Energetic-ion confinement studies by using comprehensive neutron diagnostics in the Large Helical Device

Understanding energetic particle (EP) confinement is one of the critical issues in realizing fusion reactors. In stellarator/helical devices, the research on EP confinement is one of the key topics to obtain better confinement by utilizing the flexibility of a 3D magnetic field. A study of EP transport in the Large Helical Device (LHD) has been performed by means of escaping EP diagnostics in hydrogen plasma operation. By starting deuterium operation of the LHD, the confinement study of EPs has progressed remarkably using newly developed comprehensive neutron diagnostics providing information for EPs confined in the core region. The total neutron emission rate (S n) increases due to the relatively low deviation of the beam ion orbit from the flux surface with the inward shift of the magnetic axis. The S n has a peak around the electron density of 2 × 1019 m−3 to 3 × 1019 m−3, as predicted. It is found that the fraction of beam–beam components in S n is evaluated to be approximately 20% by the Fokker–Planck models TASK/FP in the plasma with both co- and counter-neutral beam injections. The equivalent fusion gain in DT plasma achieved 0.11 in a negative-ion-based neutral beam heated plasma. Time evolution of S n following the short pulse neutral beam injection into the electron–cyclotron-heated low-beta plasma is reproduced by drift kinetic simulation, indicating that transport of a beam ion injected by a short pulse neutral beam can be described with neoclassical models in magnetohydrodynamic quiescent low-beta plasmas. The vertical neutron camera works successfully, demonstrating that in the co-neutral beam-injected plasma, the neutron emission profile shifts according to the magnetic axis position. The shift of the neutron emission profile is reproduced by orbit-following models. The triton burnup study is performed for the first time in a stellarator/heliotron to understand the alpha particle confinement. It is found that the triton burnup ratio, which largely increases at inward-shifted configurations due to the better triton orbit and better plasma performance in the inward-shifted configuration, is similar to that measured in a tokamak having a similar minor radius to the LHD. We study the confinement capability of EPs toward a helical reactor in the magnetohydrodynamic quiescent region and expansion of the energetic ion physics study in toroidal fusion plasmas.