Ballooning instabilities in a Heliotron J plasma

Ideal magnetohydrodynamic stability analysis of local pressure-driven modes in an L=1 heliotron, Heliotron J [M. Wakatani , Nucl. Fusion 40, 569 (2000 )], is investigated by means of three-dimensional (3D) ballooning formalism and the Mercier criterion. In 3D systems such as heliotrons, the balloonin g modes are separated into two categories: One is tokamak-like ballooning m odes which are localized only in the poloidal direction, and the other is m odes inherent to 3D systems which are localized on the specific flux tubes. The tokamak-like ballooning modes change to the Mercier modes in the limit that the mode is sufficiently extended along the field line, but the nonax isymmetric ballooning mode does not so. The L=1 Heliotron J equilibrium inv estigated here has weak global shear and the dominant Fourier amplitudes of magnetic-field strength is rather different from the conventional helical systems with L=2 helical coils. Since the weak global shear causes the redu ction of integrated local shear along the field lines easily, which combine s with strongly modulated destabilizing effects on the flux surface, the no naxisymmetric ballooning modes localized on the specific flux tubes can bec ome unstable. On the other hand, the Mercier modes are suppressed due to th e deep magnetic well. The results obtained from the model equilibrium of L= 2 Large Helical Device (LHD), for which several reports have already publis hed [N. Nakajima, Phys. Plasmas 3, 4556 (1996), for example], are also show n and compared with the results of Heliotron J. The LHD equilibrium employe d here has a magnetic hill region at the outer radius, and this tends to ma ke the Mercier modes unstable. It is found that this difference of the Merc ier stability property in two equilibria is concerned in the local ballooni ng stability, and the notable difference of local dispersion relations appe ars. It is also found from the comparison of two systems that the nonaxisym metric ballooning modes have a similar property to the tokamak-like balloon ing modes, in the sense of the (s) over bar- <(<alpha>)over bar> diagram wh ere (s) over bar and <(<alpha>)over bar> are shear and pressure gradient pa rameter. (C) 2001 American Institute of Physics.