Magnetohydrodynamic stability of high-beta tokamak equilibria with force-free currents

The growth rates and eigenfunctions of the leading internal modes are calculated for some high-beta tokamak equilibria with force-free currents for beta values ranging from 5 to 20%. The force-free currents flowing between the main plasma and the conducting wall stabilize the internal hydromagnetic instabilities including the ballooning modes. Modes with low toroidal mode numbers (n<=3) are stabilized at a plasma beta of about 10% when the current density in the force-free region is comparable to that in the main column. (author)