Wall Stabilization of High Beta Tokamak Discharges in DIII-D

DIII-D discharges with values of beta (the ratio of plasma pressure to magnetic pressure) up to 12.5% demonstrate that a resistive wall can stabilize low- $n$ magnetohydrodynamic (MHD) modes. In discharges with broad current profiles, beta exceeds the ideal MHD stability limit by at least a factor of 1.3 assuming no wall, but remains below the limit calculated under the assumptions that the vacuum vessel is a perfectly conducting wall. Plasma rotation is essential to stabilization, and instabilities resembling the predicted ``resistive wall mode'' appear only when the rotation velocity approaches zero.