(TH/6-3) Nonlinear MHD Simulations of Spherical Tokamak and Helical Plasmas

**Abstract.** Nonlinear MHD simulations on relaxation phenomena in spherical
tokamak and helical plasmas, including 3D HINT equilibrium computations, are
reported. 1. The dynamics of global relaxation phenomena in spherical
tokamak have been revealed. We analyze the sawtooth and IRE. For the
sawtooth case, a new nonlinear triger mechanism is found, in which the n=1
activity in the core region is subsequently excited by development of higher
n ballooning modes in the periphery region. The overall shape of the torus
surface does not change throughout the event. On the other hand, our
previous simulations for IRE show a large distortion of the torus. The
differences arise from only slight discrepancies in the initial
profiles. 2. The 3D equilibrium code HINT, which does not assume the
existence of magnetic surfaces, is modified in a couple of directions to
extend the functions. Based on a HINT equilibrium, nonlinear evolution of
instabilities is studied in a full 3D geometry of LHD. An initial phase of
plasma deformations is dominated by medium-n resistive ballooning
instability. In the later stage, a well-confined state is recovered by
having spontaneous evolution to a broader pressure profile.

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*IAEA 2003*