Overview of studies on energetic-ion-driven MHD instabilities in stellarator/helical plasmas and comparison with tokamaks

Suppression of anomalous radial transport of alpha particles and beam ions due to energetic ion driven MHD instabilities is one of the critical issues toward D-T burning plasma experiments in toroidal magnetic confinement devices such as tokamaks and stellarator/helical devices. This paper reports the characteristics of energetic ion driven MHD modes such as Alfvén eigenmodes (AEs) and their effects on transport of energetic ions and bulk plasma in the Large Helical Device (LHD) and some different types of stellarator/helical devices. Beneficial impacts of AEs on toroidal plasma confinement are also included. These experimental results on AEs are compared with those in tokamaks, and the important similarities and differences are pointed out that can lead to better understanding of AEs in toroidal plasmas.