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) [3] 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 [4].
[1] L. C. Appel et al., Plasma Phys. Control. Fusion 50, 115011 (2008)
[2] S. E. Sharapov et al., Plys Plasmas 21, 082501 (2014)
[3] J. F. Rivero-Rodriguez et al., Rev. Sci. Instrum. 89, 10I112 (2018)
[4] W.W. Heidbrink et al., Nucl. Fusion 46, 324 (2006)