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This report presents experimental and modeling studies on the interaction between energetic particles, Alfvén eigenmodes (AEs), and thermal plasma turbulence in neutral-beam-heated MAST-U spherical tokamak plasmas. These studies address the nonlinear coupling between fast-ion-driven MHD activity and microturbulence, with emphasis on the impact of energetic particles on turbulence regulation and transport. While previous experiments on conventional aspect-ratio tokamaks have shown evidence of AE-mediated turbulence suppression [1] and nonlinear wave coupling leading to zonal-flow generation [2], this work provides the first detailed investigation of these processes in a spherical tokamak configuration, which possesses a fundamentally different turbulence spectra. Results from MAST-U discharges are presented with distinct beam-on and beam-off phases containing altered fast ion driven modes and turbulence characteristics. Preliminary analysis indicates that the turbulence and AE amplitudes are weakly anti-correlated. Doppler backscattering (DBS) provides the primary turbulence measurements making it possible to study the impact of fast ions and fast ion-driven modes on: turbulence spectra, nonlinear coupling behavior, radial, temporal, and toroidal correlations, and perpendicular flow and radial electric field profiles. Macroscopic transport quantities, such as $\chi_i$- and $\chi_e$-profiles, from integrated TRANSP [3] simulations are compared against the DBS measurements. Higher fidelity is supplied from CGYRO [4] simulations which classify the turbulent spectra and dominant modes. Lastly, experimental analysis of fast ion transport is presented using data from a fast ion loss detector (FILD), neutral particle analyzer (NPA), fast ion D-$\alpha$ (FIDA) spectrometer, and neutron diagnostics. Overall, this work establishes an experimental and computational basis for multiscale energetic particle–turbulence interactions in low-aspect-ratio plasmas and provides insights relevant to the optimization of thermal confinement and fast-ion performance in spherical tokamaks.
[1] X.D. Du et al. 2025 Phys. Rev. Lett. 135 265101
[2] J. Ruiz Ruiz et al. 2025 Phys. Rev. Lett. 134 095103
[3] A.Y. Pankin et al. 2025 Comput. Phys. Commun. 312 109611
[4] J. Candy, E. A. Belli, and R. V. Bravenec 2016 J. Comput. Phys. 324 73