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T. Ozeki, Y. Ishii, S. R. Hudson, S. Tokuda,
K. Hamamatsu, M. Yamagiwa, G. Kurita,
A. Hirose1, D. Monticello2, T. Tuda,
T. Hirayama
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute,
Naka-machi, Naka-gun, Ibaraki-ken, Japan
1 University of Saskatchewan, Canada
2 Princeton Plasma Physics Laboratory, USA
Abstract. Comprehensive
stability analyses have been carried out to clarify characteristics of
MHD stability of both strongly negative and weak shear configurations for
improvement of plasma performance. Hard beta limits due to ideal MHD stability
can be improved by reducing the pressure gradient in the weak shear region and
by increasing the pressure gradient near the plasma surface in both
configurations. In the negative shear configuration, the double tearing mode
may emerge just under
qmin = 2. However, small current modulation, to
reduce the shear, at the outer resonance surface stabilizes the double tearing
mode. In the weak shear configuration, the neoclassical tearing mode, which is
induced by the helical bootstrap current along the island, reduces the
beta limit below the ideal kink limit. However, a localized additional
current at
the O-point of the island significantly stabilizes the mode. Stabilization of
the KBM requires a large shear, which may be either positive or negative. A
narrow stable window appears near null shear and expands with the pressure
gradient.
IAEA 2001
