Effects of plasma boundary shape on the βN threshold in the suppression of tearing mode in toroidal tokamak plasmas with reversed magnetic shear

The toroidal magnetohydrodynamic code MARS-F (Liu et al 2000 Phys. Plasmas7 3681) is adopted to investigate the plasma boundary shape effect on the threshold ( is the normalized plasma pressure) reported in (Liu et al 2017 Plasma Phys. Control. Fusion59 065009) in reversed magnetic shear toroidal tokamak plasmas under different separations between the two rational surfaces of the same helicity. The boundary shape effect is modeled via elongation and triangularity . Here, the study focuses on cases ( is the toroidal mode number). In the small regime, the critical value increases as increases (without triangularity), and barely changes with triangularity (at small elongation). On the other hand, at large elongation (e.g. ), the critical value of decreases with increasing . The change in the may be due to a change in the coupling strength between different poloidal harmonics, due to the fact that, besides toroidal coupling ( is the poloidal mode number), a large elongation (or triangularity ) can result in the () equilibrium component and thus enhances the coupling between the and (). In the large regime, with increasing (without triangularity), first slightly increases followed by a quick reduction. Moreover, as increases, the critical value decreases at small elongation, and first increases followed by a decrease at large elongation (e.g. ). Besides the change in the coupling strength between different poloidal harmonics, the change in the in the large regime is also related to the mode transition. The corresponding physics mechanisms underlying the shift of due to the plasma boundary shape are all discussed in detail.