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

Abstract

Abstract The toroidal magnetohydrodynamic code MARS-F (Liu et al 2000 Phys. Plasmas 7 3681) is adopted to investigate the plasma boundary shape effect on the β N threshold ( β N is the normalized plasma pressure) reported in (Liu et al 2017 Plasma Phys. Control. Fusion 59 065009) in reversed magnetic shear toroidal tokamak plasmas under different separations Δ r s between the two rational surfaces of the same helicity. The boundary shape effect is modeled via elongation κ and triangularity δ . Here, the study focuses on n = 1 cases ( n is the toroidal mode number). In the small Δ r s regime, the critical value β N c increases as κ increases (without triangularity), and barely changes with triangularity (at small elongation). On the other hand, at large elongation (e.g. κ = 2 ), the critical value of β N c decreases with increasing δ . The change in the β N c may be due to a change in the coupling strength between different poloidal harmonics, due to the fact that, besides m = ± 1 toroidal coupling ( m is the poloidal mode number), a large elongation κ (or triangularity δ ) can result in the m = 2 ( m = 3 ) equilibrium component and thus enhances the coupling between the m and m = ± 2 ( m = ± 3 ). In the large Δ r s regime, with increasing κ (without triangularity), β N c first slightly increases followed by a quick reduction. Moreover, as δ increases, the critical value β N c decreases at small elongation, and first increases followed by a decrease at large elongation (e.g. κ = 2 ). Besides the change in the coupling strength between different poloidal harmonics, the change in the β N c in the large Δ r s regime is also related to the mode transition. The corresponding physics mechanisms underlying the shift of β N c due to the plasma boundary shape are all discussed in detail.