Experimental evidence of fast reconnection via trapped electron motion

Magnetic reconnection in collisionless magnetohydrodynamic plasmas is studied under controlled conditions in toroidal X-line configurations formed in the center of the Versatile Toroidal Facility [J. Egedal , Rev. Sci. Instrum. 71, 3351 (2000)]. Profiles of key parameters are measured during driven reconnection pulses in the reproducible plasma discharges. The experimental results prove to be highly sensitive to the boundary conditions applied. In the configuration with "open" boundaries reconnection is fast; the measured current densities are about three orders of magnitude below those predicted based on classical Spitzer resistivity. These small current densities are consistent with the electrons following toroidally trapped orbits. Further, the size of the electron diffusion region scales with the drift orbit width of the trapped electrons. Experiments conducted with "closed" boundaries yield significantly different results. In this configuration the electrons are not trapped toroidally and large current densities are permitted to develop. These currents reduce significantly the reconnection rate in the closed configurations. (C) 2004 American Institute of Physics.