NIFS-Repository (National Institute for Fusion Science)
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Experimental observations and modelling of radiation asymmetries during N2 seeding in LHD
Get PDF0000-0002-4446-4603N2 gas has been seeded in the Large Helical Device (LHD) to reduce the divertor heat load through enhanced radiation. Radiation is observed by two imaging bolometers, viewing the same poloidal cross-section from top and bottom ports, at a location which is 36° toroidally removed from the N2 gas puff nozzle located at the bottom of the machine. During N2 seeding, these measurements both confirm that additional radiation from the outboard side is coming exclusively from the top of the cross-section, indicating up/down asymmetry, which is also reproduced by modelling with EMC3-EIRENE using a half torus model. In addition, a toroidally localized, magnetic field direction-dependent radiation enhancement is observed with N2 seeding, but is not reproducible by the model.journal articl
NIFS Atomic and Molecular Numerical Database for Collision Processes
Get PDF0000-0001-7544-1773The National Institute for Fusion Science (NIFS) has compiled and developed atomic and molecular numerical databases for various collision processes and makes it accessible from the internet to the public. The database contains numerical data of cross sections and rate coefficients for electron collision or ion collisions with atoms and molecules, attached with bibliographic information on their data sources. The database system provides query forms to search data, and numerical data are retrievable. The graphical output is helpful to understand energy dependence of cross sections and temperature dependence of rate coefficients obtained by various studies. All data are compiled mainly from published literature, and data sources can be tracked by the bibliographic information. We also have data of sputtering yields and back-scattering coefficients for solid surfaces collided by ions in the database. All data in the database are applicable to understand atomic and molecular processes in various plasmas, such as fusion plasma, astrophysical plasma and applied plasma, as well as for understanding plasma–surface interaction in plasmas.journal articl
Simulation Study of Trapped Electron Effects on Positron Acceleration by a Shock Wave in an Electron–Ion–Positron Plasma
Get PDF0000-0003-2691-5068Positron acceleration by a shock wave in an electron–ion–positron plasma is studied with two-dimensional (2D), relativistic, electromagnetic particle simulations with special attention to effects of electrons trapped in the shock wave. It is demonstrated that ultrarelativistic positron acceleration can occur even when the fluctuations along the shock front, which we call 2D fluctuations, excited by the trapped electrons grow to large amplitudes. Further, by comparing the results of the simulation and the calculation of test positrons not influenced by the 2D fluctuations, we show that the positrons influenced by the 2D fluctuations are accelerated to higher energies than the positrons not influenced by them. In addition to the parallel electric field formed in the shock transition region, the perpendicular electric field in the shock wave greatly contributes to the ultrarelativistic positron acceleration under the influence of the 2D fluctuations. Effects of the accelerated positrons on the 2D fluctuations are also discussed.journal articl
Characteristics of plasma parameters and turbulence in the isotope-mixing and the non-mixing states in hydrogen–deuterium mixture plasmas in the large helical device
Get PDF0000-0002-0585-4561Characteristics of plasma parameters and turbulence in the isotope-mixing and the non-mixing states in hydrogen-deuterium mixture plasmas in the large helical device are discussed. The isotope mixing state is characterized by the uniform isotope ratio profile regardless of the location of the particle source of each species in the isotope mixture plasma. The isotope non-mixing state is identified by the non-uniform isotope ratio profile measured with bulk charge exchange spectroscopy when the beam fueling isotope species differs from the recycling isotope species. The effect of collisionality, ratio, sign of density gradient on transition between isotope mixing and non-mixing is discussed. The plasma parameters preferable for the non-mixing state are found to be lower collisionality, higher , and negative or zero density gradient (peaked or flat density profile). The time scale of transition from non-mixing to mixing is evaluated by the hydrogen and deuterium pellet injection near the plasma edge and is found to be less than 5 ms, which is much shorter than the particle confinement time. The strong correlation between isotope mixing and turbulence characteristics is observed. This strong correlation suggests the change in turbulence is a strong candidate for the mechanism causing the transition between uniform and non-uniform isotope density ratio profiles.journal articl
Divertor Detachment with Multi-Species Impurity Seeding in LHD
Get PDF0000-0003-1586-1084Divertor detachment using higher-Z (Kr) and/or lower-Z (Ne) is investigated. Using Kr+Ne superimposed seeding, plasma radiation could be enhanced at the upstream region in the edge plasma with suppression of impurity accumulation toward the core plasma. Here, the pre-seeded Kr emission was drastically enhanced after the subsequent Ne seeding. Moreover, the detachment using Kr+Ne seeding could be stably sustained while the detachment using single species seeding is short-lived. Reduction of edge electron temperature due to Ne seeding can promote the Kr emission at the upstream region. It indicates the availability of multi-species impurity seeding with different cooling rate.journal articl
Radiation Resistant Camera System for Monitoring Deuterium Plasma Discharges in the Large Helical Device
Get PDF0000-0003-0655-7347Radiation resistant camera system was constructed for monitoring deuterium plasma discharges in the Large Helical Device (LHD). This system has contributed to safe operation during two experimental campaigns without serious problems due to radiation (neutrons and gamma-rays). The cameras steadily functioned even in the plasma discharge with the maximum neutron emission rate in FY 2017, though some bright specks temporarily appeared on the images. The cameras have been installed in shield boxes which consist of lead boxes covered with 10% borated polyethylene blocks in all directions. For optimizing the design of the shield box, the radiation flux distribution was calculated by MCNP-6 code, which reveals the reduction of the radiation flux and the change of the energy spectra in the shield box. Thanks to the optimization, significant extension of the lifetime of the cameras has been realized. Investigation of the influence of the radiation on the CCD image sensor shows that the number of bright specks generally increases with the radiation flux to the camera, which also indicates that some bright specks disappear by the self-annealing process on the image sensor. This phenomenon also highly contributes to the further extension of the lifetime of the radiation resistant cameras.journal articl
Development of a Compact Hard X-Ray Camera on the TST-2 Spherical Tokamak
Get PDF0000-0002-4486-7437A compact hard X-ray camera has been developed to study the behavior of fast electrons accelerated by lower hybrid waves in the TST-2 spherical tokamak. The camera consists of a pinhole made of tungsten alloy, a thin resin window, a thin LYSO scintillator disk, 5 fiber optic light guides and 5 photomultiplier tubes and shields. The camera is installed on a tangential port, and two-dimensional images are obtained by rotating the system around the axis of the port on a shot by shot basis. The typical energy range is 10 - 250 keV, and the energy resolution is about 100% (FWHM) at 100 keV. The measured images of a lower hybrid wave driven plasma show bright region at the inboard side (i.e., high field side).journal articl
Optimization of finite-sized modular coils for advanced stellarators
Get PDF0000-0002-7635-2905To date, almost all coil-design codes, e.g. NESCOIL, COILOPT, FOCUS codes, etc, have been primarily attributed to the optimization of filament coils for stellarators. However, evolving to a practical/finite-sized coil from a filament coil, the finite-size effect of coils significantly constrains the fabrication tolerances of a coil system. This paper presents a novel approach that emphasizes the optimization of practical modular coils to reduce sensitivity to fabrication tolerances and to achieve the expected magnetic configurations precisely. A new evaluation parameter, surface twist, is defined in this paper and applied to the optimization sequence in addition to the practical coil line torsion and curvature. The approach has been applied to the framework of the filament coil scheme in the Chinese first quasi-axisymmetric stellarator. This practical coil system without surface twists has been accomplished. Compared to the original finite-sized coil design, the new result is a more considerable simplification of coil shapes, such that in a certain direction view each finite-sized coil becomes a planar-like one. Moreover, this method can also be implemented for the estimation of stochastic deviations of practical coils during the fabrication and assembly of the coil system.journal articl
Microstructure and Retention in He-W Co-Deposition Layer
Get PDFA tungsten (W) deposition layer is formed while a sample is exposed to a helium (He) plasma at the surface temperature of 473 or 573 K. The formed He-W co-deposition layer was composed of fine grains, the size of which increased with the surface temperature. The samples with the co-deposition layer was exposed to a deuterium (D2) plasma, and the D retention was investigated using thermal desorption spectroscopy. It was found that the co-deposition layer has quite different D retention characteristics compared with bulk W.journal articl
Dynamics of Hydrogen Isotope Absorption and Emission of Neutron-Irradiated Tungsten
Get PDFThis overview presents recent results regarding hydrogen isotope absorption and emission dynamics in neutron-irradiated tungsten (W) using our recently developed Compact Diverter Plasma Simulator (CDPS), a linear plasma device in a radiation-controlled area. Neutron irradiation to 0.016 - 0.06 displacement per atom resulted in a significant increase in deuterium (D) retention due to trapping effects of radiation-induced defects. We analyzed the dependency of D retention on the D plasma fluence by exposing neutron-irradiated pure W to D plasma at 563 K over a range of D fluence values. The total retention was revealed to be proportional to the square root of D fluence, indicating that the implanted D atoms first occupy the defects caused by neutron-irradiation near the surface and then the defects located in deeper regions. We further investigated the effects of post-plasma annealing on D emission; neutron-irradiated pure W was exposed to D plasma at 573 K and was then annealed at the same temperature for 30 hours. Approximately 10% of the absorbed D was released by annealing, suggesting that a heat treatment of the plasma-facing component of a fusion reactor at moderately elevated temperatures could contribute to the removal of accumulated hydrogen isotopes. The experimental results obtained in this study were only available by investigating neutron-irradiated specimens with the CDPS system, which will be essential for future studies of material behavior and plasma-wall interactions in the fusion reactor environment.journal articl