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

MHD induced beam ion losses were observed on the CHS heliotron/torsatron wi th an energy and pitch angle resolved scintillator detector. Beam ions esca ping into the probe were characterized using orbit calculation codes. It wa s found that two types of ions, namely 'passing boundary ions' and 'trapped ions'. were lost periodically during an m/n = 3/2 fishbone-like instabilit y. There are thresholds in the mode amplitude fur the ejection of fast ions . Above this threshold, losses are enhanced together with an increase of th e mode amplitude. The excitation of the fishbone-like instability and the e nhancement of MHD induced losses strongly depend on the degree of accumulat ion of Learn ions in the plasma. Orbit calculations show that passing bound ary ions, which have large pitch angle and nu(phi) close to zero (or close to title propagating velocity of the magnetic fluctuation) near the q = 3/2 surface, are ejected by the instability.