Faculty Profiles - OHNO, Noriyasu

Papers - OHNO, Noriyasu

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Thermal conductivity measurement of fuzzy W using a thermoreflectance method Reviewed

Kajita S., Yagi T., Ohno N.

Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV Vol. 2023-June page： 526 - 528 2023

Publisher：Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV

Thermophysical properties of the fuzzy nanostructured tungsten (W) layer formed by helium plasma irradiation (a) are measured using a pulsed light heating thermoreflectance method. A three layered model has been developed to explain the temporal evolution of the themoreflectance signal. The thermal conductivity of the nanostructured layer was deduced to be in the range of 0.14-0.26% of the pure (ideal) W.

DOI： 10.23919/ISDEIV55268.2023.10200254

Scopus

Use of machine learning for a helium line intensity ratio method in Magnum-PSI Reviewed International coauthorship

Kajita Shin, Iwai Sho, Tanaka Hirohiko, Nishijima Daisuke, Fujii Keisuke, van der Meiden Hennie, Ohno Noriyasu

NUCLEAR MATERIALS AND ENERGY Vol. 33 page： 101281 2022.10

Plasma induced surface nanostructure on semiconductors for the application of random laser Reviewed

Shi Q., Fujiwara H., Osaka R., Kajita S., Yasuhara R., Ohno N., Uehara H.

2022 Conference on Lasers and Electro-Optics Pacific Rim, CLEO-PR 2022 - Proceedings 2022

POWER TRANSMISSION FACTOR FOR TUNGSTEN TARGET W/WO FIBER-FORM NANOSTRUCTURE IN H PLASMAS WITH HOT ELECTRON COMPONENT USING COMPACT PLASMA DEVICE AIT-PID Reviewed International journal

Takamura, S; Miyamoto, T; Ohno, N

FUSION SCIENCE AND TECHNOLOGY Vol. 63 ( 1T ) page： 225 - 228 2013.5

Simulation of dynamics of carbon dust particles in the JT-60U tokamak Reviewed International coauthorship International journal

Tanaka, Y; Smirnov, RD; Pigarov, AY; Takenaga, H; Asakura, N; Uesugi, Y; Ohno, N

JOURNAL OF NUCLEAR MATERIALS Vol. 415 ( 1 ) page： S1106 - S1110 2011.8

Dusty sheaths in plasmas with two-temperature electrons Reviewed International journal

Asano, K; Chutov, YI; Kravchenko, OY; Ohno, N; Pshenychnyj, AF; Smirnov, RD; Takamura, S

DUSTY PLASMAS IN THE NEW MILLENNIUM Vol. 649 page： 277 - 280 2002

Generation of high heat flux plasma in divertor plasma simulator by ion cyclotron wave heating Reviewed International journal

Watanabe, S; Ohsawa, S; Takagi, M; Ohno, N; Uesugi, Y; Takamura, S

ICPP 96 CONTRIBUTED PAPERS - PROCEEDINGS OF THE 1996 INTERNATIONAL CONFERENCE ON PLASMA PHYSICS, VOLS 1 AND 2 page： 1050 - 1053 1997

Generation of high heat flux plasmas by rf heating in divertor plasma simulator NAGDIS-II Reviewed International journal

Watanabe, S; Ohsawa, S; Takagi, M; Ohno, N; Uesugi, Y; Takamura, S

RADIO FREQUENCY POWER IN PLASMAS ( 403 ) page： 483 - 486 1997

STUDIES ON SHEATH FORMATION IN DIVERTOR PLASMAS BY COMPUTER-SIMULATION USING PARTICLES Reviewed International journal

OHNO, N; MASUZAKI, S; KONNO, H; TAKAMURA, S

CONTRIBUTIONS TO PLASMA PHYSICS Vol. 32 ( 3-4 ) page： 249 - 254 1992

Experiments on Subharmonic Waves in an Ion Beam-Plasma System Reviewed

Ono Noriyasu, Kasuya Toshiro, Tanaka Masayoshi, Komori Akio, Kawai Yoshinobu

Vol. 9 ( 1 ) page： 23 - 27 1987.7

Language：Japanese Publisher：Interdisciplinary Graduate School of Engineering Sciences, Kyushu University

Subharmonic waves are experimentally studied in detail by applying pump waves to an ion beam-plasma system. It is found that the subharmonic waves are excited in the characteristic region of the pump-wave frequency, accompanied with low-frequency fluctuations. These low-frequency fluctuations are considered to play an important role on excitation of the subharmonics.

DOI： 10.15017/17680

CiNii Research

TALIF measurements of atomic deuterium in toroidal divertor simulator NAGDIS-T Reviewed International journal

Kajita S., Hiraiwa K., Tanaka H., Iwai R., Aramaki M., Yasuhara R., Ohno N.

Nuclear Materials and Energy Vol. 42 2025.3

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Nuclear Materials and Energy

The behavior of neutral deuterium (D) atoms is important for understanding the physics in the divertor region of nuclear fusion reactors. It is necessary to introduce an active measurement when considering the application to detached plasmas, where recombination processes dominate the processes determining the population distribution. In this work, we have developed a two-photon absorption laser-induced fluorescence (TALIF) system in the toroidal divertor simulator NAGDIS-T to measure D atomic density. The absolute D atomic density was obtained by calibrating the signal with the krypton TALIF signal. The gas pressure and power dependence of the D atomic density is shown. The D atomic density was in the range of 1.6 × 1018–1.4 × 1019 m−3, and the temperature was estimated to be <0.4 eV. The behavior of the D atoms is discussed in terms of the production process.

DOI： 10.1016/j.nme.2025.101898

Scopus

Formation of micro-pillars on W by He plasma irradiation with N<inf>2</inf>/Ar impurities Reviewed International journal

Zhang R., Kajita S., Hwangbo D., Tanaka H., Yamamoto Y., Ohno N.

Materialia Vol. 39 2025.3

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Materialia

Tungsten samples were exposed to helium plasma with additional nitrogen and argon impurity gases. As a result, microscale pillar-like structures formed on the tungsten surface under similar conditions where nanotendril bundles can be formed. This study summarizes the condition under which those pillar-like structures form. The surface morphology and height profile were observed, and the field emission properties of the W sample with the pillar-like structure were analyzed. The possible formation mechanism of these structures was discussed.

DOI： 10.1016/j.mtla.2025.102341

Scopus

DISCOVER: New integrated code to simulate helium detached plasma in a linear device Reviewed International journal

Natsume H., Masuda S., Tanaka H., Migita R., Mano R., Sugiura K., Sawada K., Hoshino K., Hayashi Y., Ohno N.

Physics of Plasmas Vol. 32 ( 2 ) 2025.2

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Physics of Plasmas

A detached plasma integrated simulation code with various elastic/inelastic reactions (DISCOVER) was developed for better understanding of detached helium plasma experiments. DISCOVER, which includes a neutral transport code, fluid code, and collisional radiative code, considers elastic and inelastic collisions between recycling neutrals and bulk particles, as well as the transport of metastable atoms [He( 21S) and He( 23S)]. The code was used for simulating helium plasma in the linear plasma device NAGDIS-II. The transition from attached to detached plasma was demonstrated in the simulation by increasing the upstream electron density through an increase in the gas supply from the plasma source while maintaining a fixed downstream gas injection rate. In addition, similar transition was also observed by introducing neutral particles from downstream, while maintaining a fixed upstream electron density. In the detached plasma, the density of He( 23S) near the end target was approximately seven times higher than that in the attached plasma. Furthermore, it was evident that the transport of He( 23S) enhanced the electron energy rate coefficient and influenced the spatial distribution of the detached plasma.

DOI： 10.1063/5.0243036

Scopus

Dynamic response of recombining detached plasma to electron beam injection Reviewed International journal

Tanaka H., Kajita S., Kaizawa H., Ohno N.

Physics of Plasmas Vol. 32 ( 2 ) 2025.2

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Physics of Plasmas

Dynamic response of recombining detached helium plasma to high-energy electrons was investigated in the linear plasma device NAGDIS-II by introducing an electron beam source. A square wave bias was applied to the tungsten electrode to generate a pulsed electron beam with controlled beam energy. At high beam energy condition, a large increase in the target ion flux and different-shape time evolutions between emissions from low- and high-excited states were observed. From the analysis using a collisional radiative code applicable to two electron temperature components, qualitative changes were explained by contributions due to the excitation process from the ground state by the electron beam and the volume recombination process due to the parameter change of bulk plasma.

DOI： 10.1063/5.0242483

Scopus

Thermal evolution of helium bubbles in tungsten by GISAXS and TDS Reviewed International journal Open Access

Teo S.H.B., Thompson M.A.T., Kirby N., Hughes D.R., Hammoud N., Khan A., Tanaka H., Ohno N., Mummery P., Harmer S.L., Corr C.S.

Journal of Nuclear Materials Vol. 604 2025.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Nuclear Materials

Grazing-incidence small-angle x-ray scattering (GISAXS) and thermal desorption spectroscopy (TDS) were used to study the thermal evolution of helium (He) bubbles in tungsten (W). W samples were exposed to a low ion energy (∼20 eV, sheath accelerated) He plasma at two sample temperatures: 573 K (low-temperature) and 1050 K (high-temperature). They were then annealed to temperatures of 773 K, 873 K and 998 K. GISAXS analysis showed an increase in the median bubble radius of the high-temperature samples from 6.42 Å to 12.87 Å as a result of annealing to 998 K. No changes in the bubble radii distributions were observed for the low-temperature samples and the median radius remained at ∼4 Å even after annealing. TDS showed that the desorption behaviour of He in W has a dependence on the sample temperature during plasma irradiation. The higher-temperature sample experienced a maximum desorption rate that is an order of magnitude larger than the low-temperature sample. The desorption profile of the high-temperature sample spanned a wider temperature range, 500 K - 1500 K, than the low-temperature sample, 500 K - 750 K. The formation and thermal evolution of He bubbles in W have a clear dependence on sample temperature during plasma exposure. The differences in annealing behaviour observed in this work agree with prior in-situ TEM annealing experiments and help explain macroscopic plasma-material interaction effects such as recrystallisation suppression.

DOI： 10.1016/j.jnucmat.2024.155524

Open Access

Scopus

Evaluation of local erosion and deposition on the W-monoblock of JA-DEMO divertor Reviewed International journal

Oya M., Hoshino K., Asakura N., Sakamoto Y., Ohno N., Hanada K.

Nuclear Materials and Energy Vol. 41 2024.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Nuclear Materials and Energy

We developed and performed a 3D Monte-Carlo simulation on local erosion and deposition in outer divertor target of JA-DEMO. The local erosion and deposition were evaluated considering the divertor plasma parameters and the roof shape of W-monoblocks. First, for local erosion simulation, the trajectories of D ions and impurity (He and Ar) ions were traced before irradiating the monoblock surface. The physical sputtering was calculated by these ions irradiation, considering their charge states, incident-energies and -angles. The highest sputtering flux was produced by multiply-charged Ar ions (Ar3+–Ar5+). The sputtering flux strongly depended on the poloidal positions along the divertor target. Although almost no erosion (sputtering) was obtained in low-temperature zone (∼1 eV), the erosion occurred in mid- (∼10 eV) and high-temperature (>10 eV) zone. Second, for local deposition simulation, the trajectories of W atoms emitted from the monoblock surface were traced. After transport (including ionization and recombination) in the plasma, W atoms and ions were deposited on the monoblock surface. The deposition occurred even on the area where plasmas did not irradiate (shadow). This was due to perpendicular diffusion to magnetic field lines, gyro-orbit effects for W ions and straight-line motion for neutral W atoms. The poloidal distribution of the deposition fluxes was similar to that of the sputtering fluxes. In low-temperature zone, no deposition flux occurred due to very low sputtering flux. The deposition flux in mid-temperature zone increased significantly and that in high-temperature zone remained high.

DOI： 10.1016/j.nme.2024.101793

Scopus

Recent progress of JT-60SA project toward plasma operation Reviewed International coauthorship International journal

Shirai H., Takahashi K., Di Pietro E., Abate D., Abdel Maksoud W., Abe H., Aiba N., Abe T., Akimitsu M., Ayllon-Guerola J., Arai T., Artaud J.F., Asakura N., Ashikawa N., Balbinot L., Barabaschi P., Baulaigue O., Belonohy E., Belpane A., Bin W., Bombarda F., Bolzonella T., Bonne F., Bonotto M., Botija J., Buermans J., Cabrera-Pérez S., Cardella A., Carralero D., Carraro L., Cavalier J., Cavinato M., Chernyshova M., Chiba S., Clement-Lorenzo S., Cocilovo V., Coda S., Coelho R., Coffey I., Collin B., Corato V., Cucchiaro A., Czarski T., Dairaku M., Davis S., Day C., Dela Luna E., De Tommasi G., Decool P., Di Pace L., Dibon M., Disset G., D’Lsa F., Ejiri A., Endo Y., Ezumi N., Falchetto G., Fassina A., Fejoz P., Ferro A., Fietz W., Figini L., Fornal T., Frello G., Fujita T., Fukuda T., Fukui K., Fukumoto M., Funaba H., Furukawa M., Futatani S., Gabellieri L., Gaio E., Galazka K., Garcia J., Garcia-Dominguez J., Garcia-Lopez J., Garcia-Munoz M., Garzotti L., Gasparini F., Gharafi S., Giacomelli L., Ginoulhiac G., Giruzzi G., Giudicotti L., Gonzalez-Martin J., Guillén-González R., Hajnal N., Hall S., Hamada K., Hanada K., Hanada M., Hasegawa K., Hatakeyama S., Hauer V., Hayashi N., Hayashi T., Heller R., Hidalgo-Salaverri J., Higashijima S.

Nuclear Fusion Vol. 64 ( 11 ) 2024.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Nuclear Fusion

Superconducting (SC) tokamak JT-60SA plays an essential role in fusion research and development by supporting and complementing the ITER project, providing directions to the DEMO design activity and fostering next generation scientists and engineers. Since the short circuit incident at the terminal joints of equilibrium field coil #1 during the integrated commissioning (IC) in March 2021, both EU and JA implementing agencies (IAs) have examined how to ensure safe operation of JT-60SA by mitigating the risk of possible discharge occurrence inside the cryostat. Based on the experience of the global Paschen tests, the IAs have established a strategy of risk mitigation measures, which is a combination of (i) reinforcement of insulation, (ii) avoiding unnecessary voltage application to the coil systems and (iii) immediate de-energization of the coils when deteriorated vacuum conditions are detected. Thanks to the considerable efforts of the Integrated Project Team members, the IC restarted in May 2023. After confirmation of the SC state of the coil systems (TF, EF and CS), the coil energization test and the plasma operation phase 1 (OP-1) started. The first plasma was successfully achieved on 23 October 2023 with a limited value of voltage and current applied to the coils. The plasma configuration control was also confirmed with low plasma current and low auxiliary heating power conditions. Based on the IO-F4E-QST collaboration, activities of JT-60SA have been shared with the IO and provided an important lesson for ITER assembly and commissioning, and will provide an outstanding contribution to fusion research at large. After OP-1, maintenance & enhancement phase 1 (M/E-1) starts from January 2024, in which in-vessel components are installed, and heating and diagnostic systems are extensively upgraded to allow a high power heating experiment planned in OP-2. In order to make the best use of JT-60SA, a newly organized JT-60SA experiment team will refine the research plan for the future high heating power operation phase.

DOI： 10.1088/1741-4326/ad34e4

Scopus

Machine learning aided line intensity ratio method for helium-hydrogen mixed recombining plasmas Reviewed International coauthorship International journal

Kajita S., Nishijima D., Fujii K., Tanaka H., Vernimmen J., van der Meiden H., Classen I., Ohno N.

Plasma Physics and Controlled Fusion Vol. 66 ( 10 ) page： 105005 2024.8

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Plasma Physics and Controlled Fusion

The helium line intensity ratio (LIR) with the help of a collisional radiative (CR) model has long been used to measure the electron density, n e , and temperature, T e , and its potential and limitations for fusion applications have been discussed. However, it has been reported that the CR model approach leads to deviations in helium-hydrogen mixed plasmas and/or recombining plasmas. In this study, a machine learning (ML) aided LIR method is used to measure n e and T e from spectroscopic data of helium-hydrogen mixed recombining plasmas in the divertor simulator Magnum-PSI. To analyze mixed plasmas, which have more complex spectral shapes, the spectroscopy data were used directly for training instead of separating the intensities of each line. It is shown that the ML approach can provide a robust and simpler analysis method to deduce n e and T e from the visible emissions in helium-hydrogen mixed plasmas.

DOI： 10.1088/1361-6587/ad6a81

Scopus

Germanium nanostructures by helium plasma irradiation Reviewed International journal

Kajita S., Shi Q., Tabata K., Kuwabara T., Tanaka H., Ohno N.

Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers Vol. 63 ( 7 ) page： 07SP07 2024.7

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers

The effects of helium plasma irradiation, which has been widely studied on metals, on germanium have been experimentally investigated. The irradiation temperature ranges from 430 to 720 K, and the incident ion energy ranges from 30 to 100 eV. From the scanning electron microscope (SEM) observation, it was found that various morphological changes including pits, nanocones, nanopillars, and roughened surfaces occur. The spatial scale of the morphological change was analyzed by applying fast Fourier transform to SEM micrographs. Thermal desorption spectroscopy analysis suggests that He atoms implanted on Ge play major roles in forming roughened surfaces at a surface temperature higher than 500 K.

DOI： 10.35848/1347-4065/ad5afe

Scopus

Hydrogen isotope effects on recombination dominant plasmas in NAGDIS-II Reviewed International coauthorship International journal

Shi J., Kaizawa H., Uematsu Y., Tanaka H., Kajita S., Ohno N., Sawada K., Ding H.

Plasma Physics and Controlled Fusion Vol. 66 ( 8 ) page： 085006(11 pp) 2024.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Plasma Physics and Controlled Fusion

The detachment processes of the hydrogen (H) and deuterium (D) plasmas are comparatively investigated in the linear plasma device NAGDIS-II. The laser Thomson scattering measurements demonstrate that the recombination rate of the H plasma is greater than that of the D plasma as the neutral pressure increases in the molecular activated recombination (MAR) dominant detachment phase. As the recombination process by MAR is strongly dependent on the vibrational and rotationally excited states of the molecule, the rovibrational quantum state populations of the H and D molecules are measured using the Fulcher-α band spectroscopy. The results indicate that the vibrational temperature in the electronic ground state is considerably higher than the rotational temperature during detachment. The reaction rate coefficients for MARs due to charge exchange chains (CX-MAR) and dissociative attachment chains (DA-MAR) are calculated by the collision-radiation model under the measured temperature conditions. It can be observed that the CX-MAR is larger than the DA-MAR for both H and D, and that the CX-MAR of H is larger than the CX-MAR of D at electron temperatures T e above 1 eV. In consideration of the experimentally observed vibrational and rotational excitation temperatures, the reaction rate coefficients of CX-MAR and DA-MAR are increasing in the low T e region. These calculations are in accordance with the experimental results, which indicate that recombination processes due to MAR are more predominant in the H plasma compared to the D plasma. Furthermore, a transition from MAR to electron-ion recombination processes is observed in the D plasma at T e below 0.5 eV.

DOI： 10.1088/1361-6587/ad59c4

Scopus

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Papers - OHNO, Noriyasu