Measurement of Space Potential Fluctuations during MHD Activities in a Toroidal Helical Plasma with a Heavy Ion Beam Probe

A heavy ion beam probe (HIBP) is a unique diagnostic instrument which can measure local plasma potent and its fluctuations. However, in the presence of MHD activities, acceleration or deceleration of both primary and secondary beams are caused by the fluctuating vector potential ∂A/∂t along the beam trajectories (path integral effect). Such effect possibly induces a contamination to space potential measurement. We have evaluated those effects during a bursting MHD instability in the Compact Helical System (CHS), which is considered to be an m/n = 2/1 interchange instability. In this procedure, the vector potential profile is estimated both theoretically and experimentally. Contribution from path effect is found to be less than several percent o the measured space potential fluctuation. Then space potential fluctuation is found to be localized around q = 2 magnetic surface, which is consistent with a model for the interchange mode.