Ballooning MHD modes in toroidal systems with complicated magnetic-field geometry

The shear-induced instability of ideal ballooning MHD modes is studied analytically for closed magnetic traps with an arbitrary shape of the magnetic axis. A stability criterion determining the maximum pressure of a hydrodynamically stable plasma for a wide range of equilibrium toroidal configurations is derived by using the method of small oscillations. The criterion derived is used in the analysis of plasma stability in toroidal traps with the magnetic axes being simple closed spatial curves. It is shown that modulation of the magnetic-axis curvature affects the plasma stability with respect to the excitation of ideal ballooning modes. In a currentless trap with positive shear, ballooning modes are less dangerous than Mercier-type perturbations. (author)