Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session JM9: Mini-Conference on Unsteady Reconnection in Laboratory and Nature II: Impulsive Reconnection and Onset |
Hide Abstracts |
Chair: Tom Intrator, Los Alamos National Laboratory Room: Regency V |
Tuesday, November 3, 2009 2:00PM - 2:25PM |
JM9.00001: Impulsive Reconnection in the Sun's Atmosphere Spiro Antiochos Recent high-resolution observations from the Hinode mission show dramatically that the Sun's atmosphere is filled with explosive activity ranging from chromospheric explosions that reach heights of Mm, to coronal jets that can extend to solar radii, to giant coronal mass ejections (CME) that reach the edge of the heliosphere. The driver for all this activity is believed to be 3D magnetic reconnection. From the large variation observed in the temporal behavior of solar activity, it is clear that reconnection in the corona must take on a variety of distinct forms. The explosive nature of jets and CMEs requires that the reconnection be impulsive in that it stays off until a substantial store of free energy has been accumulated, but then turns on abruptly and stays on until much of this free energy is released. The key question, therefore, is what determines whether the reconnection is impulsive or not. We present some of the latest observations and numerical models of explosive and non-explosive solar activity. We argue that, in order for the reconnection to be impulsive, it must be driven by a quasi-ideal instability. We discuss the generality of our results for understanding 3D reconnection in other contexts. [Preview Abstract] |
Tuesday, November 3, 2009 2:25PM - 2:45PM |
JM9.00002: ABSTRACT WITHDRAWN |
Tuesday, November 3, 2009 2:45PM - 3:10PM |
JM9.00003: Onset of Fast Reconnection in Laboratory and Space Plasmas A. Bhattacharjee, K. Germaschewski, Yi-Min Huang, H. Yang, B. Rogers The onset of fast reconnection is widely studied in laboratory plasmas, satellite measurements in the Earth's magnetosphere, and solar flares. Using high- performance computing tools, based on Hall MHD equations, we will show that important features of such dynamics can be accounted for in one unifying framework. The model also elucidates the role of diamagnetic drifts that can quench nonlinearly the onset of fast reconnection. The problem takes on additional complexity when it is applied to large systems of high Lundquist number. It is shown that the dynamics of thin current sheets in such systems shows qualitative similarities in the collisional and collisionless regimes. In the resistive MHD model, at high-Lundquist number, the thin current sheet becomes near-explosively unstable to secondary tearing, producing plasmoids copiously. The linear instability for an entire class of such super-Alfvenic instabilities can be deduced from the classical tearing mode dispersion relation. In the nonlinear stage, one attains a new regime of fast reconnection in which the reconnection rate exceeds by far the predictions of Sweet-Parker theory, and becomes weakly dependent on the Lundquist number. [Preview Abstract] |
Tuesday, November 3, 2009 3:10PM - 3:35PM |
JM9.00004: Excitation and Removal of Neoclassical Tearing Mode Islands in Tokamaks R.J. La Haye A principal pressure limit in tokamaks is set by the onset of neoclassical tearing modes (NTMs), which are destabilized and maintained by helical perturbations to the pressure-gradient driven ``bootstrap'' current. The resulting magnetic islands break up the magnetic surfaces that confine the plasma. The NTM is linearly stable but nonlinearly unstable, and generally requires a ``seed'' to destabilize a meta-stable state. Thus a good analogy is an avalanche. Once excited NTM islands are very robust but can be removed by reducing the plasma pressure (i.e. bootstrap current) sufficiently so that the meta-stable parameter space is gone; self-stabilization then occurs. Examples from the DIII-D tokamak will be presented. [Preview Abstract] |
Tuesday, November 3, 2009 3:35PM - 4:00PM |
JM9.00005: Impulsive Magnetic Reconnection in TS-3 and TS-4 Merging Experiments Yasushi Ono Impulsive effects of magnetic reconnection have been investigated in TS-3 and TS-4 tokamak merging experiments for maximizing reconnection speed. Under low guide-field, over-acceleration of two merging tokamaks caused the 3-D local deformation of current sheet. The toroidal asymmetry grew locally around the current sheet only during the reconnection time, increasing the plasma mass ejection from the current sheet. The local compression of current sheet thickness shorter than ion gyro-radius triggered its anomalous resistivity, causing significant increase in the reconnection speed. Under high guidefield, the sheet resistivity was almost classical due to the sheet thickness larger than ion gyroradius When we over-accelerated the merging tokamaks under high-guide field, large inflow flux and low current-sheet dissipation caused plasma pileup around the sheet. When the flux pileup exceeded a critical limit, the sheet was ejected mechanically from the squeezed X-point area. The reconnection (outflow) speed was slow during the flux pileup and was fast during the ejection During the ejection, the current sheet was often transformed into several current islands, suggesting that the island size comparable with ion gyrorasius increased the sheet resistivity. Those three fast reconnection mechanisms: anomalous reisistvity, plasmoid (sheet) ejection and 3-D reconnection were observed consistently in TS reconnection experiments. [Preview Abstract] |
Tuesday, November 3, 2009 4:00PM - 4:25PM |
JM9.00006: Unsteady magnetic reconnection in laboratory experiments with current sheets Anna Frank According to present notion, unsteady magnetic reconnection in current sheets (CS) is basic to dramatic natural phenomena: solar and stellar flares, substorms in the Earth and other planetary magnetospheres, as well as to disruptive instabilities in tokamak plasmas. We present a review of laboratory experiments studying evolution of CS formed in 3D and 2D magnetic configurations with an X line, in the CS-3D device. Usually CS exists during an extended period in a metastable stage, without essential changes of its structure and parameters. Under certain conditions this stage may be suddenly interrupted by unsteady phase of magnetic reconnection, which manifests itself in a rapid change of the magnetic field topology, current redistribution, excitation of pulsed electric fields, and other dynamic effects. The unsteady phase results in effective conversion of magnetic energy into the energy of plasma and accelerated particles, and may finally bring about the CS disruption. In the context of the solar flares, a metastable CS is associated with a pre-flare situation, while CS disruption -- with the flare itself. The physical mechanisms triggering the unsteady magnetic reconnection in the laboratory produced current sheets are discussed. Supported by the Russian Foundation for Basic Research (project {\#} 09-02-00971). [Preview Abstract] |
Tuesday, November 3, 2009 4:25PM - 4:45PM |
JM9.00007: Impulsive Reconnection and the Role of Hall Physics in the Reversed Field Pinch T.D. Tharp, A.F. Almagri, M.C. Miller, V.V. Mirnov, S.C. Prager, J.S. Sarff Previous measurements have established that the RFP sawtooth relaxation is characterized by the simultaneous occurrence of multiple impulsive reconnection events. Here, we report measurements of the reconnection Ohm's Law, including the reconnection electric field and terms that balance this field. This analysis is applied to tearing modes with poloidal mode number $m=0$ and toroidal mode number $n\ge 1$. Measurements are performed with probes in the vicinity of the resonant surface, and clearly indicate that single fluid physics is not sufficient to explain this reconnection. In particular, nonlinear three-wave interaction Hall terms are a strong contributor to the dynamics of this reconnection, indicating the substantial role of collective mode phase matching during sawtooth events. [Preview Abstract] |
Tuesday, November 3, 2009 4:45PM - 5:05PM |
JM9.00008: Experiments on 3D Onset of Magnetic Reconnection N. Katz, J. Egedal, W. Fox, A. Le, A. Vrublevskis, J. Bonde, M. Porkolab We study collisionless magnetic reconnection in the laboratory using the Versatile Toroidal Facility (VTF) at MIT. In our experiments a parameter regime exists where the reconnection process appears in rapid bursts [1]. This regime provides a unique opportunity to study the unresolved ``trigger problem'' of magnetic reconnection related to the spontaneous and explosive onset of events observed on the sun, in the Earth's magneto-tail and in sawtooth oscillations in magnetic fusion devices. Although the reconnection is driven, fast reconnection is not seen immediately: instead, the plasma responds at first by building up current to oppose the drive, and only after a delay of $\sim$100 $\mu$s is there a burst of spontaneous, fast reconnection. We find that although the experiment is set up axi-symmetrically, the burst of reconnection starts at one toroidal location and propagates along X-line approximately at the Alfv\'en speed. This burst of fast reconnection is only observed in the presence of a global plasma mode ($q=2$ or $q=3$), which is seen both in the current density and in the floating potential and is related to the formation of a magnetic island. \\[0ex] [1] J. Egedal et al., Phys. Rev. Lett., {\bf 98}, 015003 (2007) [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700