Study of Alfvén eigenmodes with heavy ion beam probing in the TJ-II stellarator

The study of Alfven eigenmodes (AEs) driven by fast particles in toroidal plasmas is of a great importance for future fusion reactor with plasma, dominating by fusion alphas. The review is the first attempt to describe systematically AEs in the TJ-II stellarator with low magnetic shear, and to summarize more than 10 years of the direct application of the Heavy Ion Beam Probing (HIBP)-a diagnostic with unique capabilities to study AEs in toroidal plasmas by local measurement of the AE-excited electric potential and density perturbation in the plasma core along with magnetic potential perturbations. Experimental findings, including absolute values of plasma potential and density perturbation, and the mode radial location, are compared with numerical simulations that allows us to identify the observed modes as helicity induced Alfven eigenmodes (HAE) and global Alfven eigenmodes (GAE). On top of that, the mode poloidal rotation, mode numbers, and turbulent particle flux estimations are described. 2D poloidal map of the AEs demonstrates the ballooning character of plasma potential perturbation. AEs in both continuous frequency and chirping form shows strong evolution caused by rotational transform (iota) change, the suggested analytical model for the mode frequency describes the observation. AE frequency dynamics down to geodesic acoustic mode frequency with iota variation along with AE transformation from continuous to chirping form and back is observed.