Dependence of resonant magnetic perturbation experiments on the DIII-D plasma shape

The shape of the cross-section of fusion plasma experiments in many devices is a key determinant of the resulting plasma characteristics. Experiments to attempt suppression of edge localized modes (ELMs) by the technique of resonant magnetic perturbation (RMP) were performed in the DIII-D tokamak in varying plasma shapes. It was found that while ELM suppression in a plasma shape similar to the ITER shape with a single magnetic null was possible, ELM suppression could not be attained in a balanced or upward-biased double-null (DN) shape, although the ELMs in the DN shape were smaller and of higher frequency than the ones before RMP was applied. Calculation of the peeling–ballooning stability of the plasma showed that the ELMing plasma is stable to type-I ELMs, suggesting the ELMs are of another type.