Fast Ion-Driven MHD Instabilities and Consequent Fast Ion Losses in the Compact Helical System

Impacts of fast-ion-driven MHD modes on fast ion transport have been investigated in a medium size helical device CHS having negative magnetic shear over an entire region of plasma. Recently, it was observed that bursting toroidicity-induced Alfven eigenmodes (TAEs) excited by co-circulating beam ions enhance beam ion loss at low magnetic field strength of 0.9 T. Flux of escaping beam ions steeply increases as the magnetic fluctuation level increases. Excitation of energetic particle modes (EPMs) due to co-circulating beam ions has been also observed in CHS, of which frequency range is located appreciably below the TAE gap frequency. Bursts of EPMs and TAEs recur periodically accompanied with periodically increased beam ion loss. Fast particle diagnostics indicate that beam ions transported by these modes are promptly lost to the large major radius side of CHS plasma in the horizontally elongated section. Particle simulation suggests that a perturbed magnetic field whose amplitude is consistent with the CHS experiment can rapidly increase the fast ion loss