Neutron Spectroscopy in Perpendicular Neutral Beam Injection Deuterium Plasmas Using Newly Developed Compact Neutron Emission Spectrometers

The presence of helically trapped fast ions in helical ripples in the large helical device (LHD), resulting from a perpendicularly injected positive-ion-source-based neutral beam (P-NB)-heated plasma and/or from ion cyclotron range of frequency (ICRF) wave-heated plasma, poses a significant concern due to the system’s lack of symmetry. In response to this challenge, compact neutron emission spectrometers (CNESs) have been strategically developed in the LHD, featuring a perpendicular line of sight relative to the plasma’s magnetic field. The first perpendicular CNES offered a vertical field of view through the deuterium plasma and incorporated a liquid (EJ-301) scintillation detector optimized for efficient operation at a high neutron emission rate. The second perpendicular CNES was operated using a Cs2LiYCl6:Ce scintillation detector enriched with 7Li (CLYC7), providing a horizontal field of view through the deuterium plasma and effectively operating in a region characterized by relatively low neutron emission rates. During plasma heating through P-NB, both perpendicular CNESs showed a neutron energy distribution, featuring a double-humped profile with two peaks at energy of approximately 2.30 and 2.74 MeV. Furthermore, the deuterium-deuterium neutron energy spectrum expected to be obtained by perpendicular CNES was calculated using the 5-D orbit following code DELTA5D and considering the Larmor motion effect. The results revealed that the experimentally obtained neutron energies were almost consistent with the neutron energies calculated based on the orbit following model. These peaks aligned with the Larmor motion of deuterons resulting from P-NB injection at the helical ripple of the LHD. In addition, we conducted neutron spectroscopy involving deuterium-deuterium interactions in a deuterium plasma simultaneously heated by both P-NB and ICRF wave, characterized by a high neutron emission rate, using perpendicular CNES based on EJ-301. The observation revealed a broadening in spectrum width, attributed to the additional ICRF wave heating.