Experimental Investigation of Reconnection in a Line-tied Plasma

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

In this work, magnetic reconnection phenomena will be investigated in a line-tied pinch experiment. There is a long history of experimental studies in periodic, toroidal configurations which has built up an understanding of reconnection based on the existence of mode rational (or mode-resonant)surfaces where the parallel wave number k_|| vanishes. However, it is common in astrophysics to observe or infer reconnection in bounded systems with line-tying in which a vanishing k_ll is absent. Thus, there remain major, mostly uncharted questions of how reconnection occurs, how it differs from periodic reconnection, and what concepts will replace the k_ll identification with reconnection. It is a topic arguably of comparable scope to periodic reconnection, yet research in this area has been scarce. In this work, a diagnostic will be constructed for observing the large,localized current and resulting reconnection predicted to exist in the saturated phase of the line-tied kink instability. This will be carried out in two different magnetic equilibria. The first is a simple linear pinch in which instability has already been experimentally observed. The second will be in a zero net current equilibrium, one similar to the line-tied equilibria of flux tubes emerging from the solar corona and twisted by vortical motion of the flux tube footpoints. The experiments will be carried out in the Rotating Wall Machine (which also has a fusion science mission, supported by the DoE). The experiment is a linear screw pinch and uses electrostatic plasma guns for producing a current carrying column. Its primary mission is to investigate the stabilization of the resistive wall mode by moving metal walls. An internal kink instability has been observed to grow and saturate in the the experiment. Detailed measurements showed that an ideal,line-tied kink mode begins growing when the effective safety factor (which measures the field line twist) drops below 1 inside the plasma; the saturated state corresponds to a rotating helical equilibrium. In addition to the ideal mode, reconnection events were observed to periodically flatten the current profile and change the magnetic topology. The reconnection events strongly resemble the reconnection phenomena described in numerical simulations of a nearly identical geometry. Numerical simulations predict the existence of a large, localized current which gives rise to reconnection. These current sheets differ significantly from the current sheets predicted in toroidal experiments: their eigenfunctions have structure in the axial direction as well as in the radial direction. We propose to install a 2D array of magnetic probes on a 6 x 6 grid at the axial midpoint of the plasma column to discover whether this current sheet exists or not. We also propose to modify the plasma gun geometry such that the central gun injects current in the opposite direction to the surrounding 7 guns. This geometry more closely resembles the type of current profiles expected in the flux tubes emerging from the surface of the sun. The results will be compared to analytical theory and to a numerical model of the experiment implemented in the NIMROD code. This proposal was submitted to the NSF-DoE Partnership in Plasma Science and Engineering joint solicitation 08-589. This award is being funded jointly by the Divisions of Physics and Astronomical Sciences of the Mathematical and Physical Sciences Directorate.