EXPERIMENTS ON THE INTERACTION OF CURRENT CHANNELS IN A LABORATORY PLASMA - RELAXATION TO THE FORCE-FREE STATE

The interaction of current channels has long been a topic of interest for those working in solar physics (magnetic flux rope interactions), magnetic field line reconnection (tearing of current sheets and coalescence of current fibers), and fusion physics (the behavior of currents in tokamaks and pinches). A series of experiments on the dynamic interaction of two current channels has been performed in the LAPD, a new laboratory device at UCLA. The two current channels are formed by coating the cathode source nonuniformly and then biasing it with respect to an anode 10 m away. Volume data of the plasma parameters n(e), V(p), and T(e), and B are taken at 5355 locations within the plasma volume, and at 10 000 time steps. The plasma produced in the current channels is high beta but the plasma ions are unmagnetized. The self-generated magnetic fields of the current channels are as large as 20% of the background magnetic field. The experiment consists of two phases, one in which a net current is drawn through the plasma, and one in which the net current is zero. The current channels are observed to twist about each other, merge, and evolve toward a force-free state when a net current is drawn through the plasma. When the net current is zero the interaction between the channels is greatly reduced. The dynamics of the system are dominated by electron pressure and associated electrostatic fields. However, as the currents twist, a small amount of magnetic helicity is generated. The helicity and its temporal rate of change are computed and found to compare within experimental uncertainty to the predicted rate.