The deuterium operation of the Large Helical Device (LHD) heliotron started in March 7, 2017, after longterm preparation and commissioning works necessary to execute the deuterium experiment. A comprehensive set of neutron diagnostics was implemented to accelerate energetic-particle physics research in the LHD. The calibrated ex-vessel neutron flux monitor indicated that the total neutron emission rate in the first deuterium campaign reached 3.3×10 15 n/s in inward shifted magnetic field configuration where confinement of helically trapped energetic ions is predicted to be better. Density dependence of measured total neutron emission rate was consistent with that predicted by the calculation. The neutron decay rate analysis following perpendicular deuterium beam blips injection suggested that the confinement of helically trapped beam ions can be understood by the classical slowing down model in relatively high-electron density plasmas at inward shifted magnetic field configuration. On the other hand, loss of helically-trapped beam ions was recognized even in the inward shifted configuration in the case of low density. Performance of the vertical neutron camera was verified by changing the plasma position and/or magnetic field strength. Drastic change of neutron emission profile was observed when the resistive interchange mode driven by helically-trapped beam ions appears. It was successfully demonstrated that the vertical neutron camera can play an important role in revealing radial transport and/or loss of beam ions. Triton burnup study was also conducted. In the first deuterium campaign, the maximum triton burnup ratio of 0.45 % was obtained in inward shifted configuration. The burnup ratio decreased as a plasma was shifted outwardly as expected.

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 2 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.

The neutral beam (NB) fast ion confinement in the Large Helical Device (LHD) is studied for several full field ( $B_{t}\sim 2.75~\text{T}$ ) magnetic configurations by a combination of neutron measurement and simulations. To investigate the NB fast ion confinement, we have performed a series of short-pulse NB injection experiments. The experiment results are analysed by the integrated code TASK3D-a. From this investigation, the effective particle diffusion coefficients of the tangential and perpendicular NBs are approximately $D^{\text{eff}}\sim 0.1~\text{m}^{2}~\text{s}^{-1}$ and $D^{\text{eff}}\sim 1~\text{m}^{2}~\text{s}^{-1}$ in the standard configuration. It is clarified that the NB fast ion confinement improves when the plasmas are shifted inward. Moreover, it is also found that the simulation, which considers the deuteron dilution effect due to the presence of impurity ions, can describe a neutron emission rate consistent with the measurement.