The off-axis pressure crash associated with the nonlinear evolution of the m/n=2/1 double tearing mode

The nonlinear evolution of the m/n=2/1 double tearing mode (DTM) is investigated by the toroidal resistive magnetohydrodynamic code CLT. It is found that the m/n=2/1 DTM can lead to either a core pressure crash or an off-axis pressure crash. Unlike the core pressure crash, the plasma pressure at the magnetic axis remains almost unchanged during the off-axis pressure crash. The pressure crash only occurs in the annular region during the off-axis crash, and the on-axis plasma pressure slowly reduces after the crash, which is consistent with TFTR observations. A series of simulations are carried out to investigate the influence of the radial position of the inner resonant surface r 1, the magnetic shear at the inner resonance surface, and the spatial separation between the two resonant surfaces on nonlinear behaviors of DTMs. We find that r 1 plays a dominant role in the nonlinear DTM behaviors. It is more likely for the DTM to lead to the core pressure crash with a smaller r 1. It is also found that the magnetic shear at the inner resonant surface and the spatial separation between the two resonant surfaces can also largely influence the nonlinear evolution of the DTM. A simple theoretical formula of the transition criterion between the two pressure crashes is proposed, which agrees well with the simulation results.