NIFS-Repository (National Institute for Fusion Science)
Not a member yet
13159 research outputs found
Sort by
Modeling of heating scheme impacts on neon seeded plasmas with the resonant magnetic perturbation application in the large helical device: ECR and NBI heating
Get PDFORCID 0000-0002-9536-9841Plasma parameter profiles along the effective minor radius of flux surfaces are modeled for the conditions with neon impurity puffing and external resonant magnetic perturbations at the plasma edge in the large helical device. For the electron cyclotron heating scenario, calculations qualitatively reproduce stable steady states with plasma detached from divertor target plates and radiation losses exceeding 90% of the heating power. It is unlikely with neutral beam injection heating that the plasma collapses if the radiation approaches a critical level noticeably below 100%. The latter is explained by the dependence of neutral beam attenuation on the plasma ion effective charge.journal articl
Three-dimensional crescent-shaped ion velocity distributions created by magnetic reconnection in the presence of a guide field
Get PDFORCID 0000-0003-2605-5600By means of theory and particle simulations, crescent-shaped ion velocity distributions in the outflow region of symmetric magnetic reconnection in the presence of a guide magnetic field are investigated. Assuming a spatial one-dimensional electromagnetic field, a theoretical model accounting for the shape of crescents is derived. First, following the earlier theoretical models suggested by Bessho et al. [Geophys. Res. Lett. 43, 1828–1836 (2016)] and Zenitani et al. [J. Geophys. Res.: Space Phys. 122, 7396–7413 (2017)], we derive a theoretical model for 2D velocity distributions based on the conservation of the canonical momentum and the energy. The 2D theoretical model exhibits a two-dimensional structure of crescent-shaped velocity distributions and further demonstrates that no magnetic field reversal is required in the formation of crescents, although many researchers have considered that magnetic field reversal plays an essential role. Next, we construct a theoretical model for 3D velocity distributions based not only on the conservation of the canonical momentum in two directions and the energy but also on the conservation of the magnetic moment and the kinetic energy in the moving frame with the reconnection outflow speed, which are applied when the guide field ratio is high. The 3D theoretical model derives a three-dimensional structure of crescents by combination of an antiparallel magnetic field and a guide magnetic field comparable to or greater than the reconnection field. Both of the 2D and 3D theoretical models are consistent with ion velocity distributions found in our particle simulations.journal articl
Correction to ‘Generalization of Hamiltonian mechanics to a three-dimensional phase space’
Get PDFIn a recent paper [N. Sato, Prog. Theor. Exp. Phys. 2021, 6, 063A01 (2021)] we introduced a generalization of Hamiltonian mechanics to three-dimensional phase spaces in terms of closed 3-forms. This correction addresses an error in the proof of theorem 3, which concerns the existence of a coordinate change transforming a closed 3-form into a constant form. Indeed, invertibility of a 3-form is not sufficient to ensure the existence of a solution Xt to eq. (77) when n > 3. The theorem can be corrected by restricting the class of 3-forms to those that are relevant to generalized Hamiltonian mechanics. Although the new theorem requires a stronger hypothesis, the formulation of dynamical systems with 2 invariants in terms of closed 3-forms, which is the key contribution of the paper, holds.journal articl
Degradation of fast-ion confinement depending on the neutral beam power in MHD quiescent LHD plasmas
Get PDFORCID 0000-0003-3293-488XWe investigated the degradation of neutral beam (NB) fast-ion confinement depending on the NB power without magnetohydrodynamics instabilities in the Large Helical Device (LHD). In the LHD deuterium experiment, the neutron emission rate per NB power decreased by up to 20% with increasing injected NBs during a single discharge. Because there were no significant variations in the electron temperature and density, the NB shine-through rate, or the magnetic fluctuation due to the change in NB power, the reduction in the neutron emission rate indicates the degradation of the fast-ion confinement. In this paper, we formulated this degradation depending on the NB power and quantitatively estimated the degraded effective confinement time. In addition, we performed neutron emission rate simulations using the obtained effective confinement time. The simulation and experimental results were in good agreement, suggesting that the degraded effective confinement time is valid.journal articl
Influences of minor Ti addition on microstructure and tensile properties of high-purity V-10Cr alloys
No full textORCID 0000-0001-5706-0221Ti(CON) formation in high-purity V-10Cr-xTi alloys evidently depends on the Ti concentration. Precipitates are not observed in V-10Cr-0.1Ti alloy, whereas precipitation occurs in 0.5 wt% and higher Ti levels, providing similar precipitation strengthening. 2 wt% Ti addition retards the grain growth. Furthermore, both yield stress and ultimate tensile stress at room temperature and 973 K are steadily increased with rising Ti concentration, and elongation does not reveal clear dependence on Ti concentration. In comparison, lower Ti addition is more beneficial to high-temperature strength than that of room temperature. Additionally, V-10Cr-0.5Ti, V-10Cr-1Ti, and V-10Cr-2Ti alloys would be alternatives to the high-purity V-4Cr-4Ti alloy from the viewpoint of comparable strength and improved low-activation characteristics.journal articl
Generating optical vortex beams using cylindrical waveguides
No full textORCID 0000-0002-2757-6638As an initial step toward exciting optical vortices using high-power millimeter waves, this study developed a method for inducing vortex modes within a cylindrical waveguide using Gaussian beams. By examining the coupling between tilted and offset Gaussian beams and specific waveguide modes, appropriate tilt angles and offsets were obtained. Numerical simulations demonstrated that the vortex modes could be efficiently excited using four Gaussian beams. Furthermore, the results revealed that increasing the number of Gaussian beams improves the excitation efficiency of vortex modes.journal articl
A scintillating-fiber detector for making high-time-resolution secondary D–T neutron measurements in KSTAR
Get PDFORCID 0000-0003-4555-1837A scintillating fiber (Sci-Fi) detector for the middle neutron flux range was installed in KSTAR as part of a collaboration between the National Institute for Fusion Science and the Korea Institute of Fusion Energy. The detector could make relatively high-time-resolution measurements of secondary deuterium (D)–tritium (T) neutron fluxes to investigate the degradation of D–D-born triton confinement, which is crucial for demonstrating alpha particle confinement, particularly above 0.9 MA in KSTAR. The pulse-height spectrum of the Sci-Fi detector exhibited two peaks, the higher of which corresponded to D–T neutrons. A discrimination technique was applied to extract the D–T neutron signal, revealing the time evolution of the D–T neutron flux during relatively high plasma current discharges with a 50 ms temporal resolution. Future research will involve investigating the causes of the degradation of the triton burnup ratio above 0.9 MA in KSTAR.journal articl
Neutron Spectroscopy in Perpendicular Neutral Beam Injection Deuterium Plasmas Using Newly Developed Compact Neutron Emission Spectrometers
Get PDFORCID 0000-0002-0160-0468The presence of helically trapped fast ions in helical ripples in the large helical device (LHD), resulting from a perpendicularly injected positive-ion-source-based neutral beam (P-NB)-heated plasma and/or from ion cyclotron range of frequency (ICRF) wave-heated plasma, poses a significant concern due to the system’s lack of symmetry. In response to this challenge, compact neutron emission spectrometers (CNESs) have been strategically developed in the LHD, featuring a perpendicular line of sight relative to the plasma’s magnetic field. The first perpendicular CNES offered a vertical field of view through the deuterium plasma and incorporated a liquid (EJ-301) scintillation detector optimized for efficient operation at a high neutron emission rate. The second perpendicular CNES was operated using a Cs2LiYCl6:Ce scintillation detector enriched with 7Li (CLYC7), providing a horizontal field of view through the deuterium plasma and effectively operating in a region characterized by relatively low neutron emission rates. During plasma heating through P-NB, both perpendicular CNESs showed a neutron energy distribution, featuring a double-humped profile with two peaks at energy of approximately 2.30 and 2.74 MeV. Furthermore, the deuterium-deuterium neutron energy spectrum expected to be obtained by perpendicular CNES was calculated using the 5-D orbit following code DELTA5D and considering the Larmor motion effect. The results revealed that the experimentally obtained neutron energies were almost consistent with the neutron energies calculated based on the orbit following model. These peaks aligned with the Larmor motion of deuterons resulting from P-NB injection at the helical ripple of the LHD. In addition, we conducted neutron spectroscopy involving deuterium-deuterium interactions in a deuterium plasma simultaneously heated by both P-NB and ICRF wave, characterized by a high neutron emission rate, using perpendicular CNES based on EJ-301. The observation revealed a broadening in spectrum width, attributed to the additional ICRF wave heating.journal articl
Convergence study of multi-field singular value decomposition for turbulence fields
Get PDFORCID 0000-0001-5695-5699Convergence of a matrix decomposition technique, the multi-field singular value decomposition (MFSVD) which efficiently analyzes nonlinear correlations by simultaneously decomposing multiple fields, is investigated. Toward applications in turbulence studies, we demonstrate that SVD for an artificial matrix with multi-scale structures reproduces the power-law-like distribution in the singular value spectrum with several orthogonal modes. Then, MFSVD is applied to practical turbulence field data produced by numerical simulations. It is clarified that relative errors in the reproduction of quadratic nonlinear quantities in multi-field turbulence converge remarkably faster than the single-field case, which requires thousands of modes to converge.journal articl
Digital-LHD: LHD Visualizer for Head-Mounted Display Systems
Get PDFWe developed a virtual reality visualization program, Digital-LHD, for modern Head-Mounted Display (HMD) systems. Digital-LHD provides an immersive environment for the interactive visualization of plasma data related to the Large Helical Device (LHD). Its predecessor, Virtual-LHD, was developed for CAVE systems, which were limited in accessibility due to their cost and size. The newly developed Digital-LHD utilizes Unity and C# to offer interactive 3D visualization. Digital-LHD includes features such as isosurface rendering, magnetic field line tracing, particle trajectory visualization, and new functionalities like local arrow glyphs and a plasma pressure color contour. The enhanced interactivity and intuitive GUI have improved the user experience. Digital-LHD enables fusion plasma researchers to perform immersive visualizations with HMDs, providing a more accessible and cost-effective solution.journal articl