Hydromagnetic Instability in a Stellarator

Stability diagrams are calculated for hydromagnetic kink instabilities in a zero-pressure plasma confined in a stellarator geometry with arbitrary mixtures of l = 2 and l = 3 multipolar helical fields and with various radial distributions of the current parallel to the confining magnetic field. The introduction of small pressure gradients increases the size of the unstable regions. Modes with small azimuthal wavenumber grow more rapidly than those with large m. Experimental data, obtained with the model C Stellarator, show that the plasma is macroscopically stable except for certain intervals of rotational transform. These intervals agree qualitatively with those in which the theory predicts the plasma should be unstable against the kink instability.