parametric Instability in a Magnetised Plasma V.V. Ivanov, A.V. Maximov, A.A. Anderson, B.S. Bauer, K. Yates Generation of strong magnetic fields with a strength of 10-50MG plays a key role in some recent conceptions for controlled fusion. We suggest a laser method for measuring the local magnetic field, B\textgreater 10MG, based on the parametric decay of the laser radiation to $\omega $/2 and 3/2$\omega $ harmonics which are generated in the area with the electron density of a quarter of the critical plasma density. Spectral components of parametric harmonics carry a signature of both the plasma temperature and strong magnetic field. A two-plasmon decay of laser radiation was studied in a magnetized plasma at the 1MA pulsed power Zebra facility at the University of Nevada, Reno. Dense magnetized plasma with a magnetic field of 1-3MG was created by the 1MA current flowing in the metal rod 0.7-2mm in diameter. Radiation from the narrowband laser with intensity \textgreater 10$^{14}$ W/cm$^{2}$ was focused on the surface plasma. Spectrum of the backscattering 3/2$\omega $ harmonic included ``red'' and ``blue'' shifted components. Large 2-3nm shifts of spectral components was identified with laser heating of plasma. Components with a small 0.1nm spectral shift of may be linked to the magnetic field. [Preview Abstract] |
Monday, October 27, 2014 10:18AM - 10:30AM |
BO7.00005: The Application of Imposed Magnetic Fields to Ignition and Thermonuclear Burn on the National Ignition Facility L. John Perkins, G. Logan, D. Ho, G. Zimmerman, D. Strozzi, M. Rhodes, R. Plummer, S. Hawkins We are studying the impact of highly compressed axial magnetic fields on ignition targets for the National Ignition Facility. Both magnetized room-temperature DT gas targets and CH/diamond cryo-ignition capsules are under study. Initial seed fields of 30-70T that compress to greater than 10000T (100MG) under implosion can reduce hotspot conditions required for ignition and propagating burn [L.J.Perkins et al, \textit{Phys. Plasmas}, 2013]. The field can also reduce hohlraum laser-plasma interactions by increasing the temperature, and supress the transport of hot electron preheat to the capsule. These combined attributes of compressed B-fields may permit recovery of ignition, or at least significant alpha particle heating, in submarginal capsules that would otherwise fail because of adverse hydrodynamic conditions and, more generally, may permit attainment of ignition in targets redesigned to operate under reduced drive and/or lower convergence ratios. Present engineering studies are also assessing the maximum attainable fields for a NIF hohlraum coil driven by a pulsed power supply located in a NIF Diagnostic Insertion Module (DIM). LLNL is operated by LLNS, LLC, for the U.S.DOE, NNSA under Contract DE-AC52-07NA27344. This work supported by LLNL LDRD 14-ER-028 [Preview Abstract] |
Monday, October 27, 2014 10:30AM - 10:42AM |
BO7.00006: Laser-generated magnetic fields in quasi-hohlraum geometries Bradley Pollock, David Turnbull, Steven Ross, Andrew Hazi, Joseph Ralph, Sebastian LePape, Dustin Froula, Dan Haberberger, John Moody Laser-generated magnetic fields of 10-40 T have been produced with 100-4000 J laser drives at Omega EP and Titan. The fields are generated using the technique described by Daido et .al. [Phys. Rev. Lett. 56, 846 (1986)], which works by directing a laser through a hole in one plate to strike a second plate.~Hot electrons generated in the laser-produced plasma on the second plate collect on the first plate.~~A strap connects the two plates allowing a current of 10s of kA to flow and generate a solenoidal magnetic field. The magnetic field is characterized using Faraday rotation, b-dot probes, and proton radiography. Further experiments to study the effect of the magnetic field on hohlraum performance are currently scheduled for Omega. This work was performed under the auspices of the United States Department of Energy by the Lawrence Livermore National Laboratory under contract No. DE-AC52-07NA-27344. [Preview Abstract] |
Monday, October 27, 2014 10:42AM - 10:54AM |
BO7.00007: Weibel magnetic field amplification and saturation in expanding plasmas Kevin Schoeffler, Nuno Loureiro, Luis Silva, Ricardo Fonseca Recent laser-solid interaction experiments have been used to generate high energy density plasmas with megagauss magnetic fields. These intense magnetic fields are generated by the Biermann battery mechanism via perpendicular temperature and density gradients, and via temperature anisotropy instabilities such as the Weibel instability. Performing particle-in-cell simulations of similar expanding plasmas, we find that in some laser systems as well as in astrophysical shocks the Weibel instability may play the dominant role [Schoeffler et al. Phys. Rev. Lett. 112, 175001 (2014)]. Particularly in systems where the Biermann Battery is expected to generate only small fields with plasma $\beta \sim L/d_i$ (with system size large compared to ion inertial length), while the Weibel may reach fields close to $\beta \sim 1$. Although the Weibel instability is a popular topic regarding these systems, the mechanism for saturation is not clearly understood. We investigate this saturation, as well as uncover a striking confirmation of gyrokinetic predictions of turbulence (from the Biermann field) with a sub-$\rho_e$ (electron gyroradius) $-4/3$ power law for electric field energy and $-16/3$ for magnetic fields [Schekochihin et al. Astrophys. J. Suppl. Ser. 182, 310 (2009)]. [Preview Abstract] |
Monday, October 27, 2014 10:54AM - 11:06AM |
BO7.00008: Interpenetration, stagnation and deflection of supersonic tungsten plasma flows produced by wire-array Z-pinches George Swadling, Sergey Lebedev, Guy Burdiak, Lee Suttle, Siddharth Patankar, Roland Smith, Matthew Bennett, Garteth Hall, Francisco Suzuki-Vidal, Jianqiang Yuan, Adam Harvey-Thompson, Woichech Rozmus We present Thomson Scattering measurements [G F Swadling et al, Phys. Rev. Lett., (Accepted 17 June 2014)] of the interpenetration, stagnation and deflection of supersonic tungsten plasma flows, produced in wire array z-pinch experiments on the MAGPIE (1.4MA, 240ns) pulsed power generator at Imperial College London. These measurements were made at early times in the evolution of the arrays, prior to the formation of the dense precursor column (120ns), when the collisional scale length between the streams was still significant compared to the scale length of the array. The scattering geometry used in these experiments allowed independent measurements of the radial and axial velocity distributions of the interacting flows; temporally and spatially resolved measurements were made over seven points across the array diameter. Analysis of the Thomson spectra provides evidence of flow interpenetration; the flows decelerate and are heated over an extended distance (1.5mm) before they fully stagnate. A previously unobserved axial deflection of the plasma flow towards the anode as it approaches the array axis provides evidence of the presence of a significant (20 T) toroidal magnetic field embedded within the precursor column at early times. [Preview Abstract] |
Monday, October 27, 2014 11:06AM - 11:18AM |
BO7.00009: ABSTRACT WITHDRAWN |
Monday, October 27, 2014 11:18AM - 11:30AM |
BO7.00010: Effect of axial B-field on shock structure within gas-filled liner z-pinch experiments performed on MAGPIE Guy Burdiak, Sergey Lebedev, Francisco Suzuki-Vidal, George Swadling, Simon Bland, Lee Suttle, Matthew Bennet, Jack Hare Cylindrical liner z-pinches can be used to drive convergent shock waves through gas contained inside with a striking degree of azimuthal symmetry. Here we present data from gas-filled liner experiments that include an azimuthally anisotropic axial magnetic field. The 4-fold azimuthal symmetry of the magnetic field distribution imprints itself upon the shape of the convergent shocks. This occurs despite a ratio of shock ram pressure to magnetic pressure of order 100. Interferometry and emission imaging data that show the evolution of the shock structure as it converges are presented alongside potential explanations for the dynamics. These experiments provide a potential platform for studying magnetized plasma physics with relevance to magnetized fusion schemes. Experiments were performed on the 1.4 MA, 240 ns rise-time MAGPIE pulsed-power device at Imperial College London. [Preview Abstract] |
Monday, October 27, 2014 11:30AM - 11:42AM |
BO7.00011: High-Z Pusher Experiments on the Cobra Triple Nozzle Gas-Puff Z-Pinch Philip de Grouchy, Niansheng Qi, Bruce Kusse, Charles Seyler, Levon Atoyan, Tom Byvank, Adam Cahill, John Greenly, Cad Hoyt, Sergei Pikuz, Tania Shelkovenko, David Hammer For inertial confinement fusion application and as efficient hard x-ray sources, the imploding sheath of a gas-puff z-pinch or thin liner must be accelerated to the highest possible velocity before hydrodynamic instabilities significantly disrupt the implosion symmetry. Much recent work has focused on increasing implosion stability using radially structured mass-density profiles produced by multi-nozzle gas-puff valves. The introduction of a high-Z element such as xenon into the outer gas shells in such experiments can modify radiation output during the implosion phase as well as at stagnation. In these experiments xenon is introduced into the triple-nozzle gas valve fielded on the (1MA, 200ns) COBRA z-pinch machine at Cornell University. The xenon is introduced only in the outer shell, only in the inner shell or in both, to investigate the radiative effects on implosion hydrodynamics and x-ray yield. Results are compared to those obtained during pure argon implosions with the same mass-density profile. Sheath thicknesses and stability are recorded using laser interferometry (532nm) and multi-frame imaging systems. The distribution of flow velocities and of high-Z material across the pinch is investigated using a (5GW, 527nm) Thomson scattering probe. [Preview Abstract] |
Monday, October 27, 2014 11:42AM - 11:54AM |
BO7.00012: High energy axial ion beam generated by deuterium gas-puff Z-pinch at the current level of 3 MA K. Rezac, D. Klir, P. Kubes, J. Cikhardt, B. Batobolotova, J. Kravarik, H. Orcikova, K. Turek, A. Shishlov, A. Labetsky, V. Kokshenev, N. Ratakhin The contribution presents results from Z-pinch experiments with a plasma shell on deuterium gas-puff (with deuterium linear mass of about 100 $\mu$g/cm) carried out on the GIT-12 generator at IHCE in Tomsk at the current level slightly below 3 MA. The first purpose of experiments was to study the influence of different parameters on the production of neutrons. Neutron yield up to $5 \times 10^{12}$ neutrons/shot was measured in the shot with LiF catcher. The second purpose was the examination of high-energy ions generated on the Z-pinch axis using RCF and CR-39. Very interesting results were provided by ion pinhole camera, where the influence of magnetic field on the ion beam could be studied. One of the conclusions is that the ions with energy below 10 MeV were significantly deflected by magnetic field. [Preview Abstract] |
Monday, October 27, 2014 11:54AM - 12:06PM |
BO7.00013: Effect of Driver Impedance on Dense Plasma Focus Z-Pinch Neutron Yield and Beam Acceleration J. Sears, A. Link, J. Ellsworth, S. Falabella, B. Rusnak, V. Tang, A. Schmidt, D. Welch We explore the effect of driver characteristics on dense plasma focus (DPF) neutron yield and beam acceleration using particle-in-cell (PIC) simulations of a kJ-scale DPF [1]. Our PIC simulations are fluid for the run-down phase and transition to fully kinetic for the pinch phase. The anode-cathode boundary is driven by a circuit model of the capacitive driver, including system inductance, the load of the railgap switches, the guard resistors, and the coaxial transmission line parameters. Simulations are benchmarked to measurements of a table top kJ DPF experiment with neutron yield measured with He3-based detectors. Simulated neutron yield scales approximately with the fourth power of peak current, I$^{4}$. We also probe the accelerating fields by measuring the acceleration of a 4 MeV deuteron beam and by measuring the DPF self-generated beam energy distribution [2], finding gradients higher than 50 MV/m.\\[4pt] [1] A. Schmidt et al., PRL, 109 (2012);\\[0pt] [2] J. Ellsworth et al., RSI, 85 (2014) [Preview Abstract] |
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