Sawtooth-like oscillations and steady states caused by the m/n = 2/1 double tearing mode

The sawtooth-like oscillations resulting from the $m/n=2/1$ double tearing mode (DTM) are numerically investigated through the three-dimensional, toroidal, nonlinear resistive-MHD code (CLT). We find that the nonlinear evolution of the $m/n=2/1$ DTM can lead to sawtooth-like oscillations, which are similar to those driven by the kink mode. The perpendicular thermal conductivity and the external heating rate can significantly alter the behaviors of the DTM driven sawtooth-like oscillations. With a high perpendicular thermal conductivity, the system quickly evolves into a steady state with $m/n=2/1$ magnetic islands and helical flow. However, with a low perpendicular thermal conductivity, the system tends to exhibit sawtooth-like oscillations. With a sufficiently high or low heating rate, the system exhibits sawtooth-like oscillations, while with an intermediate heating rate, the system quickly evolves into a steady state. At the steady state, there exist the non-axisymmetric magnetic field and strong radial flow, and both are with helicity of $m/n=2/1.$ Like the steady state with $m/n=1/1$ radial flow, which is beneficial for preventing the helium ash accumulation in the core, the steady state with $m/n=2/1$ radial flow might also be a good candidate for the advanced steady state operations in future fusion reactors. We also find that the behaviors of the sawtooth-like oscillations are almost independent of tokamak geometry, which implies that the steady state with saturated $m/n=2/1$ islands might exist in different tokamaks.