Coherent Nonlinear Destabilization of Tearing Modes: The Effect of the Mass Flow

Three‐dimensional tearing modes are shown to be linearly unstable in a helical equilibrium containing large‐amplitude q=2 magnetic islands and their associated plasma flow. The source of the instability is a steep current gradient which is formed at the inner separatrix of the q=2 islands as they evolve. The coherent flow associated with the q=2 islands has a strong influence on the growth of the three‐dimensional modes. In particular, this flow can dominate the resistivity in the linear stability analysis of these modes.