Positional stability of current-carrying plasma in a helical winding

The average force acting on a current-carrying plasma column is calculated when a straight multipolar helical winding is displaced laterally with respect to the plasma, whose average position is kept fixed by a uniform external transverse magnetic field B/sub x/. This nonsymmetric equilibrium problem is solved for force-free j/B = const plasmas by Fourier-analyzing plasma and vacuum fields into helical harmonics and numerically solving the free boundary interface for their amplitudes. In the stellarator limit the average helical force is as predicted from stellarator expansion theory. In the new OHTE limit, both l = 2 and l = 3 reversed pitch pinches are repelled from the helix. A simple explanation is offered in terms of plasma diamagnetism to the helical field