NIFS-Repository (National Institute for Fusion Science)
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Numerical analysis of deuterium migration behaviors in tungsten damaged by fast neutron by means of gas absorption method
Get PDF0000-0003-0920-2154Deuterium retention behavior in tungsten damaged by fast neutrons at high temperatures (0.43 dpa at 918 K and 0.74 dpa at 1079 K) and 6.4 MeV Fe2+ (0.3 dpa at R.T.) were investigated to evaluate the tritium retention property of fusion reactor divertors. A deuterium gas absorption method was carried out to avoid additional damage that may be induced by plasma exposure, then, deuterium retention and desorption behaviors were investigated quantitatively by means of thermal desorption spectroscopy and the following simulation code. The deuterium desorption spectra for tungsten samples were analyzed by the numerical code which includes the elementary steps of hydrogen isotope migration processes including diffusion, trapping, detrapping, and surface recombination. The evaluated deuterium detrapping energy from the irradiation defects in neutron irradiated tungsten sample was larger than that in 6.4 MeV Fe2+ irradiated tungsten. It was suggested that the dominant deuterium trapping site in the neutron irradiated tungsten would be voids which was formed by the accumulation of vacancies during neutron irradiation under high temperature and long duration.journal articl
Linear MHD analyses of locked-mode-like instabilities in LHD
Get PDFTo investigate the driving mechanism of the locked-mode-like instability observed in the large helical device, we reconstruct the magnetohydrodynamic (MHD) equilibria consistent with the measurement and identify a dominant MHD instability in the precursor phase based on linear MHD analyses. From the dependence of the linear growth rate on the magnetic Reynolds number, the radial mode structure of the electrostatic potential fluctuation and other indices, the ideal interchange mode is found to be dominant. Moreover, it is found that the Mercier parameter, DI, becomes much larger than 0.3 during the phase, while the precursor has constant frequency and fluctuation amplitude. Therefore, DI ≫ 0.3 is a good index of the on-set condition of the minor collapse itself. It is also found that the achievement of DI ≫ 0.3 is due to the movement of the resonant surface to the inner plasma region, which corresponds to the larger pressure gradient region.journal articl
Data-Driven Approach on the Mechanism of Radiative Collapse in the Large Helical Device
Get PDFA radiative collapse predictor has been developed using a machine-learning model based on high-density plasma experiments in the Large Helical Device (LHD). Concurrently, the physical background of radiative collapse was discussed based on the distinct features extracted by a sparse modeling, which is one of the frameworks of data-driven science. Electron density, CIV and OV line emissions, and electron temperature at the plasma edge have been extracted as the key parameters of radiative collapse. Those parameters are relevant to the physical knowledge that the major cause of radiative collapse is the enhancement of radiative loss by light impurities in the plasma-edge region. Using these four parameters, the likelihood of occurrence of radiative collapse has been estimated. The behavior of plasma at the edge—in particular, the carbon impurities outside the last closed flux surface—has been evaluated using EMC3-EIRENE code for the phase with increasing likelihood, that is, the plasma is getting close to the collapse. It is shown that the radiation caused by the C3+ ion, which corresponds to the CIV emission, is enhanced in the region where electron temperature is around 10 eV.journal articl
A new multi-tracer pellet injection for a simultaneous study of low- and mid/high-Z impurities in high-temperature plasmas
Get PDF0000-0003-1682-1519A new multi-tracer technique in the Tracer-Encapsulated Solid Pellet (TESPEL) method has been developed in order to acquire simultaneously the information about the behaviors of various impurities, i.e., to study concurrently the behaviors of low- and mid/high-Z impurities in magnetically confined high-temperature plasmas. In this new technique, an inorganic compound (for example, lithium titanate, Li2TiO3) is proposed to be used as a tracer embedded in the core of the TESPEL, instead of pure elements. The results of the proof-of-principle experiment clearly demonstrate the applicability of the new multi-tracer technique in the TESPEL method for the simultaneous study of behaviors of low- and mid/high-Z impurities in high-temperature plasmas.journal articl
Characterization and qualification of neutron radiation effects – Summary of Japan-USA Joint Projects for 40 years –
Get PDFThe Joint Projects under the Japan-USA Fusion Cooperation Program started in 1981 and has continued for more than 40 years. In the Joint Projects, although a wide range of fusion materials and engineering issues were covered, neutron radiation effects on fusion reactor materials have always been the major research emphases, and the neutron irradiation facilities in the US were jointly used by Japanese and US researchers. Japanese test facilities including neutron and charged particle irradiation facilities were complementarily used.
The initial focus of the Joint Projects was on fundamental fusion neutron radiation effects and irradiation correlation. Systematic comparison of fission and fusion radiation effects in comparable damage levels and the effects of transmutation-induced helium were investigated. The collaboration was then focused on the effect of dynamic irradiation effects in variable conditions. In addition to the relatively fundamental studies, the Joint Projects contributed largely to development of candidate materials such as RAFM steels, vanadium alloys, SiC/SiC composites, and tungsten alloys, through a mechanism-oriented approach. The Joint Projects also covered issues specific to materials application to fusion blankets and plasma-facing components, including neutron radiation effects such as tritium retention and permeation of neutron-irradiated plasma-facing materials. Various irradiation technologies were developed and applied to the irradiation experiments, including those for in-situ testing.
Considering that high energy neutron sources, such as A-FNS and IFMIF-DONES, now have high viability, the research supporting the neutron source programs is essential. The knowledge obtained through the Joint Projects is valuable and should be advanced for this purpose. To this end, it is of urgent necessity to launch an international scientific program accumulating knowledge of fusion neutron radiation effects, including their fundamental aspects.journal articl
Change in the Positron Annihilation Lifetime of Vacancy Clusters Containing Hydrogen Atoms in Electron-Irradiated F82H
No full textThe change in the positron annihilation lifetime (PAL) of vacancy clusters before and after electrolysis hydrogen charging was determined using PAL measurements in electron-irradiated F82H. The experimental change indicated 8 hydrogen atoms were trapped in vacancy clusters; whereas the theoretical calculation resulted in approximately 14 atoms. As the samples were left at room temperature for 5 min until the start of the PAL measurements, the de-trapping effect of hydrogen atoms was also considered; approximately 13 hydrogen atoms were captured at each vacancy cluster. The PAL decreased after annealing at 148 K, which could not be explained theoretically. Therefore, further experiments and discussions are needed to obtain a precise change in the PAL of vacancy clusters containing hydrogen atoms in F82H.journal articl
Detection of Ammonia and Deuterated Hydrocarbons in Exhaust Gas by Infrared Absorption Spectroscopy during Wall Conditioning
Get PDF0000-0001-9941-1958To detect ammonia and deuterated hydrocarbons in exhaust gas from the Large Helical Device (LHD), infrared absorption spectrometry, FTIR with a long optical path gas cell, was applied. Ammonia (NH3) and deuterated hydrocarbons (CxHyDz) could be detected during the first operations of wall baking at 368 K and the D2 glow discharge conducted after vacuum vessel closure. The concentration of ammonia increased with increasing baking temperature, and deuterated ammonia was not detected. Thus, the ammonia, which likely originated from sweat of workers produced during vacuum vessel maintenance activities, was released from the vacuum vessel wall. Hydrocarbons were likely produced by chemical sputtering of carbon tiles and were deuterated by a hydrogen isotope exchange reaction due to D2 glow discharge, while H2O was released from the vacuum vessel during wall baking. It was confirmed that ammonia and various types of deuterated hydrocarbons could be measured discriminately by an FTIR spectroscopy system using a long optical path gas cell.journal articl
Research and Development of SMES for Instantaneous Voltage Drop Compensation
Get PDFSMES (Superconducting Magnetic Energy Storage System) is a power storage technology whose realization has been expected for a long time. If the efficient power storage becomes possible, that will allow to store temporarily the power generation output of renewable energy such as solar power and wind power generation, whose usage expands more and more, and supply it according to the load. It is expected that it can contribute to the stabilization of the energy supply. A major feature of SMES is that compared to other power storage technologies such as secondary batteries and capacitors, the superconducting coil, which is an energy storage unit, does not deteriorate with repeated charging and discharging, even in a short time. A compact system can be realized because the capacity of the storage unit can be designed without the excess or deficiency common for applications that require high output. From this point of view, since 2003, we have developed SMES that compensates for the instantaneous voltage drop, and after field tests, since 2007, we have been conducting a commercial operation of SMES that can output 10 MW for up to 1 second with a track record of continuous operation of 100,000 hours or more. The history of research and development of SMES as an instantaneous voltage drop compensation device, the results of research and development, and the operation results are summarized in this article.journal articl
Electro-optically Q-switched operation of a high-peak-power Tb:LiYF4 green laser
Get PDFWe report on an electro-optically Q-switched Tb:LiYF4 green laser pumped by a frequency-doubled optically pumped semiconductor blue laser. The electro-optically Q-switched characteristics were studied under a wide range of repetition rates from 200 Hz to 50 kHz using a KD2PO4 Q-switch. Up to 198 µJ of pulse energy was obtained with a pulse width of 248 ns at a repetition rate of 200 Hz, corresponding to a peak power of 797 W at 544 nm.journal articl
PIC Simulation of Energetic-ion Injection Effects on Nonlinear Development of Lower Hybrid Wave Instabilities
Get PDF0000-0001-8894-3430By means of one-dimensional, electromagnetic, particle-in-cell simulations, we investigate the nonlinear development of lower hybrid wave (LHW) instabilities driven by energetic ions with a ring-like velocity distribution, paying special attention to the effects of energetic-ion injection. We consider the LHWs propagating perpendicular to the magnetic field in a collisionless plasma into which energetic ions with a speed smaller than the Alfvén speed are continuously injected. We found that the LHWs excited by the energetic ions can maintain large amplitudes for a long time because the continuous injection causes the steep gradient of the energetic-ion velocity distribution. Furthermore, as time advances, the wavenumber of excited modes become larger. Because of this nonlinear development of the LHWs, the energy transfer from the energetic ions to bulk ions through the LHWs is enhanced. As the injection speed increases, a wide wavenumber range of the modes are excited and the bulk-ion energy change increases.journal articl