Experiments on collisionless reconnection and the interaction of current channels

A neutral magnetic sheet in which the current is carried mainly by the electrons is set up in a nearly collisionless (lmfp = 2m = machine length, Rm ∼ 10) laboratory plasma. When the current sheet is forced to be sufficiently narrow (length/thickness ∼ 30) the current is observed to tear. The tearing of the finite dimension current sheet (Lx = 40cm, Ly = 100cm, Lz = 1.3cm) is fully three dimensional. The 3D currents are evaluated from magnetic field (j = 1/μ0 ∇xB) acquired at 5,000 spatial positions and thousands of time steps. Tearing is accompanied by the generation of significant Hall currents and, magnetic disturbances are observed to propagate at the whistler wave speed. In spite of its 3D nature the tearing mode growth time is in good agreement with linear 2D theory. In a series of related experiments the interaction of two current channels with appreciable self fields (Bcurrent ∼ 0.1 By) are investigated in a newly constructed 10m long device. The current filaments are observed to merge while sprialing around each other. The system relaxes toward a force free state (j parallel B) and helicity is conserved in the process