Behaviors of low poloidal(m) and toroidal(n) Fourier modes driven by an interchange and/or ballooning instabilities are investigated by means of direct numerical simulations (DNS) of a full three-dimensional (3D) magnetohydrodynamic (MHD) equations.

Our DNS starts from an ideal equilibrium with Rax = 3.6m and β₀ = 3%. In the initial stage of the time evolution, plasma fluctuations form a ballooning-mode-like structres with moderate poloidal and toroidal mode numbers, being similar to those obtained by simulations under a stellarator symmetry.[1] Then growth of fluctuations are saturated and m/n=2/1 modes dominates the field. In the course of the time evolutions, two anti-parallel vortex pairs associated with m/n=2/1 modes are formed on a poloidal section. The two vortex pairs advect each other, transporting plasma from inner side to outer side of a poloidal section, to contribute to
redistribute plasma and stabilize the plasma motions.
The plasma motions decay after the saturation of the fluctuation, to recover a well-confined state.

In our presentation, detailed mechanism of the re-distribution of the plasma are reported. Furthermore, recent developments of our numerical code in order to achieve very high-accuracy of the simulation, which is required to observe long-term behavior of the plasma after the saturation, are reported, too.

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