Neutron Spectroscopy of Deuterium-deuterium Reactions in Neutral Beam-heated Plasmas on the Large Helical Device

The compact neutron emission spectrometers (CNES) system has enabled detailed and comprehensive characterization of fast ion confinement during the neutral beam injection heating in the Large Helical Device (LHD). Using high resolution neutron spectroscopy along both tangential and perpendicular lines-of-sight, CNES provides direct experimental access to deuterium–deuterium (D–D) reaction dynamics in velocity space, reflecting the behavior of fast ions injected by neutral beam injections. Significant Doppler shifts in D-D neutron energy were observed during intense tangential neutral beam injection, revealing the impact of the relative motion of the D-D reaction center-of-mass (CoM) with respect to the CNES sightline. An upshift in neutron energy was detected when the CoM moved toward the sightline, while a downshift occurred when it move away from the sighline. In plasmas heated by perpendicular neutral beam injection, a distinct double-humped neutron energy spectrum was observed, corresponding to the Larmor motion of helically trapped beam ions within the LHD’s magnetic ripple structure. The results demonstrate the capability of CNES to detect Doppler shifts in neutron energy associated with the direction and confinement of fast ions. By comparing experimental results with numerical simulations, the study confirms the reliability of both tangential and perpendicular CNESs sightline configuration in D-D neutron emission spectroscopy in LHD. The results improve our understanding of how fast ions generated by neutral beam injection affect D-D reaction.