Simulation studies on plasma position control for the next generation tokamak machines with up-down asymmetry

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The effect of coupling between vertical and radial plasma motions, originating from the asymmetric features of plasma configuration, provides new and interesting problems. A plasma column moves vertically even when it is disturbed radially by some kinds of perturbations. The tokamak with single-null poloidal divertor configuration and up—down asymmetry, has two types of coupling effects: one is due to the asymmetry of external equilibrium field and the other is due to the asymmetry of eddy currents induced in structures surrounding the plasma, such as first wall and blanket. Stability analysis and simulations on plasma position control have been carried out for the tokamak reactor with up—down asymmetric configuration peculiar to the single-null poloidal divertor. The results indicate that the effect of the asymmetry of the equilibrium field is governing; the up—down asymmetry enhances a growth rate of plasma vertical instability compared to a symmetric case; and the stability criteria for the asymmetric system should be modified. Large vertical displacements toward the divertor plates are observed in the simulations on plasma position control when large disturbances are applied to the plasma, e.g. 40% reduction of poloidal beta, βp, and normalized internal plasma inductance, li. It is quite difficult to suppress the vertical movement with a practical control power level, although the radial movement could be controlled. Plasma with the single-null divertor would therefore interact strongly with the divertor plates, or the first wall near the divertor plates, and would be disrupted at some level of radial perturbations, while for such perturbations a plasma with up—down symmetry (e.g. double-null divertor configuration) could attain equilibrium and be shut down in a controlled manner.