NIFS-Repository (National Institute for Fusion Science)
Not a member yet
13159 research outputs found
Sort by
Energy Dependence of the Line Ratio I(233.9 Å)/I(243.8 Å) in Fe xv Observed with an Electron Beam Ion Trap
Get PDF0000-0003-2209-3255We present the energy dependence of the intensity ratio between the 3s3p3P2—3s3d3D3 transition at 233.9 Å and the 3s3p1P1—3s3d1D2 transition at 243.8 Å in Fe xv studied with an electron beam ion trap over an energy range that spans resonance excitation regions. Clear resonance structures are observed in the electron energy range of 400–600 eV. The energy dependence obtained in the experiment is compared with a collisional-radiative model calculation, including resonance excitations, and overall agreement is found. It is shown that the ratio strongly reflects the population of the 3s3p3P2 metastable state, which is the lower state of the 233.9 Å transition.journal articl
Effects of toroidally-distributed-divertor biasing on scrape-off-layer (SOL) current drive, divertor particle flux and fast electron confinement in the QUEST spherical tokamak
Get PDFORCID 0000-0002-8244-8305A novel divertor biasing by four biasing plates distributed toroidally (TDDB) on the upper divertor target plate is applied to low density tokamak plasmas started-up by 28 GHz 2nd harmonic electron cyclotron current drive (ECCD) in a quasi-double null configuration of the QUEST spherical tokamak (ST). In the ST plasmas of line averaged electron density ∼0.7–1 × 1018 m−3, about 20%–40% of the current driven by ∼85 V sawtooth bias voltage reaches the lower divertor plate along the biased scrape-off-layer (SOL) flux tube as the SOL current . The fact is noticeably lower than indicates an appreciable leakage of parallel current from the biased SOL flux tube. The leakage currents in the toroidal and radial directions are confirmed by detection of them using the unbiased plates in the TDDB experiments. From the ion saturation current density profile obtained by a divertor Langmuir probe array, the fall-off lengths of divertor particle flux are estimated together with the strike line position. Total particle flux to the upper divertor, evaluated by the integrated ion saturation current density profile is reduced by up to 45% during positive biasing of the TDDB, depending on the position of the strike line to the biased plate. In addition, the TDDB also induces a noticeable loss of fast electrons produced by ECCD, leading to an ∼2% reduction in the maximum toroidal current of the ST plasma compared to a shot without the TDDB. Reduction of the divertor particle flux and enhancement of the fast electron losses are thought to be dominantly caused by E drift induced by the TDDB. In the present experimental conditions, the effects of magnetic perturbations produced by the SOL currents on the fast electron losses can be neglected because of a too small SOL current.journal articl
Development of a High Sampling Rate Data Acquisition System Working in a High Pulse Count Rate Region for Radiation Diagnostics in Nuclear Fusion Plasma Research
Get PDFORCID 0000-0003-4555-1837In this study, a high sampling rate data acquisition system with the ability to provide timestamp, pulse shape information, and waveform simultaneously under a sub megahertz pulse counting rate was developed for radiation diagnostics for magnetic confinement nuclear fusion plasma research. The testing of the data acquisition system under the high pulse counting rate condition using real signals was performed in an accelerator-based deuterium-deuterium fusion neutron source (Fast Neutron Source) at the Japan Atomic Energy Agency. We found that the pulse counts acquired by the system linearly increased up to 6 × 105 cps, and the count loss at 106 cps was estimated to be ~10%. The data acquisition system was applied to deuterium-deuterium neutron profile diagnostics in the deuterium gas operation of a helical-type magnetic confinement plasma device, called the Large Helical Device, to observe the radial profile of neutron emissivity for the first time in a three-dimensional magnetic confinement fusion device. Time-resolved measurements of the deuterium-deuterium fusion emission profile were performed. The experimentally observed radial neutron emission profile was consistent with numerical predictions based on the orbit-following models using experimental data. The data acquisition system was shown to have the desired performance.journal articl
Study on the State Prediction of a Pool-Cooled Large Superconducting Coil Using Machine Learning
Get PDFORCID 0000-0002-1989-867XFor the LHD subcooling system composed of pool-cooled large superconducting coils wound with NbTi superconductors, a machine learning technique was introduced to increase the reliability of the system. The machine learning model for the state prediction of the system was developed using the technique, together with the data accumulated in the LHD plasma experimental campaign. Regarding the temperature changes in the system due to coil excitation and discharging, it is possible to make predictions using the model. Especially for the usual coil current waveform in a helical coil operation, which is a trapezoidal waveform, the model achieved high prediction accuracy.journal articl
Chromatographic Analysis of Molecular Hydrogen (H2) in the Atmosphere for Understanding Atmospheric Tritiated Hydrogen (HT)
Get PDF0000-0001-9941-1958Tritium monitoring would be required at the site of the fusion test facility from the viewpoints of radiation safety and public acceptance. As one of the tritium monitoring items, atmospheric tritium monitoring in the environment is conducted and has three chemical forms: tritiated water vapor (HTO), tritiated molecular hydrogen (HT), and tritiated methane (CH3T). It is well-known that the specific activity of HT and CH3T is much higher than that of HTO. To investigate the cause of high specific activity, we focus on the behavior of molecular hydrogen (H2) in the atmosphere and develop a measurement system based on a gas chromatograph. Monitoring results showed that the range of the H2 mixing ratio at the NIFS Toki site was 0.4 ∼ 0.6 ppm over the observation period. Atmospheric H2 concentrations were higher during the daytime and long daylight seasons, suggesting the generation of hydrogen by photochemical reactions. Preliminary results suggest that there does not appear to be a clear correlation between the concentration of atmospheric HT and the concentration of H2 in the atmosphere. This suggests that the source of HT is different from the source of H2 in the atmosphere.journal articl
Study on Fast Deuteron Diagnostics Method Using Fast 3He Visible Spectra in the Large Helical Device Deuterium Plasma
Get PDF0000-0003-3198-042XFast ion diagnostic is one of the most crucial plasma diagnostics for nuclear fusion investigation. A new diagnostic method for fast ions has been proposed using visible spectra of 3He produced by a deuteron-deuteron reaction. This diagnostic method has a better energy resolution than methods using neutron/γ-ray and is superior to conventional spectroscopy in measuring high energy (MeV order) ions. This diagnostic method has been predicted using numerical analysis for ITER, but no verification experiments have been performed yet. In this study, we examined the measurability of this diagnostic method in the large helical device (LHD) deuterium plasma. Although very dependent on the measurement geometry and the spectrometer performance, it may be possible to measure the fast 3He visible spectrum.journal articl
LHD 重水素プラズマにおける核弾性散乱効果の観測実験
Get PDF0000-0002-7498-4191高温プラズマの散乱過程としてはCoulomb 散乱が支配的であるが，イオンのエネルギーが高まると散乱過程に核力の影響（核弾性散乱）が現れる．核弾性散乱が核燃焼プラズマに及ぼす影響については，古くより数値解析による予測がなされてきたが，実験による検証は十分とは言い難い．近年，著者等はLHD（Large Helical Device）重水素プラズマを対象に核弾性散乱効果の検証実験に取り組んだ．核融合研究は，MeV オーダーのイオンによって核燃焼を維持する核融合プラズマの実現に挑戦する段階にあり，核弾性散乱の影響はその大小はあるにせよ計測値に様々な形で現れる可能性がある．本稿では，最近のLHD 実験における，核弾性散乱効果を対象とした研究の現状を紹介する．journal articl
Gamma ray diagnostics for high time resolution measurement in large helical device
Get PDFORCID 0000-0003-4555-1837A 1-inch LaBr3:Ce gamma ray scintillation detector, characterized by high time and energy resolution, was installed to advance energetic particle physics studies in Large Helical Device (LHD). We reduced the size of the scintillator from 3 inches to 1 inch to reinforce the radiation shielding to reduce the unwanted signal in the detector induced by fast neutrons and stray gamma rays according to the commissioning results. The radiation shielding composed of 10% borated polyethylene and lead was redesigned to suppress the gamma-ray induced signal based on the Monte Carlo three-dimensional neutron and gamma-ray transport code MCNP6. We increased the lead thickness from 50 mm to 77.5 mm to suppress the stray gamma ray effect. The gamma ray spectrum was measured in the hydrogen neutral beam heated deuterium plasma with 6LiF pellet injection. We might obtain a gamma ray peak near 0.48 MeV due to the 6Li(d,p'γ)7Li reaction.journal articl
Characterization of CLYC7 Scintillation Detector in Wide Neutron Energy Range for Fusion Neutron Spectroscopy
Get PDFORCID 0000-0002-0160-0468The newly developed Cs2LiYCl6:Ce crystal with7Li-enrichment (CLYC7) scintillators had been utilized as a key component in a compact neutron emission spectrometer (CNES) at the large helical device (LHD). The CNES was employed to enhance understanding the slowing-down process of neutral-beam-injected energetic beam deuterons by measuring the Doppler broadening of beam-driven deuterium–deuterium neutrons. In order to assess the detection capabilities in fusion neutron spectroscopy and to gain understanding into the energy spectrum at various neutron energies, an understanding of the response function of the detector across a wide neutron energy range is essential. Therefore, before implementing the CNES at the LHD, the detector was tested at high-performance neutron source facilities to evaluate its response to both thermal and fast neutrons. The detector exhibited excellent pulse shape discrimination (PSD) properties. However, it also exhibited complexity through the 35Cl(n, p)35S and 35Cl(n, α )32P reactions in the fast neutron measurement. To emphasize the accuracy and reliability of the detector’s response, particularly in the fast neutron energy range, Monte Carlo N-Particle (MCNP) transport simulations were employed. In order to validate the experimental results against the calculations, an energy-dependent quenching factor was determined. This analysis provided a clear understanding of the CLYC7 scintillation detector and its potential in the context of fusion neutron spectroscopy for applications in the LHD and future fusion devices.journal articl
Verdet constant dispersion of magnesium fluoride for deep-ultraviolet and vacuum-ultraviolet Faraday rotators
Get PDF0000-0002-7220-0613The Verdet constant dispersion in magnesium fluoride (MgF2) crystals was evaluated over a wavelength range of 190–300 nm. The Verdet constant was found to be 38.7 rad/(T·m) at a wavelength of 193 nm. These results were fitted using the diamagnetic dispersion model and the classical Becquerel formula. The fitted results can be used for the designing of suitable Faraday rotators at various wavelengths. These results indicate the possibility of using MgF2 as Faraday rotators not only in deep-ultraviolet regions, but also in vacuum-ultraviolet regions owing to its large bandgap.journal articl