Developing a physics understanding of the quasi-continuous exhaust regime: pedestal profile and ballooning stability analysis

Abstract It has been experimentally observed that at ASDEX Upgrade (AUG) plasmas at relatively high shaping, an increase of gas fuelling corresponds to an increase of the frequency and intensity of the type-I edge localised modes (ELMs). At high enough fuelling, the plasma enters the quasi continuous exhaust (QCE) regime. We have performed ideal ballooning n → ∞ stability analysis on four AUG discharges, comparing the type-I ELM dominated phases, with the phases that are in the QCE regime. The results of this study show that as the gas puff increases, the plasma gets more ballooning unstable in the pedestal region, especially very close to the separatrix, at the pedestal bottom. On the contrary, in the middle of the pedestal, the discharges are more ballooning stable. Here the locally negative magnetic shear has a stabilising effect on ballooning modes, allowing access to the second stability region. Our analysis of the ballooning stability and the confinement factor H98 suggest that with optimisation of the pedestal shape, good confinement without type-I ELMs can be achieved. Necessary ingredients are that the region of the highest pressure gradient is not ideal ballooning limited, while the pedestal bottom is ballooning unstable. Ideal stability analysis of 36 simulated ITER profiles shows that, similarly to the experimental cases from AUG, a high pedestal top pressure can be maintained concomitant with a ballooning instability at the pedestal bottom, making QCE a promising scenario.