Born-locked neoclassical tearing mode triggered by error field penetration

We investigate magnetic islands in the coexisting system of the error field and the perturbed bootstrap current. In numerical simulations, it is observed that the born-locked neoclassical tearing mode, which is locked at its onset, is triggered by the penetration of the error field. The necessary vacuum island width to directly trigger the locked neoclassical tearing mode is found to be smaller than the seed island width to trigger the original neoclassical tearing mode. This implies that the error field penetration is an efficient trigger mechanism of the neoclassical tearing mode. The error field penetration threshold is weakly affected by the perturbed bootstrap current. This implies that, the conventional theory about the penetration threshold in the low-beta regime may be applicable to the high-beta regime, where the neoclassical tearing mode threshold is approached. However, once the error field penetration occurs, the perturbed bootstrap current enhances the saturated magnetic island width which overwhelms the vacuum island width. This enhancement is directly linked to the excitation of the born-locked neoclassical tearing mode.