Testing for chaos in type-I ELM dynamics on JET with the ILW

In H-mode plasmas, Edge Localized Modes are one of the main causes of the first wall erosion, especially in the divertor tiles. On the other hand, ELMs contribute to maintaining a stable plasma density, reducing the risks of disruptions. Moreover ELMs play a complex role in the amount and spatial distribution of impurities. Understanding and controlling ELMs are crucial issues for the operations of ITER where the type-I ELMy H-mode has been chosen as the standard operation scenario. The new full-metal ITER-like wall (ILW) at JET was found to have a deep impact on the physics of ELMs in JET. The change in the wall composition was found to be at the root of the different baseline H-mode behaviour in ILW, in which a significant change of the ELM dynamics is observed for spontaneous type-I ELMs. The ELM duration for example is much longer for the ILW than in similar cases with a C wall (CW). [1] In this paper, the differences between the dynamical properties of ELMs in both the CW and ILW shots have been investigated by means of surrogate data analysis. In order to make this comparison, several nonlinear measures, such as the mutual information, the correlation dimension and the maximum Lyapunov exponent have been evaluated for the original series and for the surrogate series. These measures have been severely limited by some typical experimental conditions, e.g. measurement noise, making a signal pre-processing stage necessary to draw conclusions. The results obtained by the analysis of the Be-II and D? radiations confirm the suitability of D?also for the characterisation of ELMs in the ILW campaigns, and generally confirm the results obtained in the CW campaigns on the pseudoperiodic dynamics of ELMs.