ABSTRACT
This chapter analyses the structure of the Mercier stability criterion for the general case of systems of circular cross-section with a longitudinally inhomogeneous magnetic field in the approximation of a finite plasma pressure. The small-scale stability studies in stellarators with helical windings go back to. The chapter discusses stability in systems homogeneous along the magnetic axis with a low- and a finite-pressure plasma. It also discusses the stability of systems of circular cross-section with a longitudinally inhomogeneous magnetic field. The chapter considers the influence of straight sections on the plasma stability in Drakon. As a result, the stability of systems is defined by the vacuum magnetic well and shear. Applying the stability criterion of to systems of Drakon type, one can see that straight sections do not influence the plasma stability. In the case of a high-pressure plasma the stability criterion should be augmented by terms of higher orders in the plasma pressure.
