Abstract. Plasma sheet stability to the ballooning mode is analyzed using several physical formulations: ideal MHD, stochastic theory, fast-MHD, and the Kruskal-Oberman formulation. It is shown that the major difference among them lies in the plasma compression expression. Explicit computations using the corresponding ballooning equations were performed for two different types of model field lines. For very high/• field lines that are excessively stretched where the stochastic description is most appropriate, the ballooning mode is found to be stable or at best weakly unstable.For field lines that are not too much stretched but even rather round, the ballooning instability can be triggered, within both ideal MHD and the stochastic theory, when fie > /•' Here the threshold value of the equatorial beta,/•, is roughly less than unity and practically set by the ideal MHD limit. Also in contrast to a recent suggestion, the fast-MHD description where the time scale of interest is too short to allow plasma parallel motion is shown to be more stable than ideal MHD. It indicates no instability in all the equilibria that were tested. The Kruskal-Oberman description is even more stable than fast-MHD.

The results of two-dimensional calculations of eddy currents induced on external conducting walls surrounding a tokamak are reported. The computed eddy currents are generated by low-n (n=1,2,3) external ideal-magnetohydrodynamic (MHD) instabilities. For a given toroidal mode number n the eddy current patterns are found to be very similar in a variety of plasma configurations, e.g., different edge safety factors and different plasma–wall separation distances, in high beta plasmas. This result is promising for the design of active feedback coils for the stabilization of the resistive wall mode. Also, the effects of having a partial wall that has a poloidal gap on the outboard side are considered. Using the expected gap size in the proposed Korea Superconducting Tokamak Advanced Research (KSTAR) [“The KSTAR tokmak,” in Proceedings of the 17th Symposium on Fusion Engineering, San Diego, 1997 (Institute of Electrical and Electronics Engineers, New York, in press), Paper No. O3.1], the calculation shows that active coils mounted behind the partial walls (the KSTAR passive plates) cover an adequate portion of the eddy current dominant region, enabling feedback stabilization.

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Ideal magnetohydrodynamic stability limits for various profiles of pressure and current density in reversed magnetic shear toroidal plasmas are numerically investigated. The plasma beta limits are calculated for n = 1 and ballooning modes, and the bootstrap current contribution is also computed. The results in the high-value regime of the plasma internal inductance l i contrast well with those in the low-l i regime and are very dependent on the choice of the pressure profile. In the high-l i regime the stability limit set by the n = 1 external mode without any external stabilizing wall can be significantly improved compared to that in the low-l i regime. This is more significant for a broader pressure profile. However, the bootstrap current profile in the high-l i regime is generally not well aligned with the total current profile. This is worse for broader pressure profiles. Also, if the pressure profile is not suitably chosen, the ballooning mode rather than the n = 1 mode can become the beta limiting mode.

Resistive wall kink mode is studied in cylindrical plasma that is surrounded by two resistive walls. The outer wall is regarded as active coils for feedback stabilization, which can be made to fake-rotate. The impact of plasma rotation on such a scheme is investigated. It is found that plasma rotation at some sufficient rate can destabilize the resistive wall mode, which would otherwise remain stable by the fake-rotating coil feedback scheme.