Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session CI3: High Energy Density HydrodynamicsInvited
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Chair: Forrest Doss, Los Alamos National Laboratory Room: 210 ABEF |
Monday, October 31, 2016 2:00PM - 2:30PM |
CI3.00001: Turbulent Dynamo Amplification of Magnetic Fields in Laser-Produced Plasmas Invited Speaker: Petros Tzeferacos Magnetic fields are ubiquitous in the Universe, as revealed by diffuse radio-synchrotron emission and Faraday rotation observations, with strengths from a few nG to tens of $\mu$G. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter in the Universe. The standard model for the origin of these intergalactic magnetic fields is through the amplification of seed fields via turbulent dynamo to the level consistent with current observations. We have conceived and conducted a series of experiments using high-power laser facilities to study the amplification of magnetic fields via turbulence. In these experiments, we characterize the properties of the fluid and the magnetic field turbulence using a comprehensive suite of plasma and magnetic field diagnostics. We describe the large-scale 3D simulations we performed with the radiation-MHD code FLASH on ANL's Mira to help design and interpret the experiments. We then discuss the results of the experiments, which indicate magnetic Reynolds numbers above the expected dynamo threshold are achieved and seed magnetic fields produced by the Biermann battery mechanism are amplified by turbulent dynamo. We relate our findings to processes occurring in galaxy clusters. [Preview Abstract] |
Monday, October 31, 2016 2:30PM - 3:00PM |
CI3.00002: Three-Dimensional Hydrodynamic Simulations of OMEGA Implosions Invited Speaker: I.V. Igumenshchev The effects of large-scale (with Legendre modes less than $\sim 30\mbox{)}$ asymmetries in OMEGA direct-drive implosions caused by laser illumination nonuniformities (beam-power imbalance and beam mispointing and mistiming) and target offset, mount, and layers nonuniformities were investigated using three-dimensional (3-D) hydrodynamic simulations. Simulations indicate that the performance degradation in cryogenic implosions is caused mainly by the target offsets $\left( {\sim 10\mbox{\thinspace to\thinspace 20\thinspace }\mu \mbox{m}} \right),$ beam{\-}power imbalance $\left( {\sigma _{\mbox{rms}} \sim 10\% } \right),$ and initial target asymmetry $\left( {\sim 5\% \mbox{\thinspace }\rho R\mbox{\thinspace variation}} \right),$ which distort implosion cores, resulting in a reduced hot-spot confinement and an increased residual kinetic energy of the stagnated target. The ion temperature inferred from the width of simulated neutron spectra are influenced by bulk fuel motion in the distorted hot spot and can result in up to $\sim 2\mbox{-keV}$ apparent temperature increase. Similar temperature variations along different lines of sight are observed. Simulated x-ray images of implosion cores in the 4- to 8-keV energy range show good agreement with experiments. Demonstrating hydrodynamic equivalence to ignition designs on OMEGA requires reducing large-scale target and laser-imposed nonuniformities, minimizing target offset, and employing high-efficient mid-adiabat $\left( {\alpha =4} \right)$ implosion designs that mitigate cross-beam energy transfer (CBET) and suppress short-wavelength Rayleigh--Taylor growth. These simulations use a new low-noise 3-D Eulerian hydrodynamic code \textit{ASTER}. Existing 3-D hydrodynamic codes for direct-drive implosions currently miss CBET and noise-free ray-trace laser deposition algorithms. \textit{ASTER} overcomes these limitations using a simplified 3-D laser-deposition model, which includes CBET and is capable of simulating the effects of beam-power imbalance, beam mispointing, mistiming, and target offset. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Monday, October 31, 2016 3:00PM - 3:30PM |
CI3.00003: Increasing Plasma Parameters using Sheared Flow Stabilization of a Z-Pinch Invited Speaker: Uri Shumlak Recent experiments on the ZaP Flow Z-Pinch at the University of Washington have been successful in compressing the plasma column to smaller radii, producing the predicted increases in plasma density ($10^{18}$ cm$^{-3}$), temperature (200 eV), and magnetic fields (4 T), while maintaining plasma stability for many Alfven times (over 40 $\mu$s) using sheared plasma flows. These results indicate the suitability of the device as a discovery science platform for astrophysical and high energy density plasma research, and keeps open a possible path to achieving burning plasma conditions in a compact fusion device. Long-lived Z-pinch plasmas have been produced with dimensions of 1 cm radius and 100 cm long that are stabilized by sheared axial flows for over 1000 Alfven radial transit times. The observed plasma stability is coincident with the presence of a sheared flow as measured by time-resolved multi-chord ion Doppler spectroscopy applied to impurity ion radiation. These measurements yield insights into the evolution of the velocity profile and show that the stabilizing behavior of flow shear agrees with theoretical calculations and 2-D MHD computational simulations. The flow shear value, extent, and duration are shown to be consistent with theoretical models of the plasma viscosity, which places a design constraint on the maximum axial length of a sheared flow stabilized Z-pinch. Measurements of the magnetic field topology indicate simultaneous azimuthal symmetry and axial uniformity along the entire 100 cm length of the Z-pinch plasma. Separate control of plasma acceleration and compression have increased the accessible plasma parameters and have generated stable plasmas with radii below 0.5 cm, as measured with a high resolution digital holographic interferometer. [Preview Abstract] |
Monday, October 31, 2016 3:30PM - 4:00PM |
CI3.00004: Rayleigh Taylor growth at an embedded interface driven by a radiative shock Invited Speaker: Channing Huntington Radiative shocks are those where the radiation generated by the shock influences the hydrodynamics of the matter in the system. Radiative shocks are common in astrophysics, including during type II supernovae, and have also been observed in the rebound phase of a compressed inertial confinement fusion (ICF) capsule. It is predicted that the radiative heating serves to stabilize hydrodynamic instabilities in these systems, but studying the effect is challenging. Only in recent experiments at the National Ignition Facility has the energy been available to drive a radiative shock across a planar, Rayleigh-Taylor unstable interface in solid-density materials. Because the generation of radiation at the shock front is a strong function of shock velocity (v$^{8})$, the RT growth rates in the presence of fast and slow shockas were directly compared. We observe reduced RT spike development when the driving shock is expected to be radiative. Both low drive (225 eV) hydrodynamic RT growth and high drive (325 eV), radiatively-stabilized growth rates are in good agreement with 2D models. This NIF Discovery Science result has important implications for our understanding of astrophysical radiative shocks, as well as the dynamics of ICF capsules. [Preview Abstract] |
Monday, October 31, 2016 4:00PM - 4:30PM |
CI3.00005: Supersonic, shockwave-driven hydrodynamic instability experiments at OMEGA-EP Invited Speaker: Willow Wan Hydrodynamic instabilities play a dominant role in the transport of mass, momentum, and energy in nearly every plasma environment, governing the dynamics of natural and engineering systems such as solar convective zones, magnetospheric boundaries, and fusion experiments. In past decades, limitations in our understanding of hydrodynamic instabilities have led to discrepancies between observations and predictions. Since then, significant improvements have been made to our available experimental techniques, diagnostics, and simulation capabilities. Here, we present a novel experimental platform that can sustain a steady, supersonic flow across a precision-machined, well-characterized material interface for unprecedented durations \\ \\ We applied this platform to a series of Kelvin-Helmholtz instability experiments. The Kelvin-Helmholtz instability generates vortical structures and turbulence at an interface with shear flow. In a supersonic flow, the growth rate is inhibited and the instability structure is altered. The data were obtained at the OMEGA-EP facility by firing three laser beams in sequence to produce a 12 kJ, 28 ns stitched laser pulse. The ablation pressure sustained a steady shockwave for $\sim$70 ns over a foam-plastic, single-mode or dual-mode interface. A spherical crystal imager was used to measure the evolution of these modulations with high-resolution x-ray radiography using Cu K$_{\alpha}$ radiation at 8.0 keV. The observed structure was reproduced with 2D hydrodynamic simulations. \\ \\ References: \\ 1. W.C. Wan, G. Malamud, et al., Physical Review Letters, 115, 145001, (2015). \\ 2. G. Malamud, et al., High Energy Density Physics, 9, 672-686, (2013) [Preview Abstract] |
Monday, October 31, 2016 4:30PM - 5:00PM |
CI3.00006: Laser-Driven Magnetized Collisionless Shocks Invited Speaker: Derek Schaeffer Collisionless shocks -- supersonic plasma flows in which the interaction length scale is much shorter than the collisional mean free path -- are common phenomena in space and astrophysical systems, including the solar wind, coronal mass ejections, supernovae remnants, and the jets of active galactic nuclei. These systems have been studied for decades, and in many the shocks are believed to efficiently accelerate particles to some of the highest observed energies. Only recently, however, have laser and diagnostic capabilities evolved sufficiently to allow the detailed study in the laboratory of the microphysics of collisionless shocks over a large parameter regime. We present experiments that demonstrate the formation of collisionless shocks utilizing the Phoenix laser laboratory and the LArge Plasma Device (LAPD) at UCLA. We also show recent observations of magnetized collisionless shocks on the Omega EP laser facility that extend the LAPD results to higher laser energy, background magnetic field, and ambient plasma density, and that may be relevant to recent experiments on strongly driven magnetic reconnection. Lastly, we discuss a new experimental regime for shocks with results from high-repetition (1 Hz), volumetric laser-driven measurements on the LAPD. These large parameter scales allow us to probe the formation physics of collisionless shocks over several Alfv\'{e}nic Mach numbers ($M_A$), from shock precursors (magnetosonic solitons with $M_A<1$) to subcritical ($M_A<3$) and supercritical ($M_A>3$) shocks. The results show that collisionless shocks can be generated using a laser-driven magnetic piston, and agree well with both 2D and 3D hybrid and PIC simulations. Additionally, using radiation-hydrodynamic modeling and measurements from multiple diagnostics, the different shock regimes are characterized with dimensionless formation parameters, allowing us to place disparate experiments in a common and predictive framework. [Preview Abstract] |
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