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 UM9: Mini-Conference: Flux Ropes and 3D Dynamics IV |
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Chair: Bill Daughton, Los Alamos National Laboratory Room: 553AB |
Thursday, November 1, 2012 2:00PM - 2:20PM |
UM9.00001: Large Reconnection Experiment (LRX): A Major Next-Step for Laboratory Studies of Magnetic Reconnection Hantao Ji, Masaaki Yamada, Stewart Prager, William Daughton A new large plasma experiment, called the Large Reconnection Experiment or LRX, has been proposed to study magnetic reconnection in regimes directly relevant to fusion, space, and astrophysical plasmas. There are at least two possibly coupled mechanisms to explain why reconnection is fast as compared to the MHD predictions: one by physics beyond MHD, and the other by breakdown of the MHD current sheet via plasmoid instabilities into a state of interacting flux ropes. However, the former works only on the ion scales, much smaller than the plasma size, while the latter is only predicted theoretically. Further progress to study these is currently impeded by several severe limitations (1) in realistic simulations (especially in 3D), (2) in space observations due to a small numbers of in-situ measurements, (3) in solar observations due to limited spatial resolution of remote-sensing techniques, (4) in fusion plasmas due to the limited diagnostic accessibility, and (5) in the existing basic laboratory experiments due to limited scale separations. All of these strongly motivate the well-controlled/diagnosed, collaborative LRX project to simultaneously achieve large scale separations and high Lundquist numbers. Major questions and conceptual design for the LRX project will be discussed. [Preview Abstract] |
Thursday, November 1, 2012 2:20PM - 2:40PM |
UM9.00002: Interaction Between Flux Ropes in Three-Dimensional Simulations of the Solar Corona C.S. Ng, T.J. Dennis, L. Lin In three-dimensional (3D) reduced magnetohydrodynamics (RMHD) simulations of the solar corona, interaction between magnetic flux ropes is a fundamental process that leads to current-sheet formation, 3D magnetic reconnection, and coronal heating [Ng et al., ApJ, 747, 109 (2012)]. In the case of long flux ropes, this process essentially reduces to the coalescence of magnetic islands in 2D, which we have also studied extensively using MHD simulations. In the high-Lundquist number limit, which requires high-resolution to simulate, the reconnection rate between the flux ropes becomes small following the Sweet-Parker description, and thus they bounce back and forth while they reconnect. We will present our latest simulations to demonstrate this process, which can potentially have great implications in the generation of Alfv\'{e}n waves and MHD turbulence. [Preview Abstract] |
Thursday, November 1, 2012 2:40PM - 3:00PM |
UM9.00003: Flux Rope Dynamics in 3D Kinetic Simulations Stefano Markidis, Giovanni Lapenta, Pierre Henri, Homa Karimabadi, Tom Intrator, Erwin Laure The dynamics of multiple flux ropes is studied with three-dimensional Particle-in-Cell simulations. The initial plasma configuration is chosen to mimic the Earth's magnetotail environment. The evolution of the system with and without a uniform guide field Bg (1/3 the asymptotic magnetic field B0) is investigated. It is found that in both cases tearing instability rapidly occurs and generates several flux rope plasmoids, that rapidly grow in size coalescing. The magnetic field is characterized by the Hall quadrupolar structure with no guide field, while by a unipolar strong core magnetic field in guide field simulations. The interchange instability occurs on the reconnection fronts of multiple flux ropes in absence of guide field. Lower hybrid waves are present in both simulations: they appear early in simulations with no guide field, while later in time in simulations with guide field. The most intense electric fields ($\sim$ three times the reconnecting electric field) develop during the coalescing process in the contact point of merging flux ropes. These electric field are probably caused by the disruption of the Hall electric field configuration during the coalescence. The intense electric field at the contact point of flux ropes might lead to localized particle acceleration. [Preview Abstract] |
Thursday, November 1, 2012 3:00PM - 3:20PM |
UM9.00004: Saturated external kink instability of a laboratory plasma column J. Sears, T.P. Intrator, G. Wurden, T.E. Weber, W. Daughton, J. Klarenbeek, K. Gao A column of plasma generated in a longitudinal magnetic field in the Reconnection Scaling Experiment suffers from a catastrophic external kink instability when sufficient current density is driven along its length. At slightly lower current density but still above the Kruskal-Shafranov stability limit, we observe the amplitude of the kink to saturate at $\approx$ {\emph{a}}, where {\emph{a}} is the radius of the current distribution, and the column to gyrate at a steady rate for many periods. We evaluate how saturation of the kink mode is influenced by axial flow and shear therein, by rotation and Coriolis force, and by kinetic effects beyond the fluid regime. The plasma column of length l = 0.48 m has electron temperature T$_e$ = 10 eV and density n$_e$ = 1e19 m$^{-3}$. The background axial field is B = 0.01 T, and the saturated steady state occurs for current I = 300 A. We measure the vector magnetic field and the plasma temperature and density in a cubic volume measuring 0.1 m on a side with resolution on the order of the electron skin depth. From these measurements we derive the flow. We present also results of a 2D numerical model simulated with the VPIC code. Study of the saturated kink mode in laboratory plasma may offer clues to the long lifetime of astrophysical jets. [Preview Abstract] |
Thursday, November 1, 2012 3:20PM - 3:40PM |
UM9.00005: Three-Dimensional Dynamics of Solar Coronal Mass Ejections: Radial and Transverse Expansion in an Asymmetric Ambient Flow Field Valbona Kunkel, James Chen The magnetic field structure underlying coronal mass ejections (CMEs) is that of a 3D magnetic flux rope with its fixed footpoints anchored in the photosphere. The momentum of an expanding flux rope is coupled to the ambient plasma via drag. The corona and solar wind plasma is characterized by a radially outward flow field so that the dominant flow is parallel to the motion of the apex of an expanding CME flux rope but is orthogonal to the expansion in the transverse direction. The gravitational force is also parallel to the apex motion and orthogonal to the transverse expansion motion. Thus, the apex and the flanks experience significantly different drag and gravity forces and therefore different net force. We have extended the existing erupting flux rope (EFR) model of CMEs, which assumes the toroidal axis of the expansion flux rope to be a segment of circular arc with two fixed footpoints, to self-consistently calculate the forces acting on the apex and the flanks. This extension allows one to calculate the coupled expansion for the apex and flanks. We characterize the resulting structure with a semi-major axis and semi-minor radii, i.e., as an ellipse. It is shown that the 3D dynamics are critically determined by the inductance of the new geometry. [Preview Abstract] |
Thursday, November 1, 2012 3:40PM - 4:00PM |
UM9.00006: Three-dimensional merging in a closed simply-connected volume Vyacheslav S. Lukin, Michael R. Brown, Tim Gray, Mark G. Linton This talk will describe numerical simulations of reconnection and relaxation of two magnetized plasma plumes in the Swarthmore Spheromak eXperiment (SSX). We model the dynamical evolution of global magnetic fields and show it to be closely related to the reconnection rate and dynamics of localized magnetic reconnection regions. The simulations are performed using the fully implicit 3D high-order spectral element HiFi framework and follow previous work on 3D merging of quasi-stationary spheromaks, including validation of HiFi simulations against SSX experimental data [Gray et al, PoP (2010)] and a systematic numerical study of 3D reconnection through a non-stationary reconnection region [Lukin \& Linton, Nonlin. Processes Geophys. (2011)]. Recently, the SSX group has begun experiments on high-velocity plasma merging in an MHD wind-tunnel, focusing on merging of two strongly magnetized high kinetic energy plasmoids. Comparison of ongoing HiFi resistive MHD and Hall MHD simulations of the SSX wind tunnel indicate that inclusion of Hall effects is crucial to reproducing the experimental data in this geometry and plasma parameter regime. Test-particle simulations in evolving HiFi-calculated electro-magnetic fields to study ion heating for a variety of species will also be discussed. [Preview Abstract] |
Thursday, November 1, 2012 4:00PM - 4:20PM |
UM9.00007: The Dynamics of Magnetic Flux Ropes from a Topological Two-Fluid Point-of-View Setthivoine You The dynamics of interaction between magnetic flux ropes generally involves localized reconnection regions where non-MHD regimes apply, with observations frequently showing strong flows and cascading magnetic activity. To bridge the gap between a global model based on MHD magnetic helicity conservation and localized models with non-MHD dynamics, a two-fluid global model is constructed for flux rope dynamics based on the transport of relative canonical helicity. The derivation shows that electron canonical helicity can be converted into ion canonical helicity in such a way as to preserve the total canonical helicity. In effect, the two-fluid model generalizes the ordinary transfer of magnetic helicity between two connected magnetic flux ropes to the transfer of canonical helicity between two connected canonical flux ropes. The transfer mechanism is a generalized ``battery effect'' due to enthalpy potential differences on surfaces separating the two canonical flux ropes, resulting in the coupling of helical magnetic fields to helical plasma flows. The model shows how a given helicity injection can be channelled into the magnetic flux component or the vorticity flow component of the canonical flux rope, as observed in counter-helicity merging experiments. [Preview Abstract] |
Thursday, November 1, 2012 4:20PM - 4:40PM |
UM9.00008: Flux tube formation by oblique tearing instabilities across a current sheet S.D. Baalrud, A. Bhattacharjee, Y.-M. Huang, W. Daughton Tearing modes arise at resonant surfaces where a wavevector is perpendicular to the magnetic field. In 2D this is the single reversal surface of the polodial field, but in 3D guide field geometries, resonant surfaces span the current layer. Flux tubes at each surface can interact, leading to field line stochasticity, which may limit the efficacy of particle acceleration by trapping in flux tubes. Knowing the volume of the plasma that is tearing unstable is important for understanding these effects. Using analytic and numerical approaches, we find that the most unstable modes in an MHD current sheet have a similar growth rate across the current layer. In contrast, collisionless kinetic theory for a Harris current sheet predicts that flow stabilizes modes far from the reversal surface. Since flux tubes are aligned along the total field, these modes have an oblique angle with respect to the conventional 2D geometry. The eigenfunction of oblique modes is found to have an odd parity component, in addition to the usual even parity of parallel modes. Ion-electron mass ratio is also found to have a profound effect, with the trend of a broader mode spectrum at lower mass ratios. Comparison of analytic and linear numerical theories with full-orbit Vlasov computation will also be given. [Preview Abstract] |
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