Experiments on the interaction of current channels in a laboratory plasma - Relaxation to the force-free state

The spatial evolution of two interacting current channels in a collisional environment in which the ions are unmagnetized are reported. The experiment consists of two phases, one in which a net current is drawn through a 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. 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.