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Impact of material-dependent energy reflection coefficient on divertor heat flux in the Large Helical Device
No full textA divertor module comprising actively-cooled tungsten (W) and carbon (C) materials placed side-by-side was simultaneously exposed to the divertor plasma in the Large Helical Device (LHD) to experimentally observe the difference in energy reflection coefficients, iE. The divertor temperatures were measured using infrared (IR) thermography and embedded thermocouples (TCs) and were converted into heat fluxes using the finite element method (FEM). The emissivity of the W surface was evaluated using a calibration chamber. This study demonstrated that combining TC measurements and IR thermography with FEM analysis enabled accurate evaluation of the heat flux on the W even in low-temperature regions, where IR thermography is susceptible to reflection-induced overestimation. The experiment showed that the peak heat flux on the C surface reached approximately 10 MWm−2, while that on the W surface was ∼ 75% of this value. Furthermore, the theoretical heat fluxes were estimated using plasma parameters obtained from Langmuir probes, combined with iE from an empirical formula. The observed ratio of heat fluxes between W and C was in good agreement with theoretical predictions based on material-dependent iE. These results quantitatively demonstrate that the higher iE of W can effectively reduce the absorbed heat flux by 20 to 30% compared to that with C. This finding offers important insights for the selection of plasma-facing materials and the optimization of cooling system designs.journal articl
Corrosive behavior of structural F82H RAFM steel by LTZO ceramic breeder pebbles
Full text linkORCID 0000-0001-8067-8732In this study, corrosion behavior of F82H reduced activation ferritic/martensitic steel (RAFM) by LTZO (Li2+xTiO3+y solid solution with 20 wt% Li2ZrO3) ceramic breeder pebbles was investigated at 773–998 K in an inert sweep gas condition (Ar + 0.1 % H2). Due to vapor gas release from the breeder pebbles and those penetrations, corrosion layer formed on the surface of the F82H steel. Glow discharge optical emission spectroscopy (GD-OES) and X-ray diffraction (XRD) identified the corrosion products as cubic, spinel, and rhombohedral Li–TM–O (TM: transition element in F82H such as Fe, Cr, and Mn). The growth of the corrosion layer thickness followed a parabolic curve at 833 K, yielding apparent diffusion coefficient of D = 6.95 × 10–13 cm2/s. Rapid growth was observed at 993 K after a parabolic growth which could be triggered by failure of the protective layer. A comparative analysis indicates a predominant effect of humidity and oxygen in the sweep gas on the growth rate, while the composition and shape of breeding materials have minor impacts.journal articl
Spectral line identification for highly charged heavy ions in soft X-ray and EUV regions based on Z-dependence measurements
No full textORCID 0000-0001-6536-9034This study focuses on new identifications of discrete spectral lines of heavy ions with one or two outermost 4s/4p subshell electrons in soft X-ray and extreme ultraviolet regions. The study is basically based on the spectroscopic data systematically taken in the last decades in high-temperature plasmas produced in the Large Helical Device using multiple grazing incidence spectrometers. In addition, we have newly taken spectroscopic data using the Tokyo electron beam ion trap (EBIT) for some of the missing elements in the past EBIT experiments. Consequently, we could identify a number of discrete lines experimentally for the first time in this study based on the detailed atomic number (Z)-dependences of the wavelengths of major resonance lines. Some transitions of Ga- and Ge-like ions show irregular Z-dependences in which the transition wavelength suddenly shifts to another curve at around a specific Z due to the effect of strong configuration interaction.journal articl
Irradiation Effects of As-Fabricated and Recrystallized 12Cr ODS Steel Under Dual-Ion Beam at 973 K
Full text linkORCID 0000-0001-5706-0221The microstructure evolution and hardness variations of as-fabricated and recrystallized 12Cr oxide dispersion strengthened (ODS) steel after dual-ion (6.4 MeV Fe3+ and energy-degraded 1 MeV He+) irradiation at 973 K up to 10.6 displacements per atom (dpa) at peak damage and 8900 appm He are investigated. Results show that the oxide particles slightly shrink in the as-fabricated specimen, while they are stable in the recrystallized specimen. Furthermore, larger helium bubbles are trapped at the grain boundaries in the as-fabricated specimen, and the size of helium bubbles in the grains is almost the same for both as-fabricated and recrystallized specimens, indicating that reduction of grain boundaries would reduce the potential nucleation sites and suppress the helium segregation. Moreover, no obvious hardening occurs in the as-fabricated specimen, whereas the hardness increases a little in the recrystallized specimen. Based on the barrier model, the barrier strength factor of helium bubbles is calculated. The value is 0.077, which is much smaller and suggests that helium bubbles seem not to significantly induce irradiation hardening.journal articl
In-situ calibration of charge exchange spectroscopy for spatially resolved measurements of helium-hydrogen density ratio in Wendelstein 7-X
Full text linkIn-situ calibration of charge exchange spectroscopy for spatially resolved measurements of helium-hydrogen density ratio is described in this paper. The helium-hydrogen density ratio in the core plasma is derived from the intensity ratio of the active helium line HeII (468.6 nm) and hydrogen line HI (656.3 nm) due to the charge exchange recombination process between fully ionized ions and neutral beam. This system is calibrated by the helium-hydrogen density ratio evaluated from HeII (656.0 nm) and HI (656.3 nm) in the recombining phase at the end of the discharge. The helium-hydrogen density ratio at the plasma center derived from the calibrated charge exchange spectroscopy is compared with the influx ratio estimated from the passive spectroscopy.journal articl
Investigation of island size effect on radiation distribution during attached and detached plasmas in the island divertor of W7-X
Full text linkORCID 0009-0002-8230-0121Mitigation of heat on the first wall through divertor operation is a key to a successful future fusion reactor. W7-X employs an island divertor to control the exhaust and heat load on the plasma impacting divertor plates. Increased radiation in the divertor reduces the heat load at the plasma contact point during detachment. In this paper we investigate the distribution of the radiation using an InfraRed imaging Video Bolometer (IRVB) that views the divertor region in two dimensions giving information on both the poloidal and toroidal variation of the radiation in comparison to conventional resistive bolometer arrays that typically only give poloidal variation information. Experiments were carried out using a standard magnetic configuration modified by changing control and planar coil currents to achieve three different island sizes without changing the strike line location. For each island size low and high density (ne = ∼4 and ∼ 7 x 1019/m3, respectively) plasmas were created with ∼ 2 MW of ECH input power, which correspond to attached and detached plasmas with radiated power fractions (frad) of ∼ 20–25 % and ∼ 90 %, respectively.
Results indicate an increase in density led to an increase in the IRVB radiation signals as seen in the total radiated power (and frad) and a slight broadening in the signals indicating less radiation from the target locations, especially the lower right location in the IRVB field of view when compared with the corresponding thermography images. However, no noticeable difference in the IRVB radiation pattern or intensity is seen with the change of the island size.journal articl
Metriplectic bracket for guiding-center Vlasov–Maxwell–Landau theory
No full textThe metriplectic formulation of collisional guiding-center Vlasov–Maxwell–Landau theory is presented. The guiding-center Landau collision operator, which describes collisions involving test-particle and field-particle guiding-center orbits, is represented in terms of a symmetric dissipative bracket involving functional derivatives of the guiding-center Vlasov phase-space density Fgc and the electromagnetic fields ðDgc; BÞ, where the guiding-center displacement vector Dgc E þ 4pPgc is expressed in terms of the electric field E and the guiding-center polarization Pgc. This dissipative Landau bracket not only conserves guiding-center energy–momentum and angular momentum, but it also satisfies a guiding-center H-theorem.journal articl
Microscale Turbulence Measurements Using Doppler Reflectometer and millimeter-wave Scattering System
No full textORCID 0000-0001-5473-2109To study microscale turbulence, two non-invasive scattering instruments that use electromagnetic waves in the microwave to millimeter-wave range have been installed at the Large Helical Device. One instrument is a Doppler reflectometer, which is suitable for observing turbulence with relatively low wavenumbers. Three circuit systems were constructed for this instrument. The Doppler reflectometer allows a very large number of spatial points (more than 30) to be observed simultaneously in the radial direction and toroidal correlation analysis to be conducted. The other instrument is a two-frequency millimeter-wave scattering system, which was developed to observe turbulence at relatively high wavenumbers. This scattering system has multiple antennas in a vacuum vessel. It can be used to study turbulence anisotropy or, in combination with the Doppler reflectometer, the response of turbulence at various scales.journal articl
Observation of a Fast-Ion Profile Stiffness Due To the Alfvén Eigenmode
No full textFast-ion transport driven by Alfvén eigenmodes (AEs) is one critical issue facing fast-ion confinement in magnetic fusion device. In the DIII-D tokamak experiment, stiff transport of fast-ions increased with increasing neutral beam (NB) injection power when the amplitudes of multiple interacting AEs exceeded a certain threshold. These experiment results are supported by simulation studies that predict monotonically degrading fast-ion confinement and profile stiffness with increasing beam power. To investigate the universality of the fast-ion profile stiffness dependence on AE amplitude, an experiment was performed at the Large Helical Device (LHD) to scan the injection current of the NB and vary the AE amplitude. Under the experimental conditions, the AE amplitude increased linearly with NB injection power. The red shifted FIDA intensity between 663 and 665 nm, corresponding to the energy range of 98–166 keV in the ctr-direction, was used for estimating the radial profile of the fast-ion density. Evidence suggests stiffening of the fast-ion profile and degraded confinement, corroborated by a reduced neutron emission rate compared to simulations. This is consistent with the experimentally observed reduction in the expected neutron emission rate. We have demonstrated that under AE-prone confinement conditions, even if the fast-ion source increases due to NB injection, they experience enhanced transport by AEs and do not increase in density.journal articl
Engineering design and manufacturing of the modular coil system for the quasi-axisymmetric stellarator CFQS-T
Full text linkThe Quasi-axisymmetric Stellarator CFQS has been constructed as an international joint project between the National Institute for Fusion Science in Japan and Southwest Jiaotong University in China. Its physical properties are as follows: toroidal periodic number m = 2, aspect ratio Ap = 4, maximum magnetic field strength Bt= 1 T, and major radius of magnetic axis in vacuum R0 = 1 m. The CFQS employs four different types of 16 modular coils to realize a quasi-axisymmetric magnetic field configuration. The CFQS plasma experiment is planned to have two stages. The first is a 0.09 T long-pulse operation with a simplified modular coil support structure referred to as “CFQS-T”. The second is a 1 T short pulse rated operation known as “CFQS” after reinforcing the modular coil system's support structure against a large electromagnetic force. This paper reports on the finalized engineering design of the modular coil system with simplified coil support structures for CFQS-T, including a validity evaluation result with finite element analysis method software ANSYS. Furthermore, the established manufacturing method of the modular coil system for CFQS-T is described. All modular coils for CFQS-T have been manufactured and its main body assembly, with modular coils and a vacuum vessel, has been completed. Currently the commissioning, including a coil energization test and magnetic field line mapping, is ongoing.journal articl