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Glow Discharge Boronization and Real-Time Boronization Using an Impurity Powder Dropper in LHD
Full text linkORCID 0000-0003-0161-0938In the Large Helical Device (LHD), boronization is utilized for wall conditioning, employing two distinct methods: glow discharge using diborane gas and the injection of boron powder into the plasma via an impurity powder dropper (IPD). Glow discharge boronization is conducted after maintenance when the vacuum vessel is exposed to air and just before the start of an experimental campaign. In contrast, real-time boronization using the IPD is performed during discharges. The distribution of boron on plasma-facing components was assessed through long-term material probes in the case of glow discharge and through numerical simulations for the IPD method. Results indicated non-uniform boron distributions in both cases. The impact of these boronization techniques on impurity levels and the lifetime of their effects were also examined. Both methods effectively reduced oxygen and iron in the plasma, with the iron impurities originating from the first wall. For glow discharge boronization, the reduction in oxygen levels persisted until the end of the experimental campaign, whereas the reduction in iron levels only lasted a few days. In the case of boron injection during long pulse discharges exceeding 200 s, the suppression of oxygen and iron was observed. The effect on iron quickly diminished within several seconds after the injection ended, whereas the effect on oxygen lasted more than 100 s. The difference in the lifetimes of the boronization effects is further discussed.journal articl
Time-resolved deuterium–deuterium fusion born 1 MeV triton confinement study in EAST deuterium plasma
Full text linkORCID 0000-0003-4555-1837A time-resolved deuterium–deuterium (D–D) fusion-born triton confinement study, aimed at understanding alpha particle confinement ability, was performed in Experimental Advanced Superconducting Tokamak (EAST) deuterium plasmas for the first time. A scintillating fiber detector was developed for measuring the secondary deuterium–tritium (D–T) neutrons, which provide evidence of triton slowdown, in EAST. The D–D fusion-born triton confinement experiment was performed by measuring secondary D–T neutrons in D-beam-heated D plasma with a plasma current of 400 kA. The secondary D–T neutron signal and its time evolution were obtained using pulse height discrimination analysis. The D–T neutron rate was calculated using the classical energetic ion confinement model to clarify the D–T neutron measurements. The secondary D–T neutron emission rate obtained from the numerical simulation closely agrees with the experimentally obtained results when considering the prompt loss of the tritons.journal articl
Neutron irradiation effect on superconductivity of Nb3Sn wire - 50 Hz data acquisition system -
Full text linkA research facility for neutron irradiation effect on superconducting materials has been installed at Oarai center in Tohoku University in 2012 as part of a post irradiation experiment. It consists of a 15.5 T superconducting magnet, a variable temperature insert and a data acquisition and control system. Since the sample holder is cooled by thermal conduction with G-M refrigeration, there is a small temperature difference between positive and negative electrodes and the sample temperature rises during the critical current test due to joule heating. To evaluate the temperature rise precisely, a new data acquisition system with a sampling rate of 50 Hz has been installed. By measuring the temperatures of both electrodes and the voltage at the center of the sample, the temperature of the sample was estimated and the measured critical current (IC) was converted to the current at 4.2 K with a critical surface equation. The comparison of IC at different conditions including neutron irradiated samples was implemented and the neutron irradiation effect on superconductivity of Nb3Sn wire is discussed based on the 4.2 K superconductivity. This paper will describe the outline of the new data acquisition system and some results of neutron irradiated Nb3Sn wires.journal articl
Isotope effects on transport characteristics of edge and core plasmas heated by neutral beam injection (NBI) in an inward shifted configuration at the Large Helical Device
Full text linkORCID 0000-0002-8504-965XIsotope effects have been investigated in Neutral Beam Injection (NBI) heated plasmas on the Large Helical Device with similar operational parameters between Hydrogen (H) and Deuterium (D) plasmas. Experimental results show that the global energy confinement has no significant dependence on the isotope mass under similar discharge conditions with nearly the same heating power, line-averaged density (¯nₑ) and magnetic field. For both electron and ion energy transport, the transport coefficients, which are obtained based on local power balance analysis, have analogous profiles between H and D dominant plasmas. For neoclassical χₑ and χi values, they are almost equal between H and D dominant plasmas in low ¯nₑ discharges, whereas in high ¯nₑ cases they are lower in H plasmas than those in D ones. At low ¯nₑ, the electron and ion thermal transport in both H and D plasmas are dominated by neoclassical transport at a certain zone (ρ 0.6 0.85), while the anomalous transport process has primary effects in the remaining area, and the density fluctuations exhibit ion temperature gradient mode nature. With increase of n , the anomalous transport becomes prevailing and the density fluctuations propagate along electron diamagnetic drift direction. Bispectral analysis reveals that the H plasma hasresearch repor
New Silica Removal Technique by Vacuum Heating toward High-Performance Cryosorption Pumps Based on Biomass-Based Activated Carbon
Full text linkWe developed a new silica (SiO2) removal technique that can maximize biomass-based activated carbon adsorption performance. SiO2 removal is one of the key processes in making activated carbon suitable for cryosorption pumps in fusion machines. In this study, we employed an evaporation process to remove SiO2 by high-temperature vacuum heating. The charcoal made from rice straw was heated at 1800℃ for 1 h at approximately 10 Pa in a vacuum furnace. We found that SiO2 amount was significantly reduced from 15.8 wt% to 4.20 wt% due to vacuum heating. In addition, the result of surface element mapping analysis using energy-dispersive X-ray spectroscopy (EDX) indicated a considerable decrease in the oxygen content of SiO2. We demonstrated the great potential of activated carbon derived from rice straw as an adsorbent for high-performance cryosorption pumps.journal articl
The multi-sightline compact D-D neutron spectrometers based on CLYC7-scintillator for beam ion anisotropy study in LHD
Full text linkORCID 0000-0002-2771-6979Three compact neutron emission spectrometers (CNESs) based on the novel inorganic scintillator Cs2LiYCl6:Ce with 99% 7Li-enrichment (CLYC7) were newly installed in different lines of sight in the Large Helical Device (LHD) to study the anisotropy of the velocity distribution of the beam ions. Neutron energy spectra measurements using multi-sightline CNESs were performed under neutral-beam (NB)-heated deuterium plasma. A significant Doppler shift of the deuterium-deuterium (D-D) neutron energy induced by different NB heating and different lines of sight was observed. The measured peak neutron energies were nearly consistent with those calculated by the simple two-body kinematics of the D-D reaction using the tangency radii of CNESs and NBs. These results demonstrated the capabilities of the CNESs for neutron energy spectra measurements and the study of fast-ion physics in magnetic confinement fusion research.journal articl
Observation of energetic ion anisotropy using neutron diagnostics in the Large Helical Device
Full text linkORCID 0000-0003-4555-1837Energetic ion anisotropy was observed by tangential sightline compact neutron energy spectrometers (CNESs) in tangential neutral beam heated deuterium plasmas in Large Helical Device. Significant upper and lower energy shifts in D–D neutron energy from 2.45 MeV were measured according to the beam ion injection directions and CNES sightline using a conventional liquid scintillation detector with the unfolding technique and a novel Cs2LiYCl6:Ce with a 7Li-enrichment (CLYC7) scintillation detector without unfolding. The observed neutron energy spectrum was compared with that predicted by a numerical simulation based on orbit following models. Numerical simulation revealed that the Doppler shift in D–D neutron energy results from energetic ion anisotropy.journal articl
Observation of tungsten emission spectra up to W46+ ions in the Large Helical Device and contribution to the study of high-Z impurity transport in fusion plasmas
Full text linkORCID 0000-0002-1171-8603Spectroscopic studies of emissions released from tungsten ions combined with a pellet injection technique have been conducted in Large Helical Device for contribution to the tungsten transport study in tungsten divertor fusion devices and for expansion of the experimental database of tungsten line emissions. The spectral intensities of W5+, W24+–W28+, W37+, W38+, W41+–W43+, W45+, and W46+ emission lines were measured simultaneously over a wide wavelength range from x-ray to visible. Time evolutions of the various tungsten line spectra indicate that the tungsten confinement time depends on the electron density of the plasma and is long in high density plasmas, on the order of seconds, and short in low density plasmas, on the order of sub-seconds. When the confinement time was long, the tungsten ions remained in the plasma until the end of the discharge, changing their dominant charge with the change in electron temperature. When the confinement time was short, the tungsten ions rapidly decreased in all charge states and disappeared. Space-resolved EUV and visible spectroscopy measurements have revealed that tungsten ions stayed in the core region of the plasma with changing their dominant charge state depending on the electron temperature in the discharges with the long confinement time. Detailed analysis of soft x-ray emission suggested that the confinement time increases with density and becomes saturated when the central electron density exceeds 2 × 1013 cm−3.journal articl
Design and initial results of the imaging neutral particle analyzer in large helical device
Full text linkORCID 0000-0002-0801-5106A novel Imaging Neutral Particle Analyzer (INPA) was newly installed in early 2024 to enhance the understanding of fast ion confinement on Large Helical Devices (LHDs). This diagnostic system, based on a magnetic spectrometer using a scintillator, provides energy-resolved radial profiles of confined fast ions by measuring charge-exchanged fast neutrals escaping from the plasma. The system utilizes a 100 nm thick carbon foil to ionize the fast neutrals, subsequently deflecting the ions toward a scintillator via the existing local magnetic field. The fast ion energy and sightline determine the position of the scintillation, while the light intensity depends on the flux of the fast ions. The INPA features two apertures, facilitating effective measurements in both clockwise and counterclockwise magnetic field directions in the LHD. This INPA was designed as a passive measurement system that measures fast ions charge exchange with background neutrals, focusing on perpendicular beam ions from 5 to 100 keV with an energy resolution of about 5.75 keV. This paper describes the details of the design, installation, and the initial results of the INPA on the LHD. This work will contribute significantly to a deeper understanding of fast ion transport due to magnetohydrodynamic instabilities.journal articl
Reactive molecular dynamics simulation on DNA double-strand breaks induced by hydrogen elimination
Full text linkORCID 0000-0002-0593-8810We propose a scar model to simulate double-strand breaks (DSBs) in telomeric DNA induced by the β-decay of tritium into helium. The two hydrogens bonded to the carbon connecting the pentasaccharides and phosphate are removed in this scar model. Molecular dynamics simulations using a reactive force field were conducted across ten cases involving telomeric DNA composed of 16 base pairs (32 nucleotides). DSBs are observed in structures with more than 24 scars, whereas only single-strand breaks (SSBs) occur in the case with 16 scars. Moreover, in the case of {16,0} and {0,16}, where only one of the strands had scars, SSB occurs only in the scarred strand. Second, in the {16,8} and {8, 16} cases, DSBs occur. Thus, we conclude that the following conditions are necessary for DSBs: (i) scars must be present on both the left (L) and right (R) strands and (ii) a substantial number of scars (24 or more) must be in close proximity.journal articl