Observation of internal coherent mode structure using heavy ion beam probe

A heavy ion beam probe (HIBP) can be used to investigate various mode structures. When a mode is distinct from the background turbulence, the spatial structure of the mode is easily determined by the fluctuation amplitude profile. On the other hand, even if the amplitude is too low to distinguish the mode from the background turbulence, the coherence between HIBP and magnetic probe signals is often sufficient to separate the coherent mode activity from the background. In the latter case, the mode profile can be inferred from the coherence. This article presents two methods for deducing the internal spatial mode structure using a HIBP in the compact helical system-a plasma toroidal confinement device.