DISTINCTIVE FEATURES OF THE STABILITY OF BALLOONING MODES IN CONVENTIONAL STELLARATORS

A three-dimensional numerical analysis of the stability of ballooning modes in conventional stellarators (see, e.g., [5]) shows that, in thr ee-dimensional systems, both the stability conditions for ballooning m odes and the structure of the most dangerous perturbations differ mark edly from those obtained in the stellarator approximation with the use of a two-dimensional description of equilibrium configurations. The i nstability of perturbations that are highly localized along a magnetic field line (see [5] and some others) cannot be treated with the help of the familiar analytical criteria based on a two-dimensional descrip tion of equilibrium configurations. Three-dimensional numerical simula tions reveal that the most dangerous perturbations are perturbations t hat are localized at the outer circumference of the torus in a transve rse cross section in which the flux surfaces are vertically elongated and the field-line curvature is most unfavorable for the onset of inst ability. The physical meaning of the results obtained was not discusse d in the above papers. In the present paper, the stability of ballooni ng modes in conventional stellarators is studied by means of a simple model in which the three-dimensional geometry of the magnetic configur ation is taken into account but equilibrium helical currents are negle cted. It is shown that this model can be applied to describe the insta bility of highly localized ballooning modes. The results obtained are interpreted.