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Indexed by:期刊论文

Date of Publication:2017-06-01

Journal:PLASMA PHYSICS AND CONTROLLED FUSION

Included Journals:SCIE、EI

Volume:59

Issue:6

ISSN No.:0741-3335

Key Words:high-beta(N); reversed magnetic shear; MHD instability

Abstract:The toroidal magnetohydrodynamic (MHD) code MARS-F (Liu et al 2000 Phys. Plasmas 7 3681) is applied to numerically investigate multiple MHD instabilities in high-beta(N) (beta(N) is the beta normalized) toroidal plasmas with reversed magnetic shear, and with different radial separations Delta r(s) between the two q = 2 rational surfaces. A resistive wall is also taken into account. In the small Delta r(s) regime, it is found that a finite beta(N) leads to multiple branches of the double tearing mode (DTM). The beta normalized has a stabilizing effect on the most unstable branch. There exists a critical value beta(Nc), above which the real frequency of the most unstable mode becomes finite due to the favorable average curvature effect (Glasser et al 1975 Phys. Fluids 18 875). Moreover, the critical value beta(Nc) decreases with increasing plasma resistivity h. In the large Delta r(s) regime, on the other hand, finite beta normalized can help to transform the two DTM branches into an external kink mode (EKM). Increasing beta(N) can also couple two single tearing modes, forming a DTM. In the intermediate Delta r(s) regime, interestingly, a new branch with EKM structure appears, which successively couples with the other two branches as Delta r(s) increases, recovering the EKM found in the large Delta r(s) limit. Characteristics of the eigenmode structures in different Delta r(s) regimes are compared and analyzed in detail. Furthermore, the properties of the high-beta(N) MHD instabilities, with higher toroidal mode number n, are also investigated. It is found that, in the small Delta r(s) limit, the growth rate always first increases and then decreases with n, forming a broad n spectrum. The critical value beta(Nc) decreases with n. In the large Delta r(s) limit, however, the growth rate of the n = 2 mode is strongly reduced with increasing beta(N).