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By studying the alpha, psi, theta(k) dependence of ballooning mode growth rates in a Mercier-unstable equilibrium case modelling the large helical device (LHD) with a broad pressure profile, it is found that two distinct topological types coexist - a topologically cylindrical branch and a topologically spherical branch. It is shown that the cylindrical branch can be described by a ripple-expansion of the ballooning equation, carried beyond lowest order in the number of field periods. However the spherical branch cannot be found from such an expansion at any finite order. According to Wentzel-Kramers-Brillouin (WKB) theory, the cylindrical and spherical branches give rise to quasi-discrete modes and continuum global modes, respectively. The cylindrical branch disappears for the Mercier-stable peaked-pressure-profile LHD cases and can thus be regarded as a finite-growth-rate interchange mode.

Additional information

Authors: DEWAR R L ET AL, Australian National University, Canberra (AU);NAKAJIMA N, National Institute for Fusion Science, Toki (JP);PERSSON M, Chalmers University of Technology, Göteborg (SE);COOPER W A, Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne (CH)
Bibliographic Reference: Article: Journal of Plasma Fusion Research SERIES, Vol. 1 (1998) pp. 108-110
Record Number: 199811015 / Last updated on: 1998-09-15
Original language: en
Available languages: en