Bulletin of the American Physical Society
2006 APS April Meeting
Saturday–Tuesday, April 22–25, 2006; Dallas, TX
Session E3: Laboratory Astrophysics: Magnetic Reconnection |
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Sponsoring Units: DPP DAP GPAP Chair: Vincent Chan, General Atomics Room: Hyatt Regency Dallas Landmark C |
Saturday, April 22, 2006 3:30PM - 4:06PM |
E3.00001: The Dynamics of Magnetic Reconnection in the Solar Atmosphere Invited Speaker: Magnetic reconnection is widely believed to play the central role in the interaction between matter and magnetic field in the Sun's corona and, therefore, to underlie most solar activity. Direct plasma heating due to reconnection has been proposed as the process that produces both the quasi-steady and the flare hot corona. Reconnection-driven flows have been proposed as the explanation for transient dynamic phenomena ranging from the smallest spicule to giant surges and sprays. Reconnection has also been proposed as the origin of the electron beams in flares, and numerous authors have argued that it is the mechanism responsible for the origin of coronal mass ejections and prominence/filament eruptions. In fact, it is difficult to find a solar phenomenon that has not been blamed on magnetic reconnection! On the other hand, there is surprisingly scarce direct evidence for reconnection in coronal observations. In this talk, I will present both the latest observations and 3D models for reconnection-driven dynamics in the corona and attempt to reconcile the data with theory. [Preview Abstract] |
Saturday, April 22, 2006 4:06PM - 4:42PM |
E3.00002: Magnetic Reconnection in a Laboratory Plasma Invited Speaker: Magnetic reconnection plays an important role in determining the evolution of magnetic topology in relaxation processes in laboratory plasmas, magnetospheric substorms, solar flares, and more distant astrophysical plasmas. Often, magnetic reconnection is invoked to explain the observed rapid release of magnetic energy. A central question concerns why the observed reconnection rates are much faster than predictions made by classical theories, such as the Sweet-Parker model based on MHD with classical Spitzer resistivity. In general, the reconnection process is determined by both local plasma dynamics in the diffusion region as well as by global boundary conditions. Presently, there are two prevailing theories to explain the mechanism of local dissipation enhancement and fast reconnection, based on the generalized Ohm's law. One is anomalous resistivity due to wave particle interactions and the other is the Hall effect originated from separation of electron motion from ions. In this talk, I would like to present recent results on these fundamental issues of reconnection from a well- controlled laboratory experiment, Magnetic Reconnection Experiment (MRX). Both electromagnetic turbulence and out-of- plane quadrupole magnetic field (a hallmark of the Hall effect) have been observed during fast reconnection process, which is also strongly affected by boundary conditions. Physics insights based on these observations as well as their comparisons and implications to space, solar and astrophysical plasmas will be discussed. [Preview Abstract] |
Saturday, April 22, 2006 4:42PM - 5:18PM |
E3.00003: Magnetic Reconnection in the DIII-D Tokamak Invited Speaker: Magnetic reconnection is an important feature of tokamak plasmas, with the resulting changes in magnetic topology often leading to redistribution of the plasma's thermal and magnetic energy. Improved experimental diagnostics, including direct measurements of the internal magnetic field, and large nonlinear 3D numerical simulations have begun to offer new views of tokamak magnetohydrodynamic (MHD) instabilities. The full potential of these tools for investigation of reconnection physics and quantitative comparison of theory and experiment has yet to be exploited. This talk summarizes several areas of research related to reconnection in the DIII-D tokamak. The sawtooth is a periodic reconnection with poloidal and toroidal mode numbers m=1, n=1 that occurs in the core of a tokamak plasma. DIII-D results demonstrate a wide range of sawtooth behavior in different plasma regimes, ranging from fast, complete reconnection of the core region (similar to the classic Kadomtsev model) to a quasi-interchange-like instability with slower reconnection. Reconnection often leads to tearing modes and magnetic islands at flux surfaces with rational safety factors q=m/n$>$1. At low plasma pressure, the onset and growth of tearing modes in DIII-D are in good agreement with the classic stability index $\Delta $'. At higher pressure the evolution and saturation of ``neoclassical tearing modes'' agrees well with the Rutherford equation, modified to include the pressure-driven bootstrap current. Forced reconnection is also important in tokamak plasmas. Tearing modes may be driven by external magnetic field errors or by other MHD events. However, plasma rotation at the resonant surface can shield the driving perturbation and inhibit reconnection, leading to nonlinear threshold behavior as observed in DIII-D and other experiments. The shielding becomes stronger as the Lundquist number increases, a possibly favorable result for larger, hotter, stronger-field fusion devices of the future. [Preview Abstract] |
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