Experimental studies of magnetic reconnection in 3D geometries

Abstract

Magnetic reconnection is an important and prevalent plasma physics phenomenon occurring in astrophysical (solar flares), space (magnetospheric substorms), and laboratory settings (tokamak sawtooth oscillations). It allows for a change in the magnetic field topology in plasmas while also transferring the magnetic field energy to plasma energy. In return, the global magnetic field topology can affect the dynamics of reconnection. Major advances have been made in understanding reconnection in two dimensions, which, however, are not necessarily applicable to fully three dimensional settings. We aim to add to the fundamental understanding of reconnection in three dimensions through observations in controlled laboratory experiments. To that effect we constructed and characterized an experimental setup at the Versatile Toroidal Facility that includes a washer plasma gun and a suite of diagnostics. We pulse a coil that drives reconnection along an X-line and also forms magnetic nulls in the plasma. We theoretically and experimentally investigated the relevant magnetic field configurations of a current coil in a background magnetic field. Notably, a coil tilted with respect to the background field produces nested separatrix surfaces consisting of field lines that encircle the null coil a certain number of times. The results are, to the best of our knowledge, original and contain features of relevance to the interaction of the Earth's magnetic field with the interplanetary magnetic field. In addition, we observed diamagnetic currents near the null points and magnetic reconnection along the X-line. The effective resistivity of the current sheet is significantly larger than the classical Spitzer resistivity. Thus, the observed reconnection cannot be explained as Sweet-Parker reconnection mediated by the classical resistivity. The reconnection current sheet extends to an azimuthal current sheet around the plasma beam. The azimuthal current sheet also is consistent with diamagnetic currents.