November 2, 2009

Ropes Of Plasma: Onset And Stagnation Of 3D Magnetic Reconnection

Experiments capture behavior of stressed magnetic fields

Magnetized plasmas occupy a large fraction of our cosmic universe; they exist on our sun, in the earth's magnetosphere, and in astrophysical plasmas. They also exist in laboratory magnetic fusion grade plasmas, and in other smaller experiments as well. Energy stored in stressed magnetic fields can produce large-scale explosive events that spontaneously evolve and energize particles, owing to unsteady and impulsive local processes in small volumes of space. The abrupt onset and cessation of these events in astrophysical and laboratory plasmas is a long-standing puzzle. The situation is inherently 3-dimensional (3D), which makes computer simulation difficult and requires experiments capable of reproducing 3D phenomena.

The Reconnection Scaling eXperiment (RSX) is a unique earthbound plasma physics experiment. It includes fully 3D plasma currents and magnetic fields.

We show the first laboratory example of the onset and 3D magnetic reconnection between flux ropes (plasma "wires") driven by the electromagnetic attraction between the plasma "wires" and a 3D plasma-current-driven instability. Oppositely directed magnetic field lines that are embedded in the colliding flux ropes get pushed into each other, mutually annihilate, and therefore create an electric field. When the inflow speed exceeds a critical threshold, a reconnection current layer is formed. Magnetic flux and pressure rush inwards, and pile up just outside this layer until the pressure grows large enough to support back-reaction forces that stall the inflow and stop the reconnection process. These laboratory results are consistent with computer fluid simulations.

The experimental and computational results exhibit unexpected features not usually considered in 2D models. For a small enough background magnetic guide field, the flux ropes merge and reconnect, but they bounce in a larger magnetic field. Asymmetric reconnection fields and forces on either side of the reconnection layer are also observed. This probably represents the typical case in nature, rather than ideal and symmetric configurations. This work was published in "Experimental Onset Threshold and Magnetic Pressure Pile-up for 3D Reconnection", Nature Physics 5, 521 (2009).

Supported by Laboratory Directed Research and Development (Los Alamos National Lab) and the Center for Magnetic Self Organization in Laboratory and Astrophysical Plasmas (National Science Foundation and U.S. Department of Energy).


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