Gyrokinetic Simulation of Ion Beam Formation During Magnetic Reconnection of Two Colliding Flux Ropes

An ion beam field-aligned to a strong background guide field was observed in a reconnection experiment on the Large Plasma Device. Magnetic reconnection occurred midway between two kink-unstable flux ropes which collided periodically with sub-Alfvénic ion beams (~15eV) observed to be correlated with the rope oscillations. 3D gyrokinetic simulations of the two reconnecting flux ropes demonstrate that ion acceleration and field-aligned ion beam formation are a consequence of the field-aligned electric potentials that arise from the merging and magnetic reconnection of the flux ropes. A gyrokinetic model is well-suited for the large guide field regime of magnetic reconnection where both charged particle species are strongly magnetized. In this model, electrons are treated as drift-kinetic particles and ions gyrokinetic, which also include gyro-averaged cross-field E×B drifts and exact parallel dynamics. Ion-electron collisions and logical sheath boundary conditions are included. A detailed analysis of the particle dynamics in the merging region indicates a nonlocal acceleration process whereby ions in the merging region were expelled into surrounding regions where the effects of the space charge fields are dominant before drifting back to the reconnection region further downstream.