Laboratory Observation of Transition from Collisional Slow to Collisionless Fast Reconnection

Temporal transition of an externally-driven anti-parallel asymmetric magnetic reconnection from collisional slow to collisionless fast regime is observed in a laboratory plasma for the first time. This transition is initiated when the two-fluid Hall effect begins to dominate over collisional effects at the X-point, characterized by the ratio of electron-ion collision mean free path to current sheet thickness exceeding unity. Prior to the transition, an enhanced reconnection electric field develops on the low-density side where electrons are heated both ohmically and by large-amplitude lower-hybrid drift waves (LHDW) before collisionality at the X-point drop significantly. These LHDWs generate anomalous resistivity accounting for 30% of reconnection electric field. The observed time evolution is consistent with the hybrid collisionless-collisional reconnection scenario relevant to onset of asymmetric reconnection in natural plasmas.