Design Study of a Lost Fast-Ion Probe in Large Helical Device

A scintillator-based lost fast-ion probe (LIP) was designed and constructed to measure energetic-ion-losses caused by energetic-ion-drivenMHD instabilities such as Alfvén eigenmodes in the Large Helical Device (LHD). It provides information about the pitch angle and gyroradius of the lost energetic ions simultaneously. It is installed at the outboard side of a horizontally elongated poloidal cross section to detect co-going energetic ions. Two apertures of the LIP were designed to detect energetic ions, whose pitch angle and gyroradius are 35-50 degrees and 1.5-15 cm at the detector position, respectively. The scintillator P46 was adopted because it has high luminosity even in a high temperature environment. The scintillation light emitted from the scintillator surface by the impact of lost energetic ions is detected using an array of photomultiplier tubes with high time-resolution (up to 200 kHz) and an image-intensified C-MOS camera (up to 2 kHz).