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≅90eV and E _max≥200eV 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 E=vBL≤(10 ⁵m/s)(0.05T)(0.1m)=500V 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 (S=1000, β=0.1) show acceleration of ions up to 2v _Alf 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.