Description
Experiments on ASDEX Upgrade investigate how plasma shaping and the poloidal beta (βpol) affect pedestal behaviour in H-mode plasmas. The focus is placed on the transition from type-I ELMy H-mode to the quasi-continuous exhaust (QCE) regime, which is accessed experimentally by increasing plasma shaping while keeping βpol approximately constant. The dataset spans a wide range of shaping parameters and covers the full transition between these confinement regimes, providing a controlled framework to isolate the role of geometry in pedestal formation and stability.
A versatile combination of experimental diagnostics and numerical tools is employed to analyse this dataset. High-resolution profile measurements are used to reconstruct kinetic pedestal profiles, which are then assessed using a comprehensive modelling approach. This includes local and global MHD stability analysis, transport simulations, and gyrokinetic calculations addressing both ion- and electron-scale turbulence. The joint application of these complementary methods allows stability limits, transport channels, and profile evolution to be examined, offering a detailed view of pedestal dynamics across the ELMy–QCE transition.
The analysis demonstrates that pedestal formation is not governed by a single dominant mechanism, but rather by the interplay of multiple instabilities and stabilising effects acting across different radial regions of the pedestal. The interplay between local magnetic shear is found to strongly stabilise kinetic ballooning modes (KBMs), which play a central role in regulating both the density gradient and turbulent ion heat transport. Increased shaping facilitates access to the second stability regime, allowing larger pressure gradients while simultaneously shifting the most unstable region radially.
The behaviour of KBMs at the pedestal foot emerges as a key element in setting the boundary conditions for accessing and sustaining the QCE regime. Complementary MHD and gyrokineticanalysis, further elucidates the role of shaping in modifying magnetic shear and stability boundaries.
Taken together, these results provide a detailed, multi-scale view of pedestal structure and establish a foundation for understanding the transition from ELMy H-mode to QCE. The presented analysis is used as the groundwork for the development of a predictive pedestal model for QCE-relevant regimes, with direct relevance for future devices and reactor-oriented operation.