Study of Helically-Trapped Beam Ion Behaviour in Neutral-Beam and Ion Cyclotron Rnage of Frequency Wave-Heated Deuterium Plasmas of the Large Helical Device using Compact Neutron Emission Spectrometers

The confinement of fast-ions in the Large Helical Device (LHD), generated through perpendicular positive-ion-source- based neutral beam (P-NB) injectors and ion cyclotron range of frequency (ICRF) wave heating, is a significant concern due to the lack of symmetry of the system. To address this issue, compact neutron emission spectrometers (CNESs) with perpendicular line-of-sight geometry have been deployed since 2021. These CNESs utilize both a conventional liquid (EJ-301) scintillation detector and a newly developed Cs2LiYCl6:Ce scintillation detector enriched with 7Li (CLYC7). The primary purpose of those CNESs was to gain insights into the behavior of fast-ions trapped in the helical configuration of the LHD. During P-NB heating, both CNESs reveal a unique neutron energy spectrum characterized by a double-humped shape, with peaks approximately 2.3 MeV and 2.7 MeV. These peaks correspond to the Larmor motion of P-NB deuterons. Additionally, the expected neutron energy spectrum was calculated using the five-dimensional orbit following code DELTA5D. This calculation accounts for the influence of Larmor motion and considers the detector’s energy resolution. Notably, the calculated deuterium-deuterium neutron energy spectrum closely aligns with experimental results, demonstrating the accuracy and reliability of using CNESs to study the behavior of helically-trapped beam ions in the LHD. The deuterium-deuterium neutron spectrometry in ICRF wave superimposed on P-NB heated deuterium plasma with a high neutron emission rate was performed by EJ-301-based CNES. It was found that the peaks become broader due to the ICRF wave heating.