Modelling of edge localized modes with a current relaxation model on JET and TEXTOR

The mitigation of edge localized modes (ELMs) has been investigated using external resonant magnetic perturbations (RMPs) on JET. An interesting observation encountered is the formation of resonant structures in the dependence of ELM frequency on the edge safety factor. Using a theory based on an extended Taylor relaxation process, initialized by an external peeling mode, this behaviour is modelled and analysed leading to a prediction of a condition that a low edge current density is required for the resonant nature to be observed. This model is outlined and the results compared with recent experimental observations from JET. The TEXTOR tokamak already has a low edge current density due to its limiter configuration and so is predicted to show the resonant behaviour even without the RMP fields being applied. Similar experiments investigating the effect of the edge safety factor on the ELM frequency are conducted on TEXTOR to validate this prediction. It was found from the model that a case with a low normalized edge current density of 0.01 shows good agreement with observations. For such a case the multi-resonance effect is less well defined than observed on JET due to the appearance of highly detailed structure in the model predictions. (paper)