Effect of plasma shaping on MAST and MAST-U pedestal stability

Modern tokamaks can operate in high-confinement (H-)modes, in which a steep edge plasma pressure gradient is established, creating a "pedestal". However, H-modes are subject to a class of explosive edge localized modes (ELMs), which could cause serious damages to the vessel walls of large tokamaks, like ITER. Large ELMs must therefore be mitigated or suppressed.

According to the "peeling-ballooning" theory, the pedestal stability is bounded by high-n ballooning modes at high pedestal pressure gradient (α), and by peeling modes at high pedestal plasma current density (J_ped) (n = toroidal mode number). This boundary depends on the plasma cross-sectional shape, defined by elongation, triangularity and squareness. Experiments and theoretical studies show that strong shaping can push the stability boundary to higher J_ped and α, improving performance and potentially influencing ELM dynamics.

In the first campaign of the new MAST-Upgrade tokamak, some H-mode shots were peeling-limited; a stark contrast to the old MAST, which was always ballooning-limited. This could be a result of different shaping parameters between them, particularly the squareness. This paper reports on the comparison between several MAST and MAST-U shots of pedestal analysis using ELITE code. If shaping plays a significant role, then it could open a new pathway for improving H-mode performance, with applications to future machines like ITER.