Initial operation of perpendicular line-of-sight compact neutron emission spectrometer in the large helical device.

The perpendicular line-of-sight compact neutron emission spectrometer (perpendicular CNES) was newly installed to understand the helically trapped fast-ion behavior through deuterium-deuterium (D-D) neutron energy spectrum measurement in the Large Helical Device (LHD). The energy calibration of the EJ-301 liquid scintillation detector system for perpendicular CNES was performed on an accelerator-based D-D neutron source. We installed two EJ-301 liquid scintillation detectors, which view the LHD plasma vertically from the lower side through the multichannel collimator. The D-D neutron energy spectrum was measured in a deuterium perpendicular-neutral-beam-heated deuterium plasma. By the derivative unfolding technique, it was found that the D-D neutron energy spectrum had a double-humped shape with peaks at ∼2.33 and ∼2.65 MeV. D-D neutron energy spectrum was calculated based on the fast ion distribution function using guiding center orbit-following models considering the detector's energy resolution. The calculated peak energies in the D-D neutron energy spectrum almost match the experiment. In addition, a feasibility study toward the measurement of the energy distribution of ion-cyclotron-range-of-frequency-wave-accelerated beam ions was performed.