Search

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 (Sn) 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 Sn 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 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 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 inc, source:https://doi.org/10.1088/1741-4326/ab14bc, identifier:0000-0003-4555-1837

In the currently performed neutral beam (NB) -heated deuterium plasma experiments, neutrons are mainly produced by a beam-plasma reaction. Therefore, time-resolved measurement of the neutron emission profile can enhance the understanding of the classical and/or anomalous transport of beam ions. To measure radial neutron emission profiles as a function of time, the vertical neutron camera (VNC) capable of operation with a counting rate in the MHz range was newly installed on the Large Helical Device (LHD). This is the world’s first neutron camera for stellarator/heliotron devices. The VNC consists of a multichannel collimator, eleven fast-neutron detectors, and the digital-signal-processing-based data acquisition system (DAQ). The multichannel collimator having little cross talk was made from hematite-doped heavy concrete, which has a high shielding performance against both neutrons and gamma-rays. A stilbene crystal coupled with a photomultiplier having high-gain-stability in the high-count rate regime was utilized as a fast-neutron scintillation detector because it has a high neutron-gamma discrimination capability at high count rates. The DAQ system equipped with a field programmable logic controller was developed to obtain the waveform acquired with a 1 GHz sampling rate and the shaping parameter of each pulse simultaneously at up to 106 cps (counts per second). Neutron emission profiles were successfully obtained in the first deuterium campaign of LHD in 2017. The neutron emission profile was measured in tangentially co-injected NB-heated plasma with different magnetic axes (Rax). The neutron counts became larger in the inward-shifted configuration, which was consistent with the total neutron rate measured by the neutron flux monitor. The radial peak position of the line-integrated neutron profile which changed according to Rax showed that the VNC worked successfully as designed. The VNC demonstrated the expected performance conducive to extending energetic-part, source:https://aip.scitation.org/doi/abs/10.1063/1.5054818, identifier:0000-0003-4555-1837

Shot-integrated measurement of the triton burnup ratio has been performed in the Large Helical Device. It was reported that the triton burnup ratio, defined as total DT neutron yield divided by total DD neutron yield, increases significantly in inward shifted configurations. To understand the magnetic configuration dependence of the triton burnup ratio, the first orbit loss fraction of 1 MeV tritons is evaluated by means of the Lorentz orbit code for various magnetic configurations. The first orbit loss of 1 MeV tritons is seen at t of less than 10−5 s and loss points of the triton are concentrated on the side of the helical coil case where the magnetic field is relatively weak. The significant decrease of the first orbit loss fraction by 15% is obtained with the inward shift of the magnetic axis position from 3.90 to 3.55 m. It is found that the decrease of first orbit loss is due to the reduction of the first orbit loss of transition and helically trapped tritons., source:Kunihiro OGAWA et al 2019 Plasma Sci. Technol. 21 025102, source:https://doi.org/10.1088/2058-6272/aaeba8, identifier:0000-0003-4555-1837

The effect of the helically-trapped energetic-ion-driven resistive interchange modes (EICs) on energetic ion confinement is studied in the Large Helical Device deuterium plasmas. Neutron diagnostics such as the neutron flux monitor and the vertical neutron camera (VNC) are used in order to measure neutrons mainly created by beam-plasma reactions. The line-integrated neutron profiles are obtained by VNC in magnetohydrodynamic-quiet plasma with various neutral beam (NB) injection patterns. The profiles are consistent with that expected by the beam ion density calculated using orbit-following simulations. Significant decreases of the total neutron emission rate (Sn) and the neutron counting rate of the VNC (Cn) in central cords are observed to be synchronized with EIC bursts with perpendicular-NB injection. The drop rates of both Sn and Cn increase with EIC amplitude and reach around 50%. The line-integrated neutron profiles before and after EIC burst show that in the central cords, Cn decrease due to EIC burst whereas there is almost no change in the other cords. The experimental results suggests that the effect of EIC on helically-trapped beam ion is substantial, however the effect of passing beam ion is not significant., source:Ogawa K, Isobe M, Kawase H, Nishitani T, Seki R, Osakabe M and LHD Experiment Group Effect of the helically-trapped energetic-iondriven resistive interchange modes on energetic ion confinement in the Large Helical Device 2018 Plasma Physics and Controlled Fusion 60 044005., source:https://doi.org/10.1088/1361-6587/aaab1f, identifier:0000-0003-4555-1837

Time-resolved measurement of triton burnup is performed with a scintillating fiber detector system in the deuterium operation of the large helical device. The scintillating fiber detector system is composed of the detector head consisting of 109 scintillating fibers having a diameter of 1 mm and a length of 100 mm embedded in the aluminum substrate, the magnetic registrant photomultiplier tube, and the data acquisition system equipped with 1 GHz sampling rate analogies to digital converter and the field programmable gate array. The discrimination level of 150 mV was set to extract the pulse signal induced by 14 MeV neutrons according to the pulse height spectra obtained in the experiment. The decay time of 14 MeV neutron emission rate after neutral beam is turned off measured by the scintillating fiber detector. The decay time is consistent with the decay time of total neutron emission rate corresponding to the 14 MeV neutrons measured by the neutron flux monitor as expected. Evaluation of the diffusion coefficient is conducted using a simple classical slowing-down model FBURN code. It is found that the diffusion coefficient of triton is evaluated to be less than 0.2 m2 s−1., source:Ogawa K, Isobe M, Nishitani T, Murakami S, Seki R, Nakata M, Takada E, Kawase H, Pu N and LHD Experiment Group 2018 Time-resolved triton burnup measurement using the scintillating fiber detector in the Large Helical Device Nucl. Fusion 58(3) 034002., source:https://doi.org/10.1088/1741-4326/aaa585, identifier:0000-0003-4555-1837

A neutron emission rate analysis code called FBURN, based on the classical energetic ion confinement assumption with radial diffusion, is developed for the time-dependent analysis of the total neutron emission rate (Sn) in neutral beam (NB) heated deuterium plasmas. The time trend of Sn evaluated by FBURN shows good agreement with the Sn measured by the neutron flux monitor on the deuterium operation of the Large Helical Device. The dependence of Sn on line-averaged electron density (ne_avg) has a peak at ne_avg of around 2.5 × 1019 m−3 in both experiment and calculation. Here, the absolute value of Sn evaluated by calculation agrees with that obtained in experiments within a factor of two. A time trend analysis of Sn in an electron cyclotron heated plasma with a short pulse NB injection is performed. The analysis shows that the diffusion coefficient of co-going transit beam ions is 0.2 to 0.3 m2 s−1. In addition, the diffusion coefficient of helically trapped beam ions decreases from 5 to 3 m2 s−1 with the inward shift of the magnetic axis position. Time-resolved analysis of the triton burnup experiment shows that the diffusion coefficient of tritons is around 0.15 m2 s−1. It is found that the diffusion coefficients of the beam and tritons are of a similar value as obtained in JT-60U. The trend of the triton burnup ratio on the ne_avg calculated by the FBURN agrees with the experiments. The results suggest that the decrease of the triton burnup ratio with the increase of ne_avg is due to the shorter slowing down time of tritons by the decrease of the electron temperature, and the increase of the triton burnup ratio with the increase of ne_avg is due to the diffusion of tritons. Time trend analysis of Sn in the Korea Superconducting Tokamak Advanced Research and the Experimental Advanced Superconducting Tokamak plasmas with a short pulse NB injection is performed. The time trend of Sn is successfully reproduced by FBURN., source:Ogawa K, Isobe M, Nishitani T, Seki R, Nuga H, Murakami S, Nakata M, Pu N, Osakabe M, Jo J, Cheon M, Kim J, Zhong G, Xiao M, Hu L and LHD Experiment Group 2018 Plasma Phys. Control. Fusion https://doi.org/10.1088/1361-6587/aad4b7, source:https://doi.org/10.1088/1361-6587/aad4b7, identifier:0000-0003-4555-1837

A deuterium experiment was initiated to achieve higher-temperature and higher-density plasmas in March 2017 in the Large Helical Device (LHD). The central ion temperature notably increases compared with that in hydrogen experiments. However, an energetic particle mode called the helically-trapped energetic-ion-driven resistive interchange (EIC) mode is often excited by intensive perpendicular neutral beam injections on high ion-temperature discharges. The mode leads to significant decrease of the ion temperature or to limiting the sustainment of the high ion-temperature state. To understand the effect of EIC on the energetic ion confinement, the radial transport of energetic ions is studied by means of the neutron flux monitor and vertical neutron camera newly installed on the LHD. Decreases of the line-integrated neutron profile in core channels show that helically-trapped energetic ions are lost from the plasma., source:Ogawa K, Isobe M, Kawase H, Nishitanai T, Seki R, Osakabe M and LHD Experiment Group 2018 Observation of enhanced radial transport of energetic ion due to energetic particle mode destabilized by helically-trapped energetic ion in the Large Helical Device Nuclear Fusion 58 044001., source:https://doi.org/10.1088/1741-4326/aaab18, identifier:0000-0003-4555-1837

High-temperature and high-density plasmas are achieved by means of real-time control, fast diagnostic, and high-power heating systems. Those systems are precisely controlled via highly integrated electronic components, but can be seriously affected by radiation damage. Therefore, the effects of irradiation on currently used electronic components should be investigated for the control and measurement of Large Helical Device (LHD) deuterium plasmas. For the precise estimation of the radiation field in the LHD torus hall, the MCNP6 code is used with the cross-section library ENDF B-VI. The geometry is modeled on the computer-aided design. The dose on silicon, which is a major ingredient of electronic components, over nine years of LHD deuterium operation shows that the gamma-ray contribution is dominant. Neutron irradiation tests were performed in the OKTAVIAN at Osaka University and the Fast Neutron Laboratory at Tohoku University. Gamma-ray irradiation tests were performed at the Nagoya University Cobalt-60 irradiation facility. We found that there are ethernet connection failures of programmable logic controller (PLC) modules due to neutron irradiation with a neutron flux of 3 × 106 cm−2 s−1. This neutron flux is equivalent to that expected at basement level in the LHD torus hall without a neutron shield. Most modules of the PLC are broken around a gamma-ray dose of 100 Gy. This is comparable with the dose in the LHD torus hall over nine years. If we consider the dose only, these components may survive more than nine years. For the safety of the LHD operation, the electronic components in the torus hall have been rearranged., source:Ogawa K, Nishitani T, Isobe M, Murata I, Hatano Y, Matsuyama S, Nakanishi H, Mukai K, Sato M, Yokota M, Kobuchi T, Nishimura T and Osakabe M 2017 Investigation of irradiation effects on highly integrated leading-edge electronic components of diagnostics and control systems for LHD deuterium operation Nuclear Fusion 57 086012., source:https://doi.org/10.1088/1741-4326/aa71e8, identifier:0000-0003-4555-1837

The enhancement of the fusion reaction rate coefficient induced by energetic protons, which is one of the typical nuclear elastic scattering (NES) effects, was observed in the large helical device (LHD) located at the National Institute for Fusion Science. An intense high purity hydrogen beam (the atomic ratio of the contained deuterium was less than 1 ppm) with ~170 keV-energy and ~10 MW-power was injected into a deuterium plasma with the electron temperature ~9 keV and density ~1019 m−3. After the hydrogen beam injection, the neutron generation rate by the D(d,n)3He fusion reaction immediately increased, and an increment of over one order of magnitude was observed, even though there was no meaningful ion temperature variation. The reasons for the increment in the neutron generation rate are discussed with reference to the Boltzmann–Fokker–Planck analysis along with LHD experimental data. It is concluded that the increment was induced by the production of energetic deuterons due to the NES of energetic protons.