Observation of MHD induced fast ion losses on the CHS heliotron/torsatron

MHD induced beam ion losses were observed on the CHS heliotron/torsatron with an energy and pitch angle resolved scintillator detector. Beam ions escaping into the probe were characterized using orbit calculation codes. It was found that two types of ions, namely `passing boundary ionsʹ and `trapped ionsʹ, were lost periodically during an m/n = 3/2 fishbone-like instability. There are thresholds in the mode amplitude for the ejection of fast ions. Above this threshold, losses are enhanced together with an increase of the mode amplitude. The excitation of the fishbone-like instability and the enhancement of MHD induced losses strongly depend on the degree of accumulation of beam ions in the plasma. Orbit calculations show that passing boundary ions, which have large pitch angle and vf close to zero (or close to the propagating velocity of the magnetic fluctuation) near the q = 3/2 surface, are ejected by the instability.