Mercier criterion for stellarator with planar circular axis

It is universally recognized that in the Stellarator with planar circular axis Mercier criterion determines plasma stability relative to local MHD perturbations. In view of importance of this criterion for plasma confinement problem it has been investigated analytically for the Stellarator with circular 'in average' magnetic surfaces. Mercier criterion has been analyzed in supposition that the toroidal effects are small in comparison with stellarator ones. The case, when toroidal and stellarator effects are the values of the same order, has been studied. However, the results differ substantially in the limit case of zero longitudinal current. A comparison of the expressions obtained leads to conclusion that the difference of the results is due to incorrect description of the magnetic well. Present report is devoted to the analysis of magnetic well influence on Mercier criterion. (author) 5 refs