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 BI2: Space, Astro, and Lab Astro |
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Chair: Amitava Bhattacharjee, Princeton Plasma Physics Laboratory Room: Bissonet |
Monday, October 27, 2014 9:30AM - 10:00AM |
BI2.00001: Turbulent amplification of magnetic fields in the laboratory Invited Speaker: Gianluca Gregori Magnetic fields exist ubiquitously in the Universe, as revealed by either diffuse radio-synchrotron emission, or 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. At present, the origin and the distribution of the magnetic fields are far from being understood. The standard model for the origin of these intergalactic magnetic fields is through the amplification of seed fields via turbulent processes to the level consistent with current observations. We have conducted a series of laboratory experiments using high power laser facilities [1,2] to exploit the scale invariance of the magneto-hydrodynamics equations. While the scaling is not perfect (e.g., in what concerns dissipation coefficients such as resistivity or viscosity), the similarity is sufficiently close to make such experiments interesting -- and the results have been showing up the fundamental physical process at play. Our results indicate the magnetic field is indeed amplified by turbulent mechanisms. We relate our findings with processes occurring in supernova remnants and in cluster of galaxies. These experiments provide an example of magnetic field amplification by turbulence in plasmas, a physical process thought to occur in many astrophysical phenomena. \\[4pt] [1] G. Gregori et al., Nature 481, 480 (2012);\\[0pt] [2] J. Meinecke et al., Nature Phys. 10, 520 (2014). [Preview Abstract] |
Monday, October 27, 2014 10:00AM - 10:30AM |
BI2.00002: Observation of electromagnetic Weibel instabilities in laser-driven high Mach number counter-streaming plasma flow experiments Invited Speaker: Hye-Sook Park Astrophysical collisionless shocks are ubiquitous, occurring in supernova remnants, gamma ray bursts, and protostellar jets. They appear when the ion-ion collision mean free path is much larger than the system size. Here we present laboratory experiments using the high-power lasers and investigate the dynamics of high Mach number collisionless shock formation in two interpenetrating plasma streams. It is believed that in the astrophysical environment such shocks are the sites where seed magnetic fields are generated on a cosmologically fast timescale [1] via the Weibel [2] (or filamentation) instability. Particle-in-cell (PIC) numerical simulations have confirmed that the strength and scale of the generated magnetic field [3,4] are consistent with this concept. Our recent proton probe experiments on Omega show filamentary structures of Weibel instabilities, that are from electromagnetic nature and the inferred magnetization level could be as high as $\sim$ 1{\%} [5]. These results imply significance of electromagnetic instabilities in the plasma interactions in the ICF and astrophysical conditions. This paper will review the recent experimental results from various laser facilities as well as the simulation results and the theoretical understanding of these observations. The planned NIF experiments will be presented where it will be possible to observe the fully formed shocks. \\[4pt] [1] M. V. Medvedev and A. Loeb, ApJ. 526(2), 697 (1999).\\[0pt] [2] E. S. Weibel, Phys. Rev. Lett., 2, 83 (1959).\\[0pt] [3] A. Spitkovsky, ApJ. Lett. 673(1), L39 (2008).\\[0pt] [4] T. N. Kato and H. Takabe, ApJ. Lett. 681, L93 (2008).\\[0pt] [5] C. M. Huntington, \textit{et al}., ``\textit{Observation of magnetic field generation via the Weibel instability in interpenetrating plasma flows},'' in preparation (2014). [Preview Abstract] |
Monday, October 27, 2014 10:30AM - 11:00AM |
BI2.00003: Astrophysical Weibel instability in counter-streaming laser-produced plasmas Invited Speaker: W. Fox Astrophysical shock waves play diverse roles, including energizing cosmic rays in the blast waves of astrophysical explosions, and generating primordial magnetic fields during the formation of galaxies and clusters. These shocks are typically collisionless and require collective electromagnetic fields to couple the upstream and downstream plasmas. The Weibel instability has been proposed to provide the requisite interaction mechanism for shock formation in weakly-magnetized shocks by generating turbulent electric and magnetic fields in the shock front. This work presents the first laboratory identification of this Weibel instability between counterstreaming supersonic plasma flows and confirms its basic features, a significant step towards understanding these shocks. In the experiments, conducted on the OMEGA EP laser facility at the University of Rochester, a pair of plasmas plumes are generated by irradiating of a pair of opposing parallel plastic (CH) targets. The ion-ion interaction between the two plumes is collisionless, so as the plumes interpenetrate, supersonic, counterstreaming ion flow conditions are obtained. Electromagnetic fields formed in the interaction of the two plumes were probed with an ultrafast laser-driven proton beam, and we observed the growth of a highly striated, transverse instability with extended filaments parallel to the flows. The instability is identified as an ion-driven Weibel instability through agreement with analytic theory and fully kinetic particle-in-cell simulations of colliding ablation flows, which include a collision operator. The experimental proton-radiography results are compared with synthetic ray-tracing through 3-D simulations. \\[4pt] [1] W. Fox, G. Fiksel, A. Bhattacharjee, et al, \textit{Phys. Rev. Lett.} \textbf{111}, 225002 (2013). [Preview Abstract] |
Monday, October 27, 2014 11:00AM - 11:30AM |
BI2.00004: Turbulence in the Solar Wind from MHD to Kinetic Scales Invited Speaker: Christopher Chen The solar wind provides one of the best opportunities to investigate plasma turbulence with a range of detailed in situ measurements. In this talk, I will describe some recent progress that has been made in understanding this turbulence, both at MHD scales and at small kinetic scales, where recent high resolution measurements have led to a rapid increase in our understanding. In particular, I will discuss measurements of the energy spectrum, anisotropy, intermittency and the interplay between linear and non-linear dynamics in the cascade. I will also compare these results to modern theories of plasma turbulence and discuss the implications for our understanding of how turbulent plasmas are heated. [Preview Abstract] |
Monday, October 27, 2014 11:30AM - 12:00PM |
BI2.00005: Experimental study of energy conversion in the magnetic reconnection layer Invited Speaker: Masaaki Yamada Magnetic reconnection, in which magnetic field lines break and reconnect to change their topology, occurs throughout the universe: in solar flares, the earth's magnetosphere, star forming galaxies, and laboratory fusion plasmas [1]. The essential feature of reconnection is that it energizes plasma particles by converting magnetic energy to particle energy; this process both accelerates and heats the plasma particles. Despite the recent advances of reconnection research, the exact mechanisms for bulk plasma heating, particle acceleration, and energy flow channels remain unresolved. In this work, the mechanisms responsible for the energization of plasma particles in the magnetic reconnection layer are investigated in the MRX device together with a quantitative evaluation of the conversion of magnetic energy to ions and electrons. A comprehensive analysis of the reconnection layer is made in terms of two-fluid physics based on the measurements of two-dimensional profiles of 1) electric potential, 2) flow vectors of electrons and ions, and 3) the electron temperature, T$_{e}$ and the ion temperature, T$_{i}$ in the layer. It is experimentally verified that a saddle shaped electrostatic electric potential profile is formed in the reconnection plane. Ions are accelerated across the separatrices by the strong electrostatic field and enter the exhaust region where they become thermalized [2,3]. Electron heating is observed to extend beyond the electron diffusion region, and non-classical heating mechanisms associated with high frequency fluctuations is found to play a role. Our quantitative analysis of the energy transport processes and energy inventory concludes that more than 50 {\%} of magnetic energy is converted to plasma particles, of which 2/3 transferred to ions and 1/3 to electrons. The results which demonstrate that conversion of magnetic energy occurs in a significantly larger region than theoretically considered before, are compared with the two-fluid simulations and the recent space measurements [4]. Broader implication of the present results will be discussed.\\[4pt] [1] M. Yamada, R. Kulsrud, {\&} H. Ji, \textit{Rev. Mod. Phys.} \textbf{82}, 603--664 (2010).\\[0pt] [2] J. Yoo et al, \textit{Phys. Plasmas }\textbf{21}, 055706 (2014).\\[0pt] [3] M. Yamada et al, Submitted \textit{to Nature Communications} (2014).\\[0pt] [4] J. P. Eastwood \textit{et al.}, \textit{Phys. Rev. Lett. }\textbf{110}, 225001 (2013). [Preview Abstract] |
Monday, October 27, 2014 12:00PM - 12:30PM |
BI2.00006: 3D MHD simulation of Caltech Plasma Jet Experiment and Implications for Astrophysical Jets Invited Speaker: Xiang Zhai Magnetic fields are believed to play an essential role in astrophysical jets with observations suggesting the presence of helical magnetic fields. In this talk we present 3D ideal MHD simulations of the Caltech plasma jet experiment using a magnetic tower scenario as the baseline model. Magnetic fields consist of an initially localized dipole-like poloidal component and a toroidal component that is continuously being injected into the domain. This flux injection mimics the poloidal currents driven by the anode-cathode voltage drop in the experiment. The injected toroidal field stretches the poloidal fields to large distances, while forming a collimated jet along with several other key features. Detailed comparisons between 3D MHD simulations and experimental measurements provide a comprehensive description of the interplay among magnetic force, pressure and flow effects. In particular, we delineate both the jet structure and the transition process that converts the injected magnetic energy to other forms. With suitably chosen parameters that are derived from experiments, the jet in the simulation agrees quantitatively with the experimental jet in terms of magnetic/kinetic/inertial energy, poloidal current, jet radius and jet propagation velocity. Specifically, the jet velocity in the simulation is proportional to the poloidal current divided by the square root of the jet density, in agreement with both the experiment and analytical theory. This work provides a new and quantitative method for relating experiments, numerical simulations and astrophysical observation, and demonstrates the possibility of using terrestrial laboratory experiments to study astrophysical jets. [Preview Abstract] |
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