Energetic particles from three-dimensional magnetic reconnection events in the Swarthmore Spheromak Experiment

Measurements are presented from the Swarthmore Spheromak Experiment ~SSX! @M. R. Brown, Phys. Plasmas 6, 1717 ~1999!# showing a population of superthermal, super-Alfvénic ions with Ē >90 eV and Emax>200 eV accelerated by reconnection activity in three-dimensional magnetic structures. These energetic ions are temporally and spatially correlated with three-dimensional magnetic reconnection events ~measured with a 3D probe array! and are accelerated along the X-line normal to the local 2D plane of reconnection. In a typical SSX discharge, the peak reconnection electromotive force E5vBL<(105 m/s)(0.05 T)(0.1 m)5500 V consistent with our observations. In addition, test particle simulations using magnetohydrodynamic ~MHD! data from SSX simulations and run with dimensionless parameters similar to the experiment ~S51000, b50.1! show acceleration of ions up to 2vAlf in a few Alfvén times consistent with the measurement. The process includes two phases—a strong but short duration direct acceleration in the quasi-steady reconnection electric field, and a weaker longer lived sub-diffusive component associated with turbulence. © 2002 American Institute of Physics. @DOI: 10.1063/1.1458589#