Experimental observations of fast-ion losses induced by neoclassical tearing modes in the MAST-U spherical tokamak

Neoclassical tearing modes (NTMs) have been identified as the most deleterious perturbations in high-performance plasmas at Mega Amp Spherical Tokamak Upgrade (MAST-U). They produce magnetic islands that flatten the electron temperature profile and enhance the fast-ion transport. Understanding the NTM-induced losses can reveal paths to mitigate them, thus increasing the energy available to heat up the plasma. The MAST-U fast-ion loss detector (FILD) is equipped with a high-resolution camera and a high-speed camera that simultaneously measure the fast-ion losses in MAST-U. The combination of both systems makes it possible to infer the velocity-space of the losses fluctuating at the frequency of the NTMs. The FILDSIM code is used to infer the velocity space of the fast-ion losses from the strike position in a scintillator plate. Eulerian video magnification is employed to identify the losses that oscillate at the frequencies of the NTMs. NTMs produce fast-ion losses across a broad range of velocity space, with pitch angles ranging from 35∘ to 54∘. Non-linear interactions between the fast-ion orbits and different magnetic islands have been observed. The lost fast-ion orbits meet the stringent conditions that makes it possible to measure these effects.