ELM-free H-mode phase and decoupling of peeling-ballooning stability boundary in MAST Upgrade tokamak
The linear MHD peeling-ballooning stability analysis on the ELM-free phase of MAST Upgrade H-mode plasma is presented. In contrast to other similar discharges, #47018 is found to have significantly higher and wider pedestal during an ELM-free H-mode phase that lasts for approximately 80ms, which is made possible by the reduced core MHD mode activity on q = 2 surface. During this period, there is sustained decoupling of peeling and ballooning branches of the stability boundary on J − α space, opening an access channel to the second stability regime with higher peaks in pedestal current density JN,ped and pressure gradient (α). Such decoupling of the stability boundary is not previously observed in MAST Upgrade H-modes, and if such condition can be readily reproduced, it opens a wide range of opportunity for MAST-U to explore low-collisionality peeling-limited pedestal regimes, as well as advanced scenarios such as quiescent H-modes that are relevant to future reactors, such as STEP and ITER.