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In this work, energetic-ion confinement and loss due to energetic-ion driven magnetohydrodynamic modes are studied using comprehensive neutron diagnostics and orbit-following numerical simulations for the Large Helical Device (LHD). The neutron flux monitor is employed in order to obtain global confinement of energetic ions and two installed vertical neutron cameras (VNCs) viewing different poloidal cross-sections are utilized in order to measure the radial profile of energetic ions. A strong helically-trapped energetic-ion-driven resistive interchange mode (EIC) excited in relatively low-density plasma terminated high-temperature state in LHD. Changes in the neutron emission profile due to the EIC excitation are clearly visualized by the VNCs. The reduction in the neutron signal for the helical ripple valley increases with EIC amplitude, which reaches approximately 50%. In addition to the EIC experiment, orbit-following simulations using the DELTA5D code with EIC fluctuations were performed to assess the energetic-ion transport and loss. Two-dimensional temporal evolution results show that the neutron emissivity at the helical ripple decreases significantly due to the EIC. The rapid reduction in neutron emissivity shows that the helically-trapped beam ions immediately escape from the plasma. The reduction in the VNC signals for the helical ripple valley and the total neutron emission rate increase with increasing EIC amplitude, as observed in the experiment. Calculated line-integrated neutron emission results show that the profile measured by VNC1 has one peak, whereas the profile measured by VNC2 has two peaks, as observed in the experiment. Although the neutron emission profile for VNC2 has a relatively wide peak compared with the experimental results, the significant decrease in neutron signal corresponding to the helical ripple valley was successfully reproduced.

A first attempt to observe a nuclear-elastic-scattering (NES) effect caused by energetic protons on deuteron slowing-down behaviour was made on the Large Helical Device located at the National Institute for Fusion Science. The NES effect on the slowing-down of fast ions can influence the confinement of fast ions, ion heating, fusion reaction rate coefficient, etc. An intense hydrogen beam was injected into a deuterium plasma to create a knock-on tail, i.e. a non-Maxwellian energetic component in the deuteron velocity distribution function. We conducted two types of experiment: (1) observation of the slowing-down of the knock-on tail and (2) observation of the NES effect on the slowing-down time of fast ions. The phenomena are discussed in terms of the difference in the decay process of the D(d,n)3He neutron generation rate after neutral beam heating is terminated between the cases when the knock-on effect is influential and not influential, and also from the difference in the neutron decay times. The results of a series of experiments indicate that the NES effect caused by energetic protons can have an impact on the slowing-down of fast deuterons.

Fuel-ion velocity distribution functions can be distorted from a Maxwellian by Alfven eigenmodes when the mode amplitudes are large, and the fusion reaction rate coefficient can be enhanced by the distortion. Assuming an ITER-like deuterium–tritium plasma containing an excited Alfven eigenmode, we have investigated the modified deuteron and triton velocity distribution functions and the resulting fusion reaction rate coefficient. It is found that the non-Maxwellian components in the distribution functions form in directions parallel and antiparallel to the magnetic field lines due to the toroidal Alfven eigenmodes or the ellipticity-induced Alfven eigenmodes. Focusing on the effect of the distortion of the distribution functions, we discuss the dependence of the enhancement of the fusion reaction rate coefficient on the mode amplitudes and determine the threshold amplitude necessary to enhance the rate coefficient.

Correlation between ion/electron distribution functions and device performance, i.e. potential structure, density profile and neutron production rate, in spherical inertial electrostatic confinement plasmas is studied by solving the Poisson equation for various deuteron and electron distribution functions. For several combinations of the ion and electron convergences, dependence of the total neutron production rate on discharged current is discussed. It is shown that when electrons have high convergence and energetic component compared with ions, the neutron production rate can increase in proportion to more than a power of the discharged current, even if the neutron production is sustained mainly by the fusion reactions between the beam (deuteron) and background (deuterium) gases.

The radial profile of the neutron production rate in spherical inertial electrostatic confinement plasmas is investigated. The electrostatic potential is obtained by solving the Poisson equation, and by using the potential; the fuel ion velocity distribution function is determined at each radial point. From the velocity distribution function, the neutron production rate is accurately evaluated. Numerical results show that if it is assumed that fuel ions are confined keeping the total energy and angular momentum almost constant, the double radial peak in the neutron production rate can appear without creation of the deep double potential well.

The effect of magnetic field on the velocity distribution function of tritons in D-3He fuelled/field reversed configuration (FRC) plasmas is investigated. Some of the tritons produced by D(d,p)T reactions in the FRC are immediately and asymmetrically lost from the device without any interaction with background charged particles, and then non-symmetric loss causes a distortion of the velocity distribution function as well as a decrease in the number of trapped tritons. Using the distorted triton distribution, the 14 MeV neutron generation rate is estimated and compared with the values for Maxwellian plasmas (the effect of the magnetic field is neglected). It is found that in a typical ignited D-3He/FRC plasma, for example, Ti = Te = 80 keV, nD = 2n3He = 0.5ne = 3 × 1020m-3, τE = 0.5τP = 3 s, rs = 1.6 m and BW = 6 T, the reduction in 14 MeV neutron generation is about 20%.

An expression for the average energy loss rate of beam ions due to nuclear elastic scattering (NES) in Maxwellian plasmas is derived, by taking into consideration the thermal motion of the background ions. The NES effect on deuterium beam injection plasma heating is examined using the expression derived. As a result of the scattering due to NES of the slowing down deuterons up to the higher energy range, the average energy loss rate due to NES of 1 MeV deuterons in 20 keV deuterium plasmas decreases by about 60% compared with the case of cold background plasmas. An examination is also made of the fraction of the beam energy deposited to ions. It is shown that when the beam energy is higher than 1 MeV, the increase in the fraction due to NES becomes appreciable.

The knock-on tail formations in fuel-ion velocity distribution functions by energetic alpha particles (by the T(d,n)4He reaction) and protons (by the D(d,p)T and 3He(d,p)4He reactions) are investigated by simultaneously solving the Boltzmann?Fokker?Planck (BFP) equations for deuteron, triton, 3He, alpha particle and proton in an ITER-like (DT) plasma admixed with a small amount of 3He. As a result of the 3He inclusion, a fraction of the transferred energy from energetic ions to thermal deuterons and tritons via nuclear plus interference (NI) scattering is reduced. Owing to the NI scattering of the energetic protons by fuel ions, the latter are knocked up to higher energies. The knocking-up effect of fuel ions is enhanced with increasing 3He concentration. It is shown that if 3He with relative concentration of 4.2%, i.e. , is included in Te = 20?keV, ne = 9.5 ? 1019?m?3 plasma, the magnitude of the knock-on tail in deuteron distribution function in 300?keV?3?MeV energy range is reduced by about 15% from the value when 3He is not externally supplied. Such knock-on tail reduction also results in alternation of the non-Gaussian neutron emission spectrum with energies less than ~13?MeV and above ~15?MeV.