The stability of long wavelength perturbations is analysed within the framework of the Hall model. Free boundary modes with m=1 and kz to 0 are shown to be unstable for all pressure profiles which go to zero at the plasma surface, if feedback from the wall can be neglected. The growth rate of the instability increases with decreasing plasma radius. Similar results are found for Extrap. Nonlinearities in combination with losses at the X-points are possibly responsible for the gross stability of free boundary modes in some Extrap discharges. In Extrap experiments (during 1987), where an axial field (BT=0.5 Bp) is added, the improved stability might instead be due to passive feedback.
Equilibrium properties at very low aspect ratio in the Pegasus toroidal experiment