Search

The current status and the progress of R&D activities for a Fusion DEMO reactor in the QST Rokkasho Fusion Institute is reported. In order to advance the JA DEMO activity, not only Japanese domestic activity but also international collaborations of Broader Approach Activity and ITER related activities are conducted in the QST Rokkasho Fusion Institute. Activities for DEMO design and relevant R&D, design of a fusion neutron source and development of an accelerator, ITER-TBS, tritium handling technology and information technology infrastructures including a supercomputer system and a remote experimentation system are carried out for a Fusion DEMO reactor.

We present long-term density trends of the Earth's upper atmosphere at altitudes between 71 and 116 km, based on atmospheric occultations of the Crab Nebula observed with X-ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR, and Hitomi. The combination of the five satellites provides a time period of 28 years from 1994 to 2022. To suppress seasonal and latitudinal variations, we concentrate on the data taken in autumn (49 < doy < 111) and spring (235 < doy < 297) in the northern hemisphere with latitudes of 0°–40°. With this constraint, local times are automatically limited either around noon or midnight. We obtain four sets (two seasons × two local times) of density trends at each altitude layer. We take into account variations due to a linear trend and the 11-year solar cycle using linear regression techniques. Because we do not see significant differences among the four trends, we combine them to provide a single vertical profile of trend slopes. We find a negative density trend of roughly −5%/decade at every altitude. This is in reasonable agreement with inferences from settling rate of the upper atmosphere. In the 100–110-km altitude, we found an exceptionally high density decline of about −12%/decade. This peak may be the first observational evidence for strong cooling due to water vapor and ozone near 110 km, which was first identified in a numerical simulation by Akmaev et al. (2006, https://doi.org/10.1016/j.jastp.2006.03.008). Further observations and numerical simulations with suitable input parameters are needed to establish this feature., 地球温暖化に伴う超高層大気の収縮をX線天文衛星で解明 --逆転の発想！捨てられた天体観測データを大気観測に転用--. 京都大学プレスリリース. 2023-02-24.

We report on the discovery of a new supergiant fast X-ray transient (SFXT), MAXI J0709$-$159, and its identification with LY CMa (also known as HD 54786). On 2022 January 25, a new flaring X-ray object named MAXI J0709$-$159, was detected by Monitor of All-sky X-ray Image (MAXI). Two flaring activities were observed in the two scans of $\sim 3$ hours apart, where the 2-10 keV flux reached $5\times 10^{-9}$ erg cm$^{-2}$ s$^{-1}$. During the period, the source exhibited a large spectral change suggesting that the absorption column density $N_\mathrm{H}$ increased from $10^{22}$ cm$^{-2}$ to $10^{23}$ cm$^{-2}$. NuSTAR follow-up observation on January 29 identified a new X-ray source with a flux of $6\times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$ at the position consistent with LY CMa, which has been identified as B supergiant as well as Be star, located at the 3 kpc distance. The observed X-ray activity characterized by the short ($\lesssim$ several hours) duration, the rapid ($\lesssim$ a few seconds) variabilities accompanied with spectral changes, and the large luminosity swing ($10^{32}$-$10^{37}$ erg s$^{-1}$) agree with those of SFXT. On the other hand, optical spectroscopic observations of LY CMa revealed a broad $H\alpha$ emission line, which may indicate the existence of a Be circumstellar disk. These obtained results suggest that the optical companion, LY CMa, certainly has a complex circumstellar medium including dense clumps., Comment: 13 pages, 10 figures, accepted for publication in PASJ (v2. author names in metadata are modified.)

The compatibility between the DEMO divertor’s cooling water and cooling component major candidate, the ITER-grade CuCrZr alloy, was investigated. The water chemistry is expected to be hydrogen-dissolved, yet available data that are within DEMO divertor’s design specification is very limited. This investigation aims at understanding the corrosion behavior and performance of CuCrZr alloy. Results showed that the weight loss of specimens in hydrogen-dissolved condition is smaller than that of in oxygen-dissolved and degassed condition (dissolved hydrogen/oxygen < 5ppb)., The 20th International Conference on Fusion Reactor Materials (ICFRM-20)

The water-cooled system using high-temperature pressurized water is a primary candidate for the blanket and divertor of the Japanese fusion DEMO reactor. Both reduced activation ferritic/martensitic steel (RAFM) (for blanket and divertor) and the copper alloy (for divertor) are used as the key structure of cooling pipes. The water chemistry of these systems needs to be decided based on the corrosion behavior of these materials, especially depending on the dissolved oxygen (DO) level. This study aims to summarize the evaluation results of high-temperature pressurized water corrosion of RAFM and copper alloys with varied DO conditions., The 20th International Conference on Fusion Reactor Materials (ICFRM-20)

Strict control of tritium migration is an essential requirement for every blanket concept in a fusion reactor. Tritium permeation barriers (TPBs) have been developed in particular for liquid lithium-lead blankets using ceramic coatings; however, recent DEMO reactor design activities initiate an argument that a TPB is necessary also in solid breeder blankets. The previous studies revealed that reduced activation ferritic/martensitic (RAFM) steels and ceramic coatings reacted with solid breeder lithium ceramics under the blanket conditions, indicating the reactivity of solid breeders and its effect on tritium permeation should be taken into consideration. Moreover, not only the RAFM steel but the TPB will be exposed to pressurized water in a case where the TPB is applied to the cooling side. In this study, compatibility tests with pressurized water and lithium ceramic pebbles have been carried out for the investigation of the chemical stability of the TPB coatings and the effect on hydrogen isotope permeation in the Water Cooled Ceramic Breeder blanket., The 20th International Conference on Fusion Reactor Materials (ICFRM-20)

In this study, creep-fatigue (CF) tests on F82H at 823 K were conducted according to the standard specimen and requirement of ASTM. Tension holding (TH) showed higher lifetime than compression holding (CH). Compared with 9Cr-1Mo-V steels, CF lifetime degradation was not observed at the holding time less than 0.05 h in case of CH and 0.17 h in case of TH. CF damages were summarized from the CF test results by Campbell method. The obtained damages distributed around CF damage envelope of Grade 91 in ASME NH. CF damage of cylindrical water-cooled ceramic breeder (WCCB) test blanket module (TBM) container at the zenith of hemisphere body under the ITER thermo-mechanical load was calculated. The calculated damage located within of the damage envelope of Grade 91. It was indicated that the present design of cylindrical WCCB TBM might resist CF interaction under the ITER thermo-mechanical load conditions.

Reduced activation ferritic/martensitic (RAFM) steel, e.g., F82H, is the leading candidate structural material for fusion blanket. Of many blanket concepts, the water-cooled ceramic breeder blanket is an attractive concept because of its compactness and its compatibility with the technologies which using high-temperature water as coolant in conventional light water reactor. For tritium breeding, it is necessary to manage the corrosion of F82H by controlling not chemicals but hydrogen and oxygen in the cooling water. Understanding the effects of oxygen and hydrogen on corrosion property was therefore required for the blanket design. This work aims to investigate the corrosion behavior of F82H in high-temperature water with hydrogen added. Based on the results of the corrosion test, it is speculated that hydrogen addition expected no considerable change of the flow-accelerated corrosion (FAC). In contrast, it was found that the number of cycles to fracture was decreased in the high-temperature water in comparison to the fatigue test at room temperature in air.

In Japan Demonstration fusion reactor, the divertor plays a critical role for it helps sustaining plasma shape and hence demanding high requirements on its structural materials for the cooling system. CuCrZr alloy is considered as one of candidates and would be used in one independent cooling system. Proposals of operating conditions would be similar with fission light water reactor, where experiences have shown that water chemistry significantly influences corrosion behavior. Available references on the corrosion of CuCrZr also address the importance of water chemistries. Because the design proposal could be one of proposals towards reactor, it is worthwhile an investigation to understand the corrosion behavior of CuCrZr alloy under high temperature water environments. This work aims at the effect of dissolved oxygen, and dissolved hydrogen on corrosion behavior. Tests were conducted in an autoclave with relatively static water flow. Three different water chemistries: water with dissolved oxygen 100ppb, water with dissolved hydrogen 1600ppb and degassed water were chosen. The exposure time of CuCrZr coupons was 100 hr. Results showed that specimens in water with oxygen 100ppb had highest weight loss with light-reflecting, grain-oriented surface while in water with hydrogen 1600ppb had lowest weight loss with tarnished surface and nanoscale pure copper precipitates attached. It is believed that hydrogen reduced dissolved copper ions back to copper atoms; atoms returned to surface in precipitate forms hence reducing weight loss.

A water-cooled ceramic breeder blanket system is the Japanese promising design, and the candidate structuralmaterial is a reduced activation ferritic/martensitic steel, F82H. On the other hands, the material of the pipes ofcooling system is planned to an austenitic stainless steel, SUS316 L. The cooling water will commonlyflow bothF82H cooling channels and SUS316 L cooling pipes. Appropriate water chemistry condition for each of them isdifferent, but it is necessary tofind out a reasonable and common water chemistry condition for both of them. Inthe fusion blanket, since the cooling water will be irradiated by neutrons and brought about the radiolysis ofwater, the dissolved oxygen concentration of the cooling water is considered to change by the radiolysis ofwater. This research provides the simultaneous effects of dissolved oxygen and applied stress on surface mor-phology of F82H and SUS316 L by the pressurized water immersion test in dissolved oxygen concentration up to100 ppb.

The purpose of the present study is to clarify the phase stability of the corroded layer in reduced activation ferritic/martensitic steels under irradiation. The oxide film of F82H steel was subjected to ion irradiation using 2.8 MeV-Fe2+ at 573 K. For post-irradiation experiments, grazing incident X-ray diffraction (GIXRD) and Raman analysis was conducted to clarify the features of the oxide layer with variation of the irradiation dose. The dominant phase of the oxide film was identified as FeCr2O4, which co-existed with up to ~30% of the residual matrix phase. The fraction of the residual matrix decreased as the irradiation dose increased, the phase transformation from FeCr2O4 to Fe3O4 was confirmed. The hypothesis for the phase transformation of FeCr2O4 to Fe3O4 under irradiation was proposed based on the experimental results. Specifically, FeCr2O4 phase is unstable under irradiation, and tends to become stable after absorbing Fe atoms from the residual matrix in the corrosion layer. As a result, the Fe concentration in the oxide layer increased under irradiation, the phase transformation of FeCr2O4 to Fe3O4 is presumed under irradiation. However, concerns of the mechanism about the instability of FeCr2O4 under irradiation, and better stability of Fe3O4 phase relative to FeCr2O4 phase have to be further considered to achieve a better understanding.

Reduced activation ferritic/martensitic (RAFM) steels are assumed as structural material of blanket design activities. Determining the design limit is essential for the RAFM steels to qualify their use in fusion blankets. This work investigates fatigue properties of a RAFM steel, F82H at elevated temperature up to 823 K in the air and vacuum condition. The tests were carried out with axial strain controlled condition using cylindrical specimen, and the results were summarized using the Manson’s modified universal slope method, which can describe fatigue lifetime as a function of tensile properties. Although fatigue lifetime of F82H decreased with temperature, the fatigue lifetime up to 723 K fell into factor of 2 of the lifetime at ambient temperature. The lifetime at vacuum condition was successfully described using universal slope method. On the other hand, the lifetime in the air was 1/3 or shorter than that in vacuum condition. It is notable that a heat of F82H prepared for IEA round robin tests demonstrated 1/5 of shorter lifetime than latest F82H. The difference is discussed with inclusion morphologies affected by steel manufacturing process and so on. This report has been prepared as an account of work assigned to the Japanese Implementing Agency within the ‘‘Broader Approach Agreement’’ between the Government of Japan and the European Atomic Energy Community.

This paper summarizes the current status of the material properties handbook for a structural design using Japanese reduced-activation ferritic/martensitic steel F82H. Specifically, the key structural parameters, e.g. time-independent/dependent design stresses and fatigue design curves, were determined by following the French structural design code RCC-MRx. Moreover, under the Japan–U.S. collaboration, tensile data were newly added to the benchmark heavy irradiation data up to 80 dpa, as critical input information in the intermediate check and review in Japan. Furthermore, the status of structural material data and the near-term and long-term issues were clarified by the evaluation using the attribute guides. In parallel, the structural design approaches, which were newly introduced and extended to cope with the structural design issues under the complex environmental conditions peculiar to the DEMO reactor, were noted with the initial R&D results. Of the many design issues, the multi-axial loading conditions due to the complexity of the DEMO reactor as well as the coolant compatibility and the irradiation effect are mentioned. For example, in the paper, multi-axial fatigue–creep testing and evaluation using the modified universal slope method and brittle/ductile fracture testing and evaluation using the local approach are explained toward DEMO.

The accretion-induced pulse-period changes of the Be/X-ray binary pulsar X Persei were investigated over a period of 1996 January to 2017 September. This study utilized the monitoring data acquired with the RXTE/ASM in 1.5$-$12 keV and MAXI/GSC in 2$-$20 keV. The source intensity changed by a factor of 5$-$6 over this period. The pulsar was spinning down for 1996$-$2003, and has been spinning up since 2003, as already reported. The spin up/down rate and the 3$-$12 keV flux, determined every 250 d, showed a clear negative correlation, which can be successfully explained by the accretion torque model proposed by Ghosh & Lamb (1979). When the mass, radius and distance of the neutron star are allowed to vary over a range of 1.0$-$2.4 solar masses, 9.5$-$15 km, and 0.77$-$0.85 kpc, respectively, the magnetic field strength of $B=(4-25) \times10^{13}\ \rm G$ gave the best fits to the observation. In contrast, the observed results cannot be explained by the values of $B\sim10^{12}\ \rm G$ previously suggested for X Persei, as long as the mass, radius, and distance are required to take reasonable values. Assuming a distance of $0.81\pm0.04$ kpc as indicated by optical astrometry, the mass of the neutron star is estimated as $M=2.03\pm0.17$ solar masses., 11 pages, 9 figures, accepted for publication in PASJ

著者人数: 32名, Accepted: 2011-09-20, 資料番号: SA1003145000

著者人数: 37名, Accepted: 2011-07-20, 資料番号: SA1003146000

著者人数: 12名, Accepted: 2007-09-04, 資料番号: SA1000314000

A report is made on the luminosity and pulse-period evolution of the Be binary X-ray pulsar, GX 304$-$1, during a series of outbursts from 2009 to 2013 observed by MAXI/GSC, RXTE/PCA, and Fermi/GBM. In total, twelve outbursts repeated by $\sim$ 132.2 days were observed, which is consistent with the X-ray periodicity of this object observed in the 1970s. These 12 outbursts, together with those in the 1970s, were found to all recur with a well defined period of 132.189$\pm$0.02 d, which can be identified with the orbital period. The pulse period at $\sim 275$ s, obtained from the RXTE/PCA and Fermi/GBM data, apparently exhibited a periodic modulation synchronized with the outburst period, suggesting the pulsar orbital motion, which is superposed on a secular spin-up trend throughout the entire active phase. The observed pulse-period changes were successfully represented by a model composed of the binary orbital modulation and pulsar spin up caused by mass accretion through an accretion disk. The orbital elements obtained from the best-fit model, including the projected orbital semi-major axis $a_{\rm x}\sin i \simeq 500-600$ light-s and an eccentricity $e \simeq 0.5$, are typical of Be binary X-ray pulsars., 10 pages, 6 figures, accepted for publication in PASJ

著者人数: 31名, Accepted: 2006-07-28, 資料番号: SA1000375000

著者人数: 43名, Accepted: 2014-06-13, 資料番号: SA1140202000

MAXI/GSC observed 21 outbursts from Circinus X-1 between 2009 August and 2013 December. Although 14 outbursts showed ordinary gradual decays, in 7 outbursts we found sudden luminosity decrease in a time scale of a few hours around the periastron, and then the outbursts terminated. These sudden decreases started at the estimated luminosity of a few times $10^{37}$ erg s$^{-1}$ and reached to $\lesssim3\times10^{36}$ erg s$^{-1}$. We propose three interpretations for the sudden luminosity decrease: (1) the end of the outburst during the dip, (2) the propeller effect, and (3) the stripping effect by the stellar wind of the companion star. It is difficult to explain the phenomenon with any of these interpretations alone. The interpretation of (1) is possible for only two outbursts assuming rapid decay. The propeller effect (2) is expected to occur at a constant luminosity, which is incompatible with the observed facts. In wind stripping effect (3), the ram pressure of a typical stellar wind is not sufficient to blow out most of the accretion disk. In this paper, we discuss a possibility of a modified effect of (3) assuming other additional conditions such as wind clumping and disk instability., Comment: 13 pages, 7 figures

The stellar wind around the compact object in luminous wind-accreting high mass X-ray binaries is expected to be strongly ionized with the X-rays coming from the compact object. The stellar wind of hot stars is mostly driven by light absorption in lines of heavier elements, and X-ray photo-ionization significantly reduces the radiative force within the so-called Stroemgren region leading to wind stagnation around the compact object. In close binaries like Vela X-1 this effect might alter the wind structure throughout the system. Using the spectral data from Monitor of All-sky X-ray Image (MAXI), we study the observed dependence of the photoelectric absorption as function of orbital phase in Vela X-1, and find that it is inconsistent with expectations for a spherically-symmetric smooth wind. Taking into account previous investigations we develop a simple model for wind structure with a stream-like photoionization wake region of slower and denser wind trailing the neutron star responsible for the observed absorption curve., 5 pages, 3 figures, accepted in A&A

We present analysis of 4U 1626-67, a 7.7 s pulsar in a low-mass X-ray binary system, observed with the hard X-ray detector of the Japanese X-ray satellite Suzaku in March 2006 for a net exposure of \sim88 ks. The source was detected at an average 10-60 keV flux of \sim4 x10^-10 erg cm^-2 s^-1. The phase-averaged spectrum is reproduced well by combining a negative and positive power-law times exponential cutoff (NPEX) model modified at \sim 37 keV by a cyclotron resonance scattering feature (CRSF). The phase-resolved analysis shows that the spectra at the bright phases are well fit by the NPEX with CRSF model. On the other hand, the spectrum in the dim phase lacks the NPEX high-energy cutoff component, and the CRSF can be reproduced by either an emission or an absorption profile. When fitting the dim phase spectrum with the NPEX plus Gaussian model, we find that the feature is better described in terms of an emission rather than an absorption profile. The statistical significance of this result, evaluated by means of an F-test, is between 2.91 x 10^-3 and 1.53 x 10^-5, taking into account the systematic errors in the background evaluation of HXD-PIN. We find that, the emission profile is more feasible than the absorption one for comparing the physical parameters in other phases. Therefore, we have possibly detected an emission line at the cyclotron resonance energy in the dim phase., Accepted for publication in ApJ on March 16, 2012. 12 pages, 14 figures

著者人数: 38名, Accepted: 2011-08-19, 資料番号: SA1003132000

We report the discovery of a cyclotron resonance scattering feature (CRSF) in the X-ray spectrum of GX 304-1, obtained by RXTE and Suzaku during major outbursts detected by MAXI in 2010. The peak intensity in August reached 600 mCrab in the 2-20 keV band, which is the highest ever observed from this source. The RXTE observations on more than twenty occasions and one Suzaku observation revealed a spectral absorption feature at around 54 keV, which is the first CRSF detection from this source. The estimated strength of surface magnetic field, $4.7 \times 10^{12}$ G, is one of the highest among binary X-ray pulsars from which CRSFs have ever been detected. The RXTE spectra taken during the August outburst also suggest that the CRSF energy changed over 50-54 keV, possibly in a positive correlation with the X-ray flux. The behavior is qualitatively similar to that observed from Her X-1 on long time scales, or from A 0535+26, but different from the negative correlation observed from 4U 0115+63 and X 0331+53., 7 pages, 4 figures, accepted for publication in PASJ

We report the in-orbit performance of the Gas Slit Camera (GSC) on the MAXI (Monitor of All-sky X-ray Image) mission carried on the International Space Station (ISS). Its commissioning operation started on August 8, 2009, confirmed the basic performances of the effective area in the energy band of 2--30 keV, the spatial resolution of the slit-and-slat collimator and detector with 1.5 degree FWHM, the source visibility of 40-150 seconds for each scan cycle, and the sky coverage of 85% per 92-minute orbital period and 95% per day. The gas gains and read-out amplifier gains have been stable within 1%. The background rate is consistent with the past X-ray experiments operated at the similar low-earth orbit if its relation with the geomagnetic cutoff rigidity is extrapolated to the high latitude. We also present the status of the in-orbit operation and the calibration of the effective area and the energy response matrix using Crab-nebula data., 11 pages, 14 figures, accepted for publication in PASJ

著者人数：32名, Accepted: 2009-05-28, 資料番号: SA1001001000

Spectral and timing studies of Suzaku ToO observations of two SGRs, 1900+14 and 1806-20, are presented. The X-ray quiescent emission spectra were well fitted by a two blackbody function or a blackbody plus a power law model. The non-thermal hard component discovered by INTEGRAL was detected by the PIN diodes and its spectrum was reproduced by the power law model reported by INTEGRAL. The XIS detected periodicity P = 5.1998+/-0.0002 s for SGR 1900+14 and P = 7.6022+/-0.0007 s for SGR 1806-20. The pulsed fraction was related to the burst activity for SGR 1900+14., 8 pages, 3 figures, Accepted for publication in PASJ (Suzaku 3rd special issue)

Magnetic, transport, and x-ray diffraction measurements on single-crystal CeRhAs, the so-called Kondo insulator, have revealed successive transitions at ${T}_{1}=370,$ ${T}_{2}=235,$ and ${T}_{3}=165\mathrm{K}.$ Below ${T}_{1},$ the unit cell is doubled along the b and c axes. Thereby, the resistivity jumps upwards and magnetic susceptibility in all the directions drops. Superlattice reflections at (0,1/3,1/3) and (1/3,0,0) appear at $Tl{T}_{2}.$ The latter suddenly increases below ${T}_{3}$ where a transport gap is enhanced. These observations indicate that the gap formation is intimately related to lattice modulations.

MAXI is an all sky X‐ray monitor to be mounted on the Japanese Experimental Module in the International Space Station (ISS). It scans almost all over the sky every 96 minutes, in the course of the orbital motion of the ISS. MAXI is designed to have a sensitivity, significantly higher than the previous X‐ray monitors, and then, to detect X‐ray sources as faint as 1 mCrab in a week observation. Therefore, MAXI is expected to create a novel catalogue of not only the stable X‐ray sources but also the highly variable ones in the sky, especially active galactic nuclei for the first time. If MAXI detects X‐ray phenomena, alerts will be quickly made through the Internet.

Monitor of All-sky X-ray Image (MAXI) is an X-ray all-sky scanner, which will be attached on Exposed Facility of Japanese Experiment Module dubbed "Kibo" of International Space Station (ISS). MAXI will be launched by the Space Shuttle or the Japanese H-IIA Transfer Vehicle (HTV) in 2008. MAXI carries two types of X-ray cameras: Solid-state Slit Camera (SSC) for 0.5-10 keV and Gas Slit Camera (GSC) for 2-30 keV bands. Both have long narrow fields of view (FOV) made by a slit and orthogonally arranged collimator plates (slats). The FOV will sweep almost the whole sky once every 96 minutes by utilizing the orbital motion of ISS. Then the light curve of an X-ray point source become triangular shape in one transit. In this paper, we present the actual triangular response of the GSC collimator, obtained by our calibration. In fact they are deformed by gaps between the slats, leaning angle of the slats, and the effective width of the slats. We are measuring these sizes by shooting X-ray beams into the detector behind the collimator. We summarize the calibration and present the first compilation of the data to make the GSC collimator response, which will be useful for public users.

The current status is reported of the development of Monitor of All-sky X-ray Image and the measurement of its observational response. MAXI is a scanning X-ray camera to be attached to the Japanese Experiment Module of the International Space Station in 2008. MAXI is mainly composed of two kinds of instruments, GSC which is sensitive to the 2 - 30 keV photons, and SSC to the 0.5 - 10 keV ones. As an X-ray all-sky monitor, MAXI has an unprecedented sensitivity of 7 mCrab in one orbit scan, and 1 mCrab in one week. Using the engineering mode of the proportional counter and of the collimator for GSC, the observational response of GSC is extensively measured. The acceptable performances are obtained as a whole for both the collimator and the counter. The engineering models of the other part of MAXI are also constructed and the measurement of their performance is ongoing.