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L. L. Lao, J. R. Ferron, R. L. Miller,
T. H. Osborne, V. S. Chan, R. J. Groebner,
G. L. Jackson, R. J. La Haye, E. J. Strait,
T. S. Taylor, A. D. Turnbull
DIII-D National Fusion Facility, General Atomics, San Diego,
California, U.S.A.
University of California, Los Angeles, California, U.S.A.
Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
University of Wisconsin, Madison, Wisconsin, U.S.A.
Lawrence Livermore National Laboratory, Livermore, California,
U.S.A.
C. Zhang, L. Chen
Institute of Plasma Physics, Chinese Academy of Science, Hefei,
P.R. China
Abstract. The results of recent experimental and theoretical studies
concerning the effects of plasma shape and current and pressure profiles on
edge instabilities in DIII-D are presented. Magnetic oscillations with
toroidal mode number
n 
2 - 9 and a fast growth time
= 20 - 150
s are often observed prior to the first giant type I ELM in
discharges with moderate squareness. High n ideal ballooning second
stability access encourages edge instabilities by facilitating the buildup of
the edge pressure gradient and bootstrap current density which destabilize the
intermediate to low n modes. Analysis suggests that discharges with large
edge pressure gradient and bootstrap current density are more unstable to n > 1 modes. Calculations and experimental results show that ELM amplitude and
frequency can be varied by controlling access to the second ballooning
stability regime at the edge through variation of the squareness of the
discharge shape. A new method is proposed to control edge instabilities by
reducing access to the second ballooning stability regime at the edge using
high order local perturbation of the plasma shape in the outboard bad
curvature region.
IAEA 2001
