Understanding the core confinement of spherical tokamaks is critical for achieving high-performance plasmas. MAST-Upgrade provides a valuable opportunity to benchmark first-principles-based model predictions against experimental results, given the high-quality diagnostic data available.

This study examines the nature of core turbulence that develops across different operational regimes in MAST-U. Transport analysis indicates that in both L-mode and H-mode regimes, there is significant anomalous electron heat transport, whereas ion heat transport remains at neoclassical levels. Furthermore, core performance in MAST-U often deteriorates due to the onset of an MHD-driven locked mode, which significantly reduces plasma rotation. The impact of this reduction on turbulence is explored here.

An L-mode and an H-mode discharge, corresponding to low and high achieved plasma β respectively, are analysed using gyrokinetic modelling. In the L-mode discharge, a range of instabilities are identified at both electron and ion scales, highlighting the multiscale nature of turbulence in MAST-U. At higher β, electron-scale turbulence is stabilized, with electromagnetic instabilities forming at the ion scale. In both scenarios, fast ions are found to have a much more significant impact deeper with the core plasma. Nonlinear analysis further reveals the stiff nature of turbulence in both regimes and the strong influence of rotation shear.