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We newly designed and installed a H␣ line-emission detector in order to examine neutral particle transport around the beam line of NBI in the central cell of the GAMMA10 tandem mirror. The H␣ detector consists of a H␣ interference filter, a lens, an optical fiber, and a photomultiplier. The detector is absolutely calibrated using a standard lamp. The H␣ detector is located at the angle of 24°to the beam line, which enabled us to directly view the H␣ emission due to neutral beam injection (NBI). In standard hot-ion mode plasmas, we measured the H␣ line emission in the case of NBI. The H␣ line emission during NBI strongly increased by 550% on the beam line compared with that before NBI. Moreover, it was found that hydrogen recycling played an important role in the neutral transport around the beam line. This diagnostic system is confirmed to be considerably helpful for examining the detailed behavior of the neutral particles related to NBI for the first time.

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