Observation of new pedestal regime for MAST Upgrade H-mode plasmas
The first pedestal stability analysis on the new MAST Upgrade (MASTU) spherical tokamak H-mode plasma is presented. Our results indicate that MAST-U pedestals are close to the low toroidal mode number (n) peeling branch of the peelingballooning instability, in contrast with MAST H-mode pedestals which were deeply in the high-n ballooning branch. This offers the possibility of reaching the ELM-free quiescent H-mode [1] or high-performance super H-mode [2] regimes. In addition, the coupling between the peeling and ballooning branches is weak in MAST-U, suggesting that a path to very high pedestal pressure gradient at high density may exist with sufficient heating power. A possible explanation for the differences between MAST and MAST-U pedestal stability is given in terms of plasma shaping parameters, in particular squareness and elongation.