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

Understanding of energetic particle (EP) confinement is one of the critical issues in realizing fusion reactor.In stellarator/helical devices, the research on EP confinement is one of the key topics to obtain better confinement by utilizing the flexibility of three-dimensional magnetic field.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, confinement study of EPs has progressed remarkably by using newly developed comprehensive neutron diagnostics providing the information of EPs confined in the core region.The total neutron emission rate (Sn) increases due to the relatively low deviation of beam ion orbit from the flux surface with the inward shift of the magnetic axis.Sn has the peak around the electron density of 2×10 19 m -3 to 3×10 19 m -3 , as predicted.It is found that the fraction of beam-beam components in Sn 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 Sn following the short pulse neutral beam injection into the electroncyclotron-heated low-beta plasma is reproduced by drift kinetic simulation, indicating that transport of beam ion injected by 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, neutron emission profile shifts according to 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 stellarator/heliotron so as 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 inward shifted configuration is similar to that measured in tokamak having a similar minor radius with the LHD.We study the confinement capability of EPs toward a helical reactor in magnetohydrodynamic -quiescent region and expansion of the energetic-ion physics study in toroidal fusion plasmas.