Nonlinear Double Tearing Mode in Negative Shear Cylindrical Tokamaks

Abstract

For negative shear cylindrical tokamaks, nonlinear behavior of double tearing modes (DTMs) with (m,n)=(3,1), (2,1) and (1,1) has been studied for 0.9<q_min<3, where q_min is a minimum safety factor and m (n) is a poloidal (toroidal) mode number. For Δ r<0.1, the linear growth rate of DTM, γ, scales as γ ∝ S^-1/3 approximately, where Δ r is a distance between two resonant surfaces normalized with a plasma radius. For Δ r>0.4, γ ∝ S^-3/5, and transition behavior is seen for 0.1<Δ r <0.4. It is noted that nonlinear destabilization of DTM is seen for this case, which means that DTM produces a completely flat q-profile or a full reconnection within the external resonant surface, although γ ∝ S^-3/5 roughly in the linear phase. Generally the lower m mode generates the full reconnection for the wider distance between the two resonant surfaces.