Seeding of neoclassical tearing modes by internal crash events in the ASDEX Upgrade and DIII-D tokamaks

Seeding of neoclassical tearing modes (NTMs) by perturbations from other MHD instabilities (sawteeth, fishbones, ELMs, etc) is one of the main MHD problems which has to be avoided or controlled in ITER. NTMs can lead to strong degradation of plasma confinement or even to a disruption. This paper compares the seeding of (3,2) NTMs in DIII-D and ASDEX Upgrade tokamaks. It was found in both devices that a mode can start as an ideal kink mode that converts into a tearing mode on a time-scale much longer () than the duration of the trigger event (). These findings are in good agreement with those for (2,1) mode seeding in ASDEX Upgrade as well as with non-linear MHD simulations. This result revises the simplified picture of fast island formation occurring only during the trigger event. (paper)