Local Effect of Equilibrium Current on Tearing Mode Stability.
The local effect of the equilibrium current on the linear stability of low poloidal number tearing modes in tokamaks is investigated analytically. The plasma response inside the tearing layer is derived from fluid theory and the local equilibrium current is shown to couple to the mode dynamics through its gradient, which is proportional to the local electron temperature gradient under the approximations used in the analysis. The relevant eigenmode equations, expressing Ampere's law and the plasma quasineutrality condition, respectively, are suitably combined in a single integral equation, from which a variational principle is formulated to derive the mode dispersion relations for several cases of interest. The local equilibrium current is treated as a small perturbation of the known results for the m (GREATERTHEQ) 2 and the m = 1 tearing modes in the collisional regime, and the m (GREATERTHEQ) 2 tearing mode in the semi-collisional regime; its effect is found to enhance stabilization for the m (GREATERTHEQ) 2 drift-tearing mode in the collisional regime, whereas the m = 1 growth rate is very slightly increased and the stabilizing effect of the parallel thermal conduction on the m (GREATERTHEQ) 2 mode in the semi-collisional regime is slightly reduced. The possibility of new modes, which do not exist in the absence of local current, is also discussed, but no relevant result is found in this case.
- Pub Date:
- Physics: Fluid and Plasma