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 KI3: Laboratory Astrophysical Plasmas II |
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Chair: S. Lebedev, Imperial College London Room: Ballroom BC |
Tuesday, October 30, 2012 3:00PM - 3:30PM |
KI3.00001: Supernova dynamics in the laboratory: Radiative shocks produced by ultra-high pressure implosion experiments on the National Ignition Facility Invited Speaker: Arthur Pak Thermonuclear fuel experiments on the National Ignition Facility implode 2-mm diameter capsules with a cryogenic deuterium-tritium ice layer to 1000x liquid density and pressures exceeding 100 Gbar ($10^{11}$ atm). About 200 ps after peak compression, a spherical supernova-like radiative shock wave is observed that expands with shock velocities of $u_S$ = 300 km/s, temperatures of order 1 keV at densities of 1 g/cc resulting in a radiation strength parameter of $Q \sim u_S^5 = 10^4$. Radiation-hydrodynamic simulations indicate that the shock launched at stagnation first goes down a strong density gradient while propagating outward from the highly compressed DT fuel ($\sim$ 1000g/cc) to the ablation front ($\sim$ 1 g/cc). Similar to what happens inside a star, the shock pressure drops as it accelerates and heats. The radiative shock emission is first observed when it breaks out of the dense compressed fuel shell into the low-density inflowing plasma at the ablation front mimicking the supernova situation where the shock breaks out through the star surface into surrounding in-falling matter [1,2]; the shock is subsequently approaching the supercritical state with a strong pre-cursor followed by rapid cooling. These observations are consistent with the rapid vanishing of the radiation ring 400 ps after peak compression due to strong radiation losses and spherical expansion. The evolution and brightness of the radiative shock provides insight into the performance of these implosions that have the goal to produce burning fusion plasmas in the laboratory. By modifying the capsule ablator composition and thickness, the stagnation pressure, density gradients, shock velocity and radiative properties could be tailored to study various regimes related to supernovae radiative remnants.\\[4pt] [1] W. David Arnett, Supernovae as phenomena of high-energy astrophysics, Ann NY Aca. Science 302, 90 (1977).\\[0pt] [2] L. Ensman and A. Burrows, Shock breakout in SN1987A, ApJ 393, 742. [Preview Abstract] |
Tuesday, October 30, 2012 3:30PM - 4:00PM |
KI3.00002: Radiative Reverse Shock Laser Experiments Relevant to Accretion Processes in Cataclysmic Variables Invited Speaker: Christine Krauland We present results from experiments that explore radiative reverse shock waves and their contribution to the evolving dynamics of the cataclysmic variable (CV) system in which they reside. CVs are close binary star systems containing a white dwarf (WD) that accretes matter from its late-type main sequence companion star. In the process of accretion, a reverse shock forms when the supersonic infalling plasma is impeded. It provides the main source of radiation in the binary systems. In the case of a non-magnetic CV, the impact on an accretion disk produces this ``hot spot,'' where the flow obliquely strikes the rotating accretion disk. This collision region has many ambiguities as a radiation hydrodynamic system, but shock development in the infalling flow can be modeled [1]. We discuss the production of radiative reverse shocks in experiments at the Omega-60 laser facility. The ability of this high-intensity laser to create large energy densities in targets having millimeter-scale volumes makes it feasible to create supersonic plasma flows. Obtaining a radiative reverse shock in the laboratory requires a sufficiently fast flow ($>$ 60 km/s) within a material whose opacity is large enough to produce energetically significant emission from experimentally achievable layers. We will show the radiographic and emission data from three campaigns on Omega-60 with accompanying CRASH [2] simulations, and will discuss the implications in the context of the CV system. \\[4pt] [1] Armitage, P. J. and Livio, M., ApJ, 493, 898 (1998).\\[0pt] [2] van der Holst, B., Toth, G., Sokolov, I.V., et al., ApJS, 194, 23 (2011). [Preview Abstract] |
Tuesday, October 30, 2012 4:00PM - 4:30PM |
KI3.00003: Visualizing electromagnetic fields in laser-produced counterstreaming plasma experiments for collisionless shock laboratory astrophysics Invited Speaker: Nathan Kugland In astrophysical settings, large and stable structures often emerge from turbulent supersonic plasma flows. Examples include the cosmic magnetic field and the collisionless shocks [1] in supernova remnants. In a scaled environment created with the high power lasers at OMEGA EP, proton imaging shows that large, stable electromagnetic field structures arise within counterstreaming supersonic plasmas [2]. These field structures are large compared to the fundamental turbulence scale lengths of the plasma (e.g. the Debye length and the ion skin-depth), indicating a high degree of self-organization. These features remain in place from 4 to 7 ns, indicating a high degree of stability. At early times out to at least 8 ns, \textit{intra}-jet ion collisions are strong (due to relatively low thermal velocities) but \textit{inter}-jet ion collisions are rare (due to relatively high flow velocities), permitting the evolution of both hydrodynamic and collisionless plasma instabilities [3, 4]. This paper will present detailed results from our laboratory astrophysics experiments. Prepared by LLNL for US DOE under Contract DE-AC52-07NA27344.\\[4pt] [1] H. S. Park et al, HEDP, 8, 38 (2011).\\[0pt] [2] N.L. Kugland et al, submitted to Nature Physics (2012).\\[0pt] [3] J.S. Ross et al, Phys. Plas., 19, 056501 (2012).\\[0pt] [3] D.D. Ryutov et al, Phys. Plas., 19, 076532 (2012). [Preview Abstract] |
Tuesday, October 30, 2012 4:30PM - 5:00PM |
KI3.00004: Constraints on the Dissipation of Solar Wind Turbulence using Gyrokinetic Simulations Invited Speaker: Gregory Howes Gyrokinetic simulation codes, developed to a high level of sophistication in the fusion energy science program, are well suited for the study of turbulence in weakly collisional space and astrophysical plasmas, such as the solar wind. A number of exciting results have recently been achieved in the study of the dissipation range of solar wind turbulence using the Astrophysical Gyrokinetics code, AstroGK. Important results include: (1) a magnetic energy spectrum over the entire dissipation range (from ion to electron scales) that shows striking agreement with high resolution spacecraft observations, (2) evidence for an anisotropic distribution of energy in wavevector space in agreement with arguments for critically balanced kinetic Alfven wave turbulence, (3) an exponentially decaying form of the magnetic energy spectrum at the scale of the electron Larmor radius in agreement with recent observations, and (4) constraints on the partitioning of turbulent power dissipation between collisionless wave-particle interactions and dissipation in current sheets. [Preview Abstract] |
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