Speaker
Description
The study of fast-ion transport, driven by auxiliary heating methods such as neutral beam injection (NBI) and/or ion cyclotron range of frequencies (ICRF) waves, is critical for optimizing plasma performance and advancing fusion reactor development. Neutron diagnostics, particularly neutron spectroscopy, play a pivotal role in analyzing neutron emissions to investigate fast ions. Neutron spectroscopy, traditionally used to determine fuel ion temperature via Maxwellian neutron energy distribution, has advanced with intense NBI heating in devices like the Large Helical Device (LHD), where Doppler-shifted neutron spectra reveal fast ion energy distributions. Since the initiation of deuterium plasma operations in LHD in 2017, the behavior and confinement of passing and helically-trapped fast ions during NBI heating phases, under both NBI and ICRF wave heating, have been extensively studied utilizing a wide array of neutron diagnostic techniques. To further explore the slowing-down dynamics of fast ions and the mechanisms underlying fast-ion-induced magnetohydrodynamic instabilities in the LHD, fast-ion velocity distribution measurements were performed using the compact neutron emission spectrometer (CNES). Six CNESs, equipped with EJ-301 liquid scintillation detectors and $^7$Li-enriched CLYC7 detectors, were deployed around the LHD, featuring two distinct sightline orientations: tangential and perpendicular. These configurations were specifically designed to investigate passing fast ions during the tangential injection of negative-ion-based neutral beams (N-NB) and helically-trapped fast ions during the perpendicular injection of positive-ion-based neutral beams (P-NB) and ICRF wave heated plasma, respectively. CNES based on EJ-301 operates across a wide neutron emission rate due to their fast decay time but require spectrum unfolding for D–D neutron detection via recoiled protons. CNES based on CLYC7, detecting D–D neutrons through $^{35}$Cl(n,p)$^{35}$S reactions, reduce the need for unfolding but are limited to lower neutron emission rates due to their longer decay time. The tangential-sightline CNES was utilized to investigate the behavior of passing fast ions during N-NB heating phases by analyzing the significant Doppler shift in D-D neutron energy resulting from high-energy fast-ion injection by co-going and counter-going passing beam ions. The upper and lower Doppler-shifted D-D neutron energies corresponded to fast ions moving toward and away from the tangential CNES, respectively. The perpendicular-sightline CNESs were employed to study the behavior of helically-trapped fast ions during P-NB heating phases. A double-humped neutron energy spectrum was observed, with the peaks likely corresponding to the large Larmor motion of fast ions injected by P-NB. Furthermore, the expected D-D neutron energy spectrum was calculated using the five-dimensional orbit-following code DELTA5D, incorporating the effects of Larmor motion. A consistent correlation was observed between the calculated and experimental results.
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