Modification of sawtooth crash behaviour during large error field experiments on the DIII-D tokamak

Externally applied, static, resonant low m=1, n=1 magnetic field perturbations have a strong effect on the behaviour of the sawtooth crash in the DIII-D tokamak. An external coil was used to produce magnetic field perturbations large enough to slow the core plasma rotation and greatly alter the radial electric field profiles but not large enough to induce a locked mode. In these cases the global plasma parameters and the density and temperature profiles remained unchanged; however, the sawtooth oscillation, in particular the sawtooth crash, changed character from resistive in nature to ideal as the static error field increased. The m=1, n=1 resistive MHD internal kink mode normally responsible for the sawtooth crash in DIII-D appears to be stabilized by resonance detuning caused by the combination of the applied static error field and the slower plasma toroidal rotation. The stabilization of the resistive internal kink allowed the plasma to evolve until an ideal mode caused a sawtooth crash. This effect may explain the differences in sawtooth behaviour reported between different tokamaks