Initial Achievement of ELM-free Negative Triangularity in the MAST-U Spherical Tokamak*

A negative triangularity (NT) plasma in the spherical torus is achieved and documented for the first time on the MAST-U tokamak. At powers that would typically induced H-mode operation, edge localized modes (ELMs) are eventually suppressed as the triangularity is decreased below δ≲-0.06, allowing for continued operation in an ELM-free state. However, the access criteria for this ELM-free environment appear qualitatively different than previously observed on conventional aspect ratio machines like DIII-D: as an extended period of H-mode operation with Type-III ELMs is sustained early in the discharge even through access to the second stability region for ideal ballooning modes is closed. This observation suggests that stability closure for infinite-n ballooning modes may not be the critical physics responsible for NT-based ELM avoidance in spherical tokamaks. While the electron temperature at the pedestal top drops across the transition to ELM-free operation, a steady rise in core temperature with more negative δ allows for similar normalized β in the ELM-free NT and H-mode positive triangularity shapes. Further exploration of the confinement properties and ELM-free behavior of NT discharges on spherical tokamaks is needed to better understand these phenomena in order to improve confidence in the prediction of NT fusion pilot plant dynamics.