Influence of ballooning effects on plasma stability in closed systems

Authors:
  • VSV. D. Shafranov
    V. D. Shafranov
  • EYE.I. Yurchenko
    E.I. Yurchenko
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Abstract
A general criterion of hydromagnetic plasma stability in the infinite electrical conductivity approximation is applied to smooth closed systems. As in the pioneering work of Mercier, the authors use the method of expansion with respect to the deviation from the magnetic axis. The effects of ballooning instability and of magnetic well deepening due to ballooninginduced stretching of the plasma column are clearly distinguished in this criterion. The above effects cancel out with an accuracy to within the square of the ellipticity of the magnetic surface cross-sections. Even in magnetic systems with a comparatively shallow magnetic well, ballooning instability does not therefore lead to a serious limitation on plasma pressure. The authors present estimates of the critical plasma pressure for a two-turn stellarator and for systems with helical and axial symmetry.