Abstract
The local three‐dimensional (3‐D) ideal magnetohydrodynamic (MHD) stability properties of a 12‐field period torsatron device of moderate aspect ratio are investigated as a function of an externally applied quadrupole magnetic field. It is demonstrated that with a parabolic pressure profile the volume‐averaged β limit due to the ballooning instabilities decreases from β≂2% to β≂1.5% as the current Imid in the quadrupole field coils normalized to that in the main helical coils increases from 0 to 0.19.