MERCIER STABILITY-CRITERION FOR CURRENTLESS TOROIDAL SYSTEMS

It is shown that the general-geometry Mercier criterion in currentless toroidal systems differs substantially from the corresponding criterion for tokamaks with respect to the form of the magnetic well. The stability of a large aspect ratio stellarator with a planar circular axis, low pressure and circular (on average) magnetic surfaces is considered as an example. It has been found that Mercier modes in such systems do not impose constraints on the value of the rotational transform, and, in fact, systematic consideration of the toroidal terms in the stability criterion leads to the conclusion that plasma stability is improved. The form of the Mercier stability criterion discussed here generalizes and clarifies previous published results.