Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session GO5: Shocks and HED Laboratory Astrophysics |
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Chair: Hye-Sook Park, Lawrence Livermore National Laboratory Room: Ballroom F |
Tuesday, November 15, 2011 9:30AM - 9:42AM |
GO5.00001: Numerical simulations of Z-Pinch experiments to create supersonic differentially-rotating plasma flows Matteo Bochi, Sebastiaan Ummels, Jeremy Chittenden, Sergey Lebedev Recently, it was proposed that a small number of plasma jets produced by lasers could be used to generate a plasma configuration relevant to some features of astrophysical accretion disc physics. We propose complementary experimental configurations which employ converging flows generated in a cylindrical wire array Z- pinch modified to produce a rotating plasma. In this paper we present 3D MHD simulations using the code GORGON which show how this approach can be implemented at the MAGPIE facility at Imperial College, London. We will present the general scenario and the results of a parametric study relating the parameters of the array with the features of the resulting plasma. In particular, we will show how a rotating plasma cylinder or ring, with typical rotation velocity ~30 Km/s and Mach number ~8 is formed, and how, after about 1-2 revolutions, the material of the plasma ring is ejected in a pair of thermally driven, conical outflows propagating along the rotation axis. We will discuss to what aspects of the physics of accretion discs, the results of such experiments could be relevant. We will also consider the effects of different magnetic configurations, which further expand the possibility to relate the experiments with the astrophysical discs. Experimental implementation of some of these setups is currently in progress on MAGPIE. [Preview Abstract] |
Tuesday, November 15, 2011 9:42AM - 9:54AM |
GO5.00002: Reverse Radiative Shock Experiments Relevant to Accreting Stream-Disk Impact in Interacting Binaries Christine Krauland, R.P. Drake, C.C. Kuranz, C.M. Huntington, F.W. Doss, M.J. Grosskopf, D.C. Marion, S. Klein, R.P. Young, T. Plewa In many Cataclysmic Binary systems, mass onto an accretion disk produces a ``hot spot'' where the infalling flow obliquely strikes the rotating accretion disk. It has been argued (Armitage {\&} Livio, ApJ 493, 898) that the shocked region may be optically thin, thick, or intermediate, which has the potential to significantly alter its structure and emissions. We report two experimental attempts to produce this type of radiative reverse shock in a colliding plasma stream. In the laboratory this requires producing a sufficiently fast flow ($>$100 km/s) within a material whose opacity is large enough to produce energetically significant emission from experimentally achievable layers. The experiments have been performed at the Omega-60 laser facility. We will discuss the astrophysical context, our experimental design, and the available data. [Preview Abstract] |
Tuesday, November 15, 2011 9:54AM - 10:06AM |
GO5.00003: Spike morphology in supernova-relevant hydrodynamics experiments C. Di Stefano, C.C. Kuranz, R.P. Drake, M.J. Grosskopf, C.M. Krauland, D.C. Marion, S.R. Klein, B. Fryxell, A. Budde, T. Plewa, P. Nilson This presentation describes experiments performed on the Omega and Omega EP lasers exploring the 3D Rayleigh-Taylor instability at a blast- wave-driven interface. These experiments are well-scaled to the He-H interface during the explosion phase of SN1987A. Laser energy is used to create a planar blast wave in a plastic disk, which then crosses the interface between the disk and a lower-density foam, inducing the RT instability. The plastic disk has an intentional pattern machined at this interface. This seed perturbation is three-dimensional with a basic structure of two orthogonal sine waves with a wavelength of 71 $\mu $m and amplitude of 2.5 $\mu $m. Interface structure has been detected under these conditions using dual, orthogonal radiography, and some of the resulting data will be shown. Current experiments are further examining the features of the unstable interface using proton radiography. [Preview Abstract] |
Tuesday, November 15, 2011 10:06AM - 10:18AM |
GO5.00004: Laboratory Simulations of Collisionless Shocks in Low-Density, Laser-Driven Magnetized Plasmas D.B. Schaeffer, C.G. Constantin, E.T. Everson, A.S. Bondarenko, L.A. Morton, D. Winske, D.S. Montgomery, K.A. Flippo, S.A. Gaillard, R.P. Johnson, T. Shimada, S.A. Letzring, C. Niemann We present magnetic field and electron temperature and density data from collisionless shock experiments on the Trident laser at Los Alamos National Laboratory. Experiments were performed with a graphite or CH target placed inside a static magnetic field ($\sim1$ kG) created by a $50$ cm-diameter Helmholtz coil and ablated by two sequential laser pulses at $1053$ nm. The first pulse created an ambient low-density, magnetized plasma while the second pulse created a super-Alfv\'{e}nic (M$_A$ $\sim10$) plasma to shock the ambient plasma. A separate laser beam at $527$ nm was used for Thomson scattering to characterize the ambient plasma $3 - 19$ cm radially from the target and $0.5 - 9.7$ $\mu$s after the first ablation. An array of single-axis, $1$ mm b-dot probes was used to measure magnetic field compression, expulsion, and fast diffusion inside and around the diamagnetic cavity formed by the laser-plasma expansion. Complimentary magnetic field data was obtained using proton deflectrometry. [Preview Abstract] |
Tuesday, November 15, 2011 10:18AM - 10:30AM |
GO5.00005: Perpendicular shocks in multi-component plasmas Anne Stockem, Luis O. Silva, Ricardo A. Fonseca We investigate the formation of perpendicularly magnetized collisionless shocks in multi-component plasmas with theory and particle-in-cell simulations. The magnetohydrodynamical jump conditions are extended for plasmas consisting of electrons and a mixture of positrons and ions. We find that the shock speed is decreased if either the ion fraction or the ion mass are increased. Furthermore, the dependence of the jump conditions on the actual particle distribution functions is discussed, which we observe to be low for a strongly magnetized plasma. Our two-dimensional simulations confirm the theoretical results. We find that in a mixed plasma the light positron species is accelerated efficiently in the early stage of shock formation, confirming previous 1D results. For a realistic proton to electron mass ratio we observe that the electron and positron spectra are equal, in contrast to the case of low mass ratio simulations. The dependence of particle acceleration and the shock properties on the mixture and the magnetization also shows new behavior previously not reported in electron-positron or electron-ion relativistic shock simulations. [Preview Abstract] |
Tuesday, November 15, 2011 10:30AM - 10:42AM |
GO5.00006: Design considerations for collisionless shock experiments using expanding laser plasmas R. Paul Drake Collisionless shocks are systems in which the properties of colliding plasmas change on spatial scales that are small by comparison with collision lengths, in consequence of particle interactions with electromagnetic fields. Such shocks abound in astrophysical systems, but remain elusive in the laboratory. Producing them in the laboratory would be of value, as these are complex, three-dimensional systems whose behavior involves the kinetic nature of the particles that are involved. In addition, the opportunities to observe their details are limited, even in the near-Earth space environment. The control offered by laboratory experiments could be essential to developing a clear understanding of their behavior. We consider here the problem of how to produce counterstreaming plasmas that meet the requirements for the generation of well developed collisionless shocks from initially unmagnetized plasmas, working with the homologous expanding plasmas naturally produced by laser irradiation. [Preview Abstract] |
Tuesday, November 15, 2011 10:42AM - 10:54AM |
GO5.00007: Weibel mediated collisionless shocks in laboratory with ultraintense lasers Frederico Fiuza, Ricardo A. Fonseca, Luis O. Silva, John Tonge, Warren B. Mori Weibel mediated collisionless shocks are believed to occur in many astrophysical scenarios, but the conditions for the generation of these shocks in laboratory are not yet fully understood. Using \textit{ab initio} multi-dimensional relativistic PIC simulations, we show that Weibel mediated collisionless shocks can be driven in laboratory by the interaction of current/near-future high power laser pulses with overcritical plasmas. The laser acts like a piston, pushing the plasma and generating a flow of hot electrons that propagate through the target. The hot incoming flow and the cold counterstreaming flow (associated with the return current) go Weibel unstable leading to a strong compression and to the formation of a shock. The Weibel-driven magnetic fields reach 10{\%} of equipartition with the upstream kinetic energy density, in good agreement with previous simulations of astrophysical scenarios. We demonstrate the possibility of controlling the shock properties by tuning the laser intensity and target density, opening the way for the first \textit{in situ }study of Weibel mediated shocks. [Preview Abstract] |
Tuesday, November 15, 2011 10:54AM - 11:06AM |
GO5.00008: Radiation from Weibel and Kelvin-Helmoltz unstable scenarios Joana L. Martins, Eduardo P. Alves, Ricardo A. Fonseca, Luis O. Silva Weibel and Kelvin-Helmoltz instabilities can be relevant for astrophysical and laboratory scenarios, e.g. in connection with shocks. In astrophysical scenarios they have been proposed as possible mechanisms for the generation and amplification of magnetic fields respectively. The identification of their radiation signatures can contribute to understand observations and experimental measurements. In this work, the radiation signatures from the Weibel and the Kelvin-Helmoltz instabilities are explored through particle-in-cell (PIC) simulations and the post-processing of particle trajectories, and compared with theoretical models. Simulations are performed in different geometries in 2D and compared with 3D simulations, in electron/positron and electron/proton scenarios, and the radiation spectra associated with these instabilities are determined. These spectra are then analyzed in detail and their features (e.g. isotropy/anisotropy, peak energy) are compared and correlated with the evolution of the self-consistent fields associated with each unstable scenario. [Preview Abstract] |
Tuesday, November 15, 2011 11:06AM - 11:18AM |
GO5.00009: Compressible Kelvin - Helmholtz instability in super-magnetosonic regimes Francesco Pegoraro, Francesco Califano, Matteo Faganello, Francesco Palermo, Anna Tenerani With a two fluid plasma model, we investigate the nonlinear competition of different plasma instabilities involving the interplay of large and small spatial scales in a magnetized plasma with a sheared flow and the role of the in-plane magnetic field and of the density inhomogeneity. This investigation is of interest for the study of the interaction between the solar wind and the Earth's magnetosphere in regions where the velocity shear generates rolled-up vortices. We investigate the transition from sub to super magnetosonic regimes. By varying the shear flow velocity amplitude, we show the possibility of generating quasi-perpendicular magnetosonic shock structures. The onset of the Kelvin - Helmholtz instability generates large scale vortices. The shocks are generated by those vortices for which the magnetosonic Mach number is of the order of unity or larger. Compressible effects as well as density variations play a crucial role in the vortex formation process and, in particular, on the vortex velocity propagation. [Preview Abstract] |
Tuesday, November 15, 2011 11:18AM - 11:30AM |
GO5.00010: Large-scale magnetic field generation via the Kelvin-Helmholtz instability Thomas Grismayer, Eduardo Paulo Alves, Ricardo Fonseca, Luis Silva The collisionless Kelvin-Helmholtz instability (KHI) is an important candidate to generate magnetic fields in the presence of strong velocity shears, which may naturally originate in energetic matter outburst of active galactic nuclei and gamma-ray bursters. We present the first self-consistent 3D PIC simulations with Osiris of the KHI for relativistic scenarios of shearing, unmagnetized electron-proton plasmas. Simulations reveal the emergence of a strong and large-scale DC magnetic field component, which is not captured by the standard linear fluid theory. This DC component arises from a kinetic effect that is due to the thermal expansion of the electrons of one flow into the other, whereas the protons remain unperturbed due to their inertia. The electron expansion forms DC current sheets, which induce a DC magnetic field. Our results indicate that most of the electromagnetic energy developed in the KHI is stored in the DC component, reaching values of equipartition on the order of 10$^{-3}$ in the electron time-scale. [Preview Abstract] |
Tuesday, November 15, 2011 11:30AM - 11:42AM |
GO5.00011: Self-Organization in Hypersonic Shock Driven Plasmas Kyron Williams, A.B. Alexander, M. Scott, J. Buchanan, J.A. Johnson III Evidence has been found using the arc-driven shock tube of self-induced Stark effect lines due to the production of hypersonic shock waves. We take advantage of high time resolution measurements of optical spectral lines. In addition, previous work also indicated a possible means to determine the time evolution of the internal EM field geometry on short time scales (less than 250 microseconds). Further examination of hypersonic argon and krypton plasmas using a phase transition model indicates preliminary evidence of local plasma self-organization and collective behavior. The determination of the system complexity from turbulence analysis also sheds insight into the interaction of hypersonic turbulent plasmas with external magnetic fields. [Preview Abstract] |
Tuesday, November 15, 2011 11:42AM - 11:54AM |
GO5.00012: Evolution of a photon gas in the nonlinear QED vacuum Sheldon Wu, Frederic Hartemann Thermally induced vacuum polarization stemming from QED radiative corrections to the electromagnetic field equations is studied. The physics of thermal radiation in the nonlinear vacuum first described by Heisenberg and Euler is a problem of some theoretical importance, in view of its relation to the cosmic microwave background, early universe evolution, and Hawking-Unruh radiation. In particular, the questions of the evolution toward equilibrium, stability, and invariance of thermal radiation under such conditions are of great interest. While nonlinear vacuum polarization effects in the photon gas had been previously studied, our analysis is presented in the framework of quantum kinetic theory. Within the context of the Euler-Heisenberg nonlinear QED vacuum, it is shown that a homogeneous, isotropic photon gas with arbitrary spectral distribution evolves toward an equilibrium state with a Bose-Einstein distribution. The transient evolution toward equilibrium of a gas of photons undergoing photon-photon scattering is described by the Boltzmann transport equation. [Preview Abstract] |
Tuesday, November 15, 2011 11:54AM - 12:06PM |
GO5.00013: Ion Acceleration in the Madison Helicon Experiment Matt Wiebold, Yung-Ta Sung, John Scharer The Madison Helicon Experiment consists of a 150 cm long, 10 cm inner diameter Pyrex tube connected to a stainless steel expansion chamber 60 cm long and 45 cm in diameter with an axial magnetic field, variable up to 1 kG at the source region with a nozzle profile. Supersonic ion acceleration up to $E_i$ = 160 eV at 500 W, 13.56 MHz RF power have been observed in the expanding region of the source. The effect of flow rate/pressure, RF power and magnetic field strength on the ion beam acceleration and the spatial variation of the plasma potential, electron density and temperature are explored. The ion energy distribution function is obtained by a two-grid RPA, while probe diagnostics determine $V_p$, $n_e$, and $T_e$. The effect of the operating mode of the helicon source (E, H or W) is explored. RF plasma potential fluctuations are observed which electrons can respond to but ions cannot, leading to time-averaged acceleration of the ions and time-dependent ``neutralization'' of the beam. Ion acceleration occurs over $\sim$ 400 $\lambda_D$, and the accelerated population persists for several ion charge exchange mean free paths. Both double layer and Boltzmann expansion are considered as mechanisms responsible for the acceleration. [Preview Abstract] |
Tuesday, November 15, 2011 12:06PM - 12:18PM |
GO5.00014: Molecular dynamics simulations and generalized Lenard-Balescu calculations of electron-ion temperature relaxation in plasmas Lorin X. Benedict, Michael P. Surh, Saad A. Khairallah, John I. Castor, Heather D. Whitley, David F. Richards, James N. Glosli, Michael S. Murillo, Frank R. Graziani We present classical molecular dynamics (MD) calculations of temperature relaxation in hydrogen, Ar-doped hydrogen, and SF$_6$ plasmas in which the two-particle interactions are represented by statistical potentials of the Dunn-Broyles and modified Kelbg forms. Using a multi-species generalized Lenard-Balescu theory in which the full frequency and wave-vector dependent dielectric response is included, we show that deviations of our hydrogen MD results from the weak-coupling theories such as Landau-Spitzer are due in large part to the use of the statistical potentials which approximate, in a classical way, the effects of quantum diffraction. Classical MD with Kelbg potentials is shown to be better at reproducing intermediate-to-weak-coupling results of true quantum-Coulomb plasmas, but it is also shown that MD with both types of statistical potential yield the correct quantum result in the limit of infinitesimal plasma coupling. Effects of dynamical screening in multi-component plasmas are also discussed. [Preview Abstract] |
Tuesday, November 15, 2011 12:18PM - 12:30PM |
GO5.00015: Runaway positrons in magnetized plasmas T. F\"ul\"op, G. Papp Runaway electron avalanches have been frequently observed in large tokamak disruptions. The energetic runaways produced in the avalanches may give rise to electron-positron pair production. It has been estimated that up to $10^{14}$ positrons may be created in collisions between runaway electrons and thermal particles in tokamak disruptions [1]. At birth, these positrons are highly relativistic, and either experience runaway acceleration or are thermalized in a few hundred milliseconds before being annihilated. In this work we calculate the distribution of positrons at birth and their subsequent fate in magnetized plasmas. The production rate is calculated by using a pair-production cross-section valid for arbitrary energies.\\[4pt] [1] P. Helander and D. Ward, Phys. Rev. Lett., 90 135004-1 (2003). [Preview Abstract] |
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