Bulletin of the American Physical Society
2006 APS April Meeting
Saturday–Tuesday, April 22–25, 2006; Dallas, TX
Session L16: Laboratory Astrophysics I |
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Sponsoring Units: DPP Chair: Hantao Ji, Princeton Plasma Physics Laboratory Room: Hyatt Regency Dallas Landmark D |
Sunday, April 23, 2006 3:15PM - 3:35PM |
L16.00001: Spontaneous reconnection in a laboratory experiment. Invited Speaker: A new experimental configuration has been developed for study of collisionless reconnection in the Versatile Toroidal Facility (VTF) at MIT. A parameter regime of special interest exists where the reconnection process appears in rapid bursts. This provides a unique opportunity to study the ``trigger problem'' of reconnection in current sheets confronting the unresolved issues related to the spontaneous and explosive onset of events observed both on the sun and in the Earth magnetotial. Our measurements include the detailed time evolution of the plasma density, current density, the magnetic flux function, the electrostatic potential and the reconnection rate. I will discuss the new experimental scenario and present our detail observations of fast spontaneous reconnection in the VTF current sheet. [Preview Abstract] |
Sunday, April 23, 2006 3:35PM - 3:55PM |
L16.00002: Kinking and merging of flux ropes in the Relaxation Scaling Experiment Invited Speaker: In solar, space, laboratory and astrophysics, magnetic fields and currents coexist, and frequently align and twist themselves into current ropes, and form the building blocks of magnetohydrodynamics (MHD). Their relaxation is bound up with the dynamics, which is omitted from the Taylor relaxation picture. For instance, magnetic fields and currents on the Sun are sheared and twisted as they store energy, experience instability, open into interplanetary space, eject the plasma trapped in them, and cause a flare. At LANL, the Relaxation Scaling Experiment (RSX) provides a simple means to systematically characterize the linear and non-linear evolution of driven, dissipative, unstable plasma-current filaments. The topology evolves in three dimensions, supports multiple modes, and can bifurcate to quasi-helical equilibria. We observe saturation to a helical state that is a candidate for a general relaxation process. We characterize experimentally magnetic structure of a kinking, rotating and merging flux ropes. We show that non line tied flux ropes are more unstable than line tied ones, i.e. at half the Kruskal-Shafranov threshold for J/B, and sketch a theoretical model for this. \newline \newline In collaboration with I. Furno, Los Alamos National Laboratory; D. Ryutov, Lawrence Livermore National Laboratory; L. Dorf, T. Nussinov, A. Light, and G. Lapenta, Los Alamos National Laboratory; and S. Abbate, Torino Politecnico. [Preview Abstract] |
Sunday, April 23, 2006 3:55PM - 4:15PM |
L16.00003: Laboratory experiments on nonlinear Electron MHD phenomena Invited Speaker: In a large laboratory plasma highly nonlinear magnetic field-plasma interactions are studied in the regime of Electron MHD (EMHD). A pulsed magnetic field is applied with a loop antenna and the resultant field is measured with magnetic probes. Topologies of field-reversed configurations (FRC), spheromaks or strong mirrors are generated. These fields propagate as highly nonlinear whistler modes through the stationary ion background. \textit{Whistler spheromaks} propagate along the ambient magnetic field at a speed which decreases with amplitude. The field tilts (precesses) with increasing amplitude. The field at the leading front steepens to a few $c/\omega_{pe}$, i.e. forms a \textit{whistler shock}. The electrons in the (predominantly) toroidal current ring are heated and produce visible light. The source of heating is not significantly modified by heat conduction and radiation losses. The collision of two counter-propagating whistler spheromaks leads to a \textit{whistler FRC} with net zero helicity. \textit{Whistler mirrors} are produced when the wave field adds to the ambient field. These structures propagate faster than spheromaks and linear whistlers. The toroidal current is predominantly an electron Hall current which produces no electron heating. Collisions between opposing whistler mirrors produces no significant electron heating by Fermi acceleration. Field topologies lacking axial symmetry, as in strong whistler turbulence, may be characterized by a mixture of magnetic energy convection and dissipation. [Preview Abstract] |
Sunday, April 23, 2006 4:15PM - 4:35PM |
L16.00004: High velocity plasma jets and 3D magnetic structure in the Swarthmore reconnection experiment Invited Speaker: Several new experimental results are reported from plasma merging studies at the Swarthmore Spheromak Experiment (SSX) with relevance to three dimensional magnetic reconnection in laboratory and space plasmas. First, recent high-resolution velocity measurements of impurity ions using ion Doppler spectroscopy (IDS) show bi- directional outflow jets at $40~km/s$ (nearly the Alfv\'en speed). Second, ion heating to nearly $10^6~K$ is observed after reconnection events in a low density regime. Third, the out-of-plane magnetic field in a reconnection volume shows a quadrupolar structure at the ion inertial scale ($c/\omega_{pi}$). Time resolved vector magnetic field measurements on a 3D lattice (${\bf B}({\bf r}, t)$) enables this measurement. Each of these measurements will be related to and compared with similar observations in a solar or space context. [Preview Abstract] |
Sunday, April 23, 2006 4:35PM - 4:55PM |
L16.00005: Simulation of astrophysical jets in a laboratory experiment Invited Speaker: Astrophysical jets are routinely simulated in a reproducible, well-diagnosed laboratory experiment. The experimental sequence starts by imposing a vacuum poloidal magnetic field linking a disk electrode to a co-planar annular electrode. Neutral gas (H, Ne, N, or Ar) is then injected via 8 nozzles located on the disk and 8 nozzles on the annulus. A 120 $\mu $F capacitor bank power supply charged to 4-7 kV is applied via ignitron switches across the electrodes, breaking down the injected gas to form plasma. The low impedance ($<$10 m$\Omega )$ of the highly conducting plasma causes the power supply to behave as a current source, rather than a voltage source. The discharging capacitor bank drives a $\sim $100 kA poloidal electric current through the plasma; this current initially flows in eight distinct `spider legs' (see photo in April meeting poster) that span from the disk to the annulus. The spider legs quickly merge via mutual attraction of their currents to form the simulated astrophysical jet. The axial gradient of the toroidal magnetic field energy density provides the force that accelerates the jet. The mass flux boundary condition at the electrodes is tightly coupled to the jet behavior. The jet is `fueled' by plasma ingested from the nozzles and the accumulation (pile-up) of the ingested plasma collimates the jet because of the associated pile-up of frozen-in toroidal magnetic flux convected with the plasma. The jet undergoes a kink instability when it becomes long enough to satisfy the Kruskal-Shafranov $q=$1 condition. [Preview Abstract] |
Sunday, April 23, 2006 4:55PM - 5:15PM |
L16.00006: The Liquid Sodium Dynamo Experiment, NMTech and LANL Invited Speaker: The liquid sodium $\alpha \omega$ dynamo experiment is designed to demonstrate how magnetic fields are generated in AGN and stars. Naturally occurring large scale astrophysical flows, Keplerian flow and star-disk driven plumes or scale-height buoyant plumes in stars create large scale $\alpha \omega$ dynamos where diffusive transport of magnetic flux by turbulence is much less than the advected magnetic flux, thus constraining turbulent diffusion, a common problem with Dudley James type flow experiments. The constraint on turbulence is the gradient of angular momentum in stable Couette flow in the experiment, the anologue of Keplerian flow or entropy gradient in stars. The experiment consists of two coaxial cylinders, $r_1 = 15$ cm, $r_2 = 30$ cm, $\Omega_1 / \Omega_2 = 4$ i.e., limiting stable Couette flow. We expect an $\omega$-gain of the toroidal field of $Rm/2\pi = 20$. The MRI will be tested at a sensitivity of $\Delta B/B \sim 10^{-3}$. The $\alpha$ gain is driven by off-axis, axial plumes as in convection in a rotating frame. Because of the high $\omega$ gain, the turbulence generated by the plumes should be fractionally small. The apparatus has been built and tested with hot oil and water in the laboratory and we have measured, in agreement with theory, the low turbulence expected of stable Couette flow at the level of the Ekman flow. \newline \newline In collaboration with Hui Li, Howard Beckley, LANL; David Westpfahl, Rocky Gianni, James Slutz, Zeb Westrom, and Joesph Jordon, NMIMT. [Preview Abstract] |
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