| Research Abstract |
1. Equilibrium ptoperties of several typical configurations of helical plasmas, namely, heliotron/torsatron, Helias, and Heliac, were studied by using the 3D equilibrium code HINT,which we have developed. We found (1) in some configurations, a self-healing' of pressure driven islands can occur, (2) it occurs even in an unstable plasma configuration, (3) therefore global effects of plasma currents are proved to be important as a physical mechanism of island formation, and (4) in some cases, excitation of nonresonant type instability modifies structure of helical equilibria significantly.
2. A new nonlinrear simulation code for nonaxisymmetric torus plasmas were successfully developed with a motivation to clarify whether or not self-organization of a plasma which is induced by pressure-driven instabilities can occur. The main characteristics of the developed code are that time development of the full MHD equations are solved in a full three dimensional geometry. Such a nonlinear simulation has seldom been attempted so far. We have executed simulations and found that while behavior of instabilities in an initial phase of the development agrees well with predictions of linear analyzes, strong mode coupling is eventually excited as the nonlinear development of instabilities, which results in a transition to a strongly turbulent state. It's interesting to note that in a later stage, the plasma pressure profile is broadened and the plasma spontaneously becomes quiet.
3. Efficiency of the developed codes on a parallel computer was investigated. Algorithm that keeps the form of the physics principle of action through medium was found to be effective. Several practical methods which enable large scale simulation codes to use CPUs efficiently were found.
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