NTM induced fast ion losses in ASDEX Upgrade

The loss of fast (i.e. suprathermal) ions from a magnetically confined fusion plasma due to the interaction with magnetohydrodynamic (MHD) instabilities has been experimentally characterized and interpreted by means of a numerical model. It is found that for a special class of instabilities, the so-called neoclassical tearing modes, fast ions losses are increased and modulated at the same frequency of the mode. This new experimental finding is explained as a result of the drift islands formed by energetic ions in particle phase space. An eventual overlapping of these drift islands leads to an orbit stochasticity and therefore to an enhancement of the fast ion losses. This explanation is confirmed by statistical analysis of simulations of fast ions trajectories performed with the ORBIT code. The mechanism is of general importance for understanding the interaction between MHD modes and fast particles in magnetic confinement experiments.