Plasma shaping and its impact on the pedestal of ASDEX Upgrade: edge stability and inter-ELM dynamics at varied triangularity

The plasma shape, in particular the triangularity (delta), impacts on the pedestal stability. A scan of d including a variation of heating power (P-heat) and gas puff was performed to study the behaviour of edge localised modes (ELMs) and the pre-ELM pedestal stability for different plasma shapes. Generally, at higher delta the pedestal top electron density (n(e)) is enhanced and the ELM repetition frequency (f(ELM)) is reduced. For all delta, the pedestal top n(e) is already fully established to its pre-ELM value during the initial recovery phase of the n(e) pedestal, which takes place immediately after the ELM crash. The lowering of the f(ELM) with increasing delta is related to longer pedestal recovery phases, especially the last pre-ELM phase with clamped pedestal gradients (after the recovery phases of the n(e) and electron temperature (T-e) pedestal) is extended. In all investigated discharge intervals, the pre-ELM pedestal profiles are in agreement with peeling-ballooning (PB) theory. Over the investigated range of delta, two well-separated f(ELM) bands are observed in several discharge intervals. Their occurrence is linked to the inter-ELM pedestal stability. In both kinds of ELM cycles the pedestal evolves similarly, however, the 'fast' ELM cycle occurs before the global plasma stored energy (W-MHD) increases, which then provides a stabilising effect on the pedestal, extending the inter-ELM period in the case of the 'slow' ELM cycle. At the end of a 'fast' ELM cycle the n(e) profile is radially shifted inwards relative to the n(e) profile at the end of a 'slow' ELM cycle, leading to a reduced pressure gradient. The appearance of two f(ELM) bands suggests that the pedestal becomes more likely PB unstable in certain phases of the inter-ELM evolution. Such a behaviour is possible because the evolution of the global plasma is not rigidly coupled to the evolution of the pedestal structure on the timescales of an ELM cycle.