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 BI2: Reconnection in Laboratory and Space |
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Chair: Hantao Ji, Princeton Plasma Physics Laboratory Room: Centennial I |
Monday, November 2, 2009 9:30AM - 10:00AM |
BI2.00001: Effects of Line-tying on Magnetohydrodynamic Instabilities and Current Sheet Formation Invited Speaker: The effects of line-tying on magnetohydrodynamic instabilities are an important issue for astrophysical plasmas, such as the solar corona or astrophysical jets, where magnetic field lines are deeply anchored at a dense medium. Recently, several laboratory experiments aimed at studying line-tying effects have been initiated. In this talk, the effects of line-tying on the ideal kink mode and resistive tearing mode will be presented. In general, line-tying has a stabilizing effect on the linear instability, and it also smooths out the internal layer of the linear eigenfunction. What is more interesting, however, is the nonlinear evolution of the unstable modes. It is well known that the ideal internal kink mode and ideal coalescence instability evolve nonlinearly into tangential discontinuities, or current sheets, in the absence of line-tying. Whether the same result remains true in the presence of the line-tying remains an open question. Current sheet formation in line-tied systems is essential in Parker's scenario of coronal heating. New theoretical results on nonlinear current sheet formation will be presented. [Preview Abstract] |
Monday, November 2, 2009 10:00AM - 10:30AM |
BI2.00002: Experimental measurement of quasi-separatrix layers in magnetic flux ropes Invited Speaker: A quasi-separatrix layer (QSL) is a region in a magnetic configuration where there are strong spatial gradients in the field line connectivity, and are thought to be favorable sites for 3D magnetic reconnection. Solar physicists have used QSLs extensively to identify reconnection sites in the complicated 3D magnetic configurations in solar flares, but we present the the first use of the technique in an experimental setting. In this experiment, performed in the Large Plasma Device (LAPD) at UCLA, two lanthanum hexaboride cathodes produce current channels initially parallel to the background magnetic field. The current channels create twisted helical structures, or flux ropes, in the magnetic field. The flux ropes rotate about their central axes, which causes them to periodically collide. Three dimensional magnetic measurements at 20000 spatial locations make the QSL calculation possible. During these collisions, QSLs, as well as reverse current sheets, are observed to form between the flux ropes. The structure of these QSLs is very similar to those seen in MHD simulations of merging currents. [Preview Abstract] |
Monday, November 2, 2009 10:30AM - 11:00AM |
BI2.00003: Influence of Coulomb Collisions on the Dynamics of Magnetic Reconnection in Space and Laboratory Plasmas Invited Speaker: Magnetic reconnection is the process of a rapid change in the magnetic field topology, frequently associated with a conversion of magnetic energy into various forms of plasma kinetic energy. Many systems of interest, such as the solar corona and laboratory experiments, operate in the parameter regimes inaccessible to both collisionless and fluid models, where the collisional mean free path is comparable to the characteristic scale lengths of interest and/or the reconnection electric field is of the order of the runaway field. In this work, fully kinetic simulations with a Monte-Carlo treatment of Landau collision integral are used to analyze two problems in reconnection under such conditions. Made practical by the recent progress in computing capabilities, this powerful simulation technique allows a seamless transition from collisionless to fully collisional regimes. First, we present simulations with boundary conditions~[1] mimicking the Magnetic Reconnection eXperiment (MRX). A thorough comparison of the structure of the electron reconnection layer between the experiment and the simulations allows the relative roles of the collisional dissipation and that of the collisionless effects in MRX to be quantified. Ultimately, this provides important insights into a possible role of 3D current-aligned instabilities and helps bridge the gap between a small laboratory experiment and much larger systems in Nature. As a second example, we discuss the transition between the collisional and the kinetic reconnection regimes. In relatively short systems with Lundquist number below $S \sim 10^3$ the transition, signified by a rapid increase in the reconnection rate, occurs at the temperature that corresponds to the width of the Sweet-Parker current sheet $\delta_{\mathrm{SP}}\sim d_i$, where $d_i$ is the ion inertial length. In larger systems $S > 10^3$, the transition is observed at significantly lower temperatures than are expected from a simple criterion $\delta_{\mathrm{SP}} \sim d_i$. In particular, the Sweet-Parker current sheet is found to be unstable against a tearing-like instability that leads to a significant modulation of the current sheet thickness~[2]. The transition is achieved when the {\em minimum} thickness of the current sheet falls below $d_i$. These results may have strong implications for reconnection in the solar corona, since many of the existing models are based on the premise that the Sweet-Parker scaling is relevant at practically important $S \sim 10^{12}$ and neglect the influence of secondary islands in estimating the transition to kinetic scales.\\[4pt] [1] S. Dorfman et al. Phys. Plasmas {\bf 15}, 102107 (2008)\\[0pt] [2] W. Daughton et al. {\em ``Transition from Collisional to Kinetic Regimes in Large-Scale Reconnection Layers''}, to appear in Phys. Rev. Letters [Preview Abstract] |
Monday, November 2, 2009 11:00AM - 11:30AM |
BI2.00004: Equations of State in Collisionless Reconnection Invited Speaker: Wind and Cluster spacecraft measurements of reconnecting current sheets in the Earth's magnetosphere show strong electron temperature anisotropy. This anisotropy is accounted for in a solution of the Vlasov equation recently derived for general reconnection geometries with magnetized electrons in the limit of fast transit time [1]. A necessary ingredient is an ion-scale parallel electric field, which maintains quasi-neutrality by regulating the electron density, traps a large fraction of thermal electrons, and heats electrons in the parallel direction. Based on the expression for the electron phase space density, equations of state provide a fluid closure that relates the parallel and perpendicular pressures to the density and magnetic field strength [2]. The resulting fluid model agrees well with fully kinetic simulations of guide-field reconnection, where the parallel electron temperature becomes several times greater than the perpendicular temperature. In addition, the equations of state relate features of the electron diffusion region that develop during anti-parallel reconnection to the upstream electron beta. They impose strong constraints on the electron Hall currents and magnetic fields. The equations of state are suitable for implementation in two-fluid or hybrid codes. Such codes could retain important electron kinetic effects in numerical models of large-scale collisionless plasmas, which lie beyond current computational limits in kinetic simulation. \\[4pt] [1] J. Egedal, W. Fox, N. Katz, et al., J. Geophys. Res. 113, A12207 (2008). \\[0pt] [2] A. Le, J. Egedal, et al., Phys. Rev. Lett. 102, 085001 (2009). [Preview Abstract] |
Monday, November 2, 2009 11:30AM - 12:00PM |
BI2.00005: Investigation of ion heating due to reconnection in the MST reversed-field pinch Invited Speaker: Anomalous ion heating in laboratory and astrophysical plasmas is not well understood. In the Madison Symmetric Torus (MST) reversed-field pinch experiment, ions are heated rapidly during impulsive reconnection, attaining temperatures exceeding hundreds of eV, often well in excess of the electron temperature. The energy source for this heating is the equilibrium magnetic field energy released during reconnection, but the means by which magnetic energy is converted to ion thermal energy has not yet been established. The results and diagnostic techniques reported here aim to test several distinct theoretical models that could describe the energy conversion is both laboratory and space plasmas: viscous damping of tearing flows, ion cyclotron heating, and stochastic heating. Neutral-beam-based diagnostics are used for ion temperature measurements. Rutherford scattering monitors the majority ions, while charge-exchange- recombination spectroscopy monitors the minority ions. The high spatial (several centimeters) and temporal (tens of microseconds) resolution of these diagnostics allows for detailed comparison of the dynamics of the ion heating with theoretical predictions. The energy budget of the ion heating and its mass scaling in hydrogen, deuterium, and helium plasmas was determined by measuring the fraction of the released magnetic energy converted to ion thermal energy. The fraction ranges from about 10-30\% and increases approximately as the square root of the ion mass. Ion heating increasing with ion mass agrees with observations in other laboratory experiments as well as in the solar corona. In addition, a recent upgrade of the charge-exchange diagnostic now allows simultaneous measurement of the perpendicular and parallel ion temperature, facilitating still further discrimination among the proposed heating mechanisms. [Preview Abstract] |
Monday, November 2, 2009 12:00PM - 12:30PM |
BI2.00006: Evolution to a minimum energy Taylor state in multiple flux conserving boundaries in SSX Invited Speaker: Dynamical evolution and final MHD states are measured and analyzed in four different flux conserving boundaries in SSX. Oblate (L/R = 1.2), slightly prolate (L/R = 2.0), prolate (L/R = 3.0), and super prolate (L/R = 10) geometries are studied. The flux conserving boundaries are provided by highly conducting copper walls. In each case, dynamics are initiated by injecting small, dense spheromaks into each end of the flux conserver. The helicity of the spheromaks can be individually chosen, allowing for both counter and co-helicity merging, both of which have been tried. During the merging process, complex behavior is observed; interpenetrating, two component flows are measured with a high resolution ion Doppler spectrometer (IDS), as well as dynamic activity of the magnetic fields. After the merging phase, complex activity subsides and a minimum energy Taylor state with constant helicity is reached. This state is measured and compared to the lowest energy eigenmode predicted by $\nabla \times B = \lambda B$. Eigenmodes for the various flux conserver geometries are calculated by PSI-TET. Ion temperatures are monitored throughout the process by the IDS system. Between 48 and 288 separate magnetic probes have been employed to characterize the magnetic structure of the plasma. Typical parameters of the merged low beta plasmas are $B = 0.1$~T, $n_e = 1-5 \times 10^{20}$~m$^{-3}$, $T_i = 20$~eV, and $\beta = 0.1$. [Preview Abstract] |
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