Due to their relatively higher Vbeam/VAlvén ratio, spherical tokamaks are ideal to investigate high-frequency modes such as Compressional Alfvén Eigenmodes (CAEs) and Global Alfvén Eigenmodes (GAEs), and so they have been previously studied in MAST [1, 2]. Besides, the recently installed scintillator-based Fast-Ion Loss Detector (FILD)  in MAST-U, with a sampling frequency of 4 MHz, enables the detection of fast-ion losses induced by such instabilities. In the 1st MAST-U experimental campaign (MU01), Fourier analyses of the FILD signal revealed coherent fast-ion losses in the range of 700 kHz – 2 MHz, matching the magnetic fluctuations identified as CAEs and GAEs in the OMAHA coils. To date, this is the most direct evidence of fast-ion losses induced by CAEs and GAEs, suggesting that CAEs/GAEs may have an adverse impact on the fast-ion confinement. Here, we discuss the first observations of fast-ion losses induced by CAEs/GAEs in MU01 and their loss mechanism. The understanding of these losses becomes of paramount importance for ITER, where an anisotropic fast-ion distribution with velocities above the Alfvén velocity could make up the essential ingredients to drive GAEs and CAEs unstable .
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