Stabilization of interchange modes in multiple-mirror confined plasmas

Interchange modes were investigated experimentally in a multiple-mirror field in which the field line curvature was adjustable both by varying the mirror ratio and by adding linked-quadrupole fields. Experiments were performed both with a thermal lithium plasma and a hot hydrogen plasma. The m=1 flute was observed growing at a rate which decreased strongly with decreasing magnetic field curvature. Higher order modes were observed for lower values of field curvature. These modes, which were calculated to be stabilized by finite Larmor orbit effects in the absence of line-tying, were found to be driven unstable by resistive effects from the line-tying, in qualitative agreement with observations. The quadrupole fields were observed to have a stabilizing effect on the higher order modes.