Faculty Profiles - OHNO, Noriyasu

Papers - OHNO, Noriyasu

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Measuring temperature effects on nano-bubble growth in tungsten with grazing incidence small angle X-ray scattering Reviewed Open Access

Matt Thompson, Russ Doerner, Noriyasu Ohno, Nigel Kirby, Patrick Kluth, Daniel Riley, Cormac Corr

Nuclear Materials and Energy Vol. 12 page： 1294 - 1297 2017.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier Ltd

W samples were exposed to He plasma in the MAGPIE, NAGDIS-II and PISCES-A across a range of sample temperatures between 473–1123 K. GISAXS was used to quantify the effect of plasma fluence and W surface temperature on He nano-bubble size distributions. In NAGDIS-II at 873 K nano-bubbles are exponentially distributed with mean diameters μ=0.64±0.01nm, similar to the value of μ=0.62±0.01nmfound for the MAGPIE plasma device at the much lower temperature of 473 K. Above ∼900 K nano-bubbles followed an approximately exponential distribution with μ &gt
0.72 nm demonstrating a significant increase in nano-bubble sizes at higher temperatures.

DOI： 10.1016/j.nme.2016.11.025

Open Access

Web of Science

Scopus

A Particle-In-Cell approach to particle flux shaping with a surface mask Reviewed Open Access

Kawamura G, Ohno N, Takamura S, Tomita Y

NUCLEAR MATERIALS AND ENERGY Vol. 12 page： 297 - 301 2017.8

Fractality and growth of He bubbles in metals Reviewed Open Access

Shin Kajita, Atsushi M. Ito, Noriyasu Ohno

PHYSICS LETTERS A Vol. 381 ( 29 ) page： 2355 - 2362 2017.8

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Pinholes are formed on surfaces of metals by the exposure to helium plasmas, and they are regarded as the initial process of the growth of fuzzy nanostructures. In this study, number density of the pinholes is investigated in detail from the scanning electron microscope (SEM) micrographs of tungsten and tantalum exposed to the helium plasmas. A power law relation was identified between the number density and the size of pinholes. From the slope and the region where the power law was satisfied, the fractal dimension D and s(min), which characterize the SEM images, are deduced. Parametric dependences and material dependence of D and s(min) are revealed. To explain the fractality, simple Monte-Carlo simulations including random walks of He atoms and absorption on bubble was introduced. It is shown that the initial position of the random walk is one of the key factors to deduce the fractality. The results indicated that new nucleations of bubbles are necessary to reproduce the number-density distribution of bubbles. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physleta.2017.05.028

Open Access

Web of Science

Scopus

Field Emission From Metal Surfaces Irradiated With Helium Plasmas Reviewed International coauthorship

Dogyun Hwangbo, Shin Kajita, Noriyasu Ohno, Dmitry Sinelnikov

IEEE TRANSACTIONS ON PLASMA SCIENCE Vol. 45 ( 8 ) page： 2080 - 2086 2017.8

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

Field electron emission from metal (tungsten, molybdenum, and tantalum) surfaces with different surface morphology was measured. Fuzz and loop samples had small increment in the emission current, whereas bubble samples marked huge increment in the current, accompanied by breakdown. The emission current became greater on the fuzzy samples than the loop samples probably because of more elongated morphology of fuzzy fiber. Among the materials, molybdenum had the lowest current. From Fowler-Nordheim plot analysis, it was revealed that fuzz samples had higher field enhancement factor than loop samples. Bubble samples marked the highest value in the field enhancement factor among the three types of morphology, likely due to the continuous emission from the craters formed after the breakdown. Molybdenum showed favorable properties in terms of suppressing the field emission and preventing the breakdown and the arcing in fusion reactor. From the scanning electron microscope observation, the size of the crater increases with the emission current on the bubble samples. Fuzzy structure was observed at the crater on the tantalum bubble surface after breakdown even without stationary plasma irradiation or stationary incidence of ion flux.

DOI： 10.1109/TPS.2017.2679211

Web of Science

Scopus

Effects of modified surfaces produced at plasma-facing surface on hydrogen release behavior in the LHD Reviewed Open Access

Nobuta Y, Masuzaki S, Tokitani M, Ashikawa N, Nagata D, Yoshida N, Oya Y, Yajima M, Motojima G, Kasahara H, Miyamoto M, Ohno N, Yamauchi Y

NUCLEAR MATERIALS AND ENERGY Vol. 12 page： 483 - 487 2017.8

Molecular activated recombination in divertor simulation plasma on GAMMA 10/PDX Reviewed Open Access

Sakamoto M, Terakado A, Nojiri K, Ezumi N, Nakashima Y, Sawada K, Ichimura K, Fukumoto M, Oki K, Shimizu K, Ohno N, Masuzaki S, Togo S, Kohagura J, Yoshikawa M

NUCLEAR MATERIALS AND ENERGY Vol. 12 page： 1004 - 1009 2017.8

Toroidally symmetric/asymmetric effect on the divertor flux due to neon/nitrogen seeding in LHD Reviewed Open Access

H. Tanaka, G. Kawamura, S. Masuzaki, M. Kobayashi, T. Akiyama, B.J. Peterson, K. Mukai, R. Sano, S.Y. Dai, R. Sakamoto, T. Morisaki, N. Ohno, LHD Experiment Group

Nucl. Mater. Energy Vol. 12 page： 241 - 246 2017.8

Behavior of 23S metastable state He atoms in low-temperature recombining plasmas Reviewed Open Access

S.Kajita, T.Tsujihara, M.Aramaki, H.van der Meiden, H.Oshima, N.Ohno, H.Tanaka, R.Yasuhara, T.Akiyama, K.Fujii, T.Shikama

Physics of Plasmas Vol. 24 page： 073301(8 pp) 2017.6

Influence of expanding and contracting magnetic field configurations on detached plasma formation in a linear plasma device Reviewed Open Access

Y.Hayashi, N.Ohno, S.Kajita, H.Tanaka

Physics of Plasmas Vol. 24 page： 062509(8 pp) 2017.6

Recent progress of the JT-60SA project Reviewed International coauthorship International journal

Shirai H., Barabaschi P., Kamada Y., Maksoud W.A., Aiba N., Akiyama T., Alonso J., Angioni C., Arai T., Arakawa H., Ardellier F., Artaud J.F., Asakura N., Ashikawa N., Barbato E., Barrera G., Baruzzo M., Baulaigue O., Beaufils S., Bécoulet M., Benkadda S., Benoit F., Bettini P., Beurskens M., Bierwage A., Boboc A., Boccaccini L., Bolzonella T., Bonne F., Borba D., Botija J., Bourdelle C., Brolatti G., Bruschi A., Bucalossi J., Cabrera-Pérez S., Canton A., Cardella A., Carvalho I., Challis C., Chapman I., Chiba S., Ciazynski D., Cismondi F., Clement-Lorenzo S., Coccoluto G., Cocilovo V., Coda S., Coelho R., Coletti A., Collin B., Conway G., Corato V., Costa P., Crisanti F., Cristofani C., Cruz N., Cucchiaro A., Dairaku M., Davis S., Day C., de Baar M., de la Luna E., De Tommasi G., De Vellis A., De Vries P., Decker J., Decool P., Delrez C., Di Pace L., Di Pietro E., Disset G., Donati A., Douai D., Dougnac H., Duchateau J.L., Duglue D., Dumas N., Dunai D., Ejiri A., Endo Y., Enoeda M., Falchetto G., Farina D., Farthing J., Fejoz P., Fernandez P., Ferro A., Fiamozzi-Zignani C., Fietz W., Figini L., Frello G., Frosi P., Fujita T., Fukuda T., Fukumoto M., Fukuyama A., Furukawa M., Furukawa M.

Nuclear Fusion Vol. 57 ( 10 ) 2017.6

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

The JT-60SA project has been implemented for the purpose of an early realization of fusion energy. With a powerful and versatile NBI and ECRF system, a flexible plasma-shaping capability, and various kinds of in-vessel coils to suppress MHD instabilities, JT-60SA plays an essential role in addressing the key physics and engineering issues of ITER and DEMO. It aims to achieve the long sustainment of high integrated performance plasmas under the high β N condition required in DEMO. The fabrication and installation of components and systems of JT-60SA procured by the EU and Japan are steadily progressing. The installation of toroidal field (TF) coils around the vacuum vessel started in December 2016. The commissioning of the cryogenic system and power supply system has been implemented in the Naka site, and JT-60SA will start operation in 2019. The JT-60SA research plan covers a wide area of issues in ITER and DEMO relevant operation regimes, and has been regularly updated on the basis of intensive discussion among European and Japanese researchers.

DOI： 10.1088/1741-4326/aa5d01

Scopus

Measurement of Poloidal Flow Profiles Using a Mach Probe Array in HYBTOK-II Tokamak with RMP Fields Reviewed

Okamoto Masaaki, Sakakibara Satoru, Yoshimoto Naoya, Ohno Takuya, Ohno Noriyasu

PLASMA AND FUSION RESEARCH Vol. 12 page： 1202027 2017.6

Observations of sustained phase shifted magnetic islands from externally imposed m/n=1/1 RMP in LHD Reviewed International coauthorship Open Access

Narushima Y., Sakakibara S., Suzuki Y., Watanabe K. Y., Ohdachi S., Takemura Y., Yoshinuma M., Ida K., Castejon F., Lopez-Bruna D., Hegna C. C., Kobayashi M., Tanaka H., Akiyama T., Ohno N.

NUCLEAR FUSION Vol. 57 ( 7 ) page： 076024 2017.5

Molecular activated recombination in divertor simulation plasma on GAMMA 10/PDX Reviewed Open Access

M.Sakamoto, A.Terakado, K.Nojiri, N.Ezumi, Y.Nakashima, K.Sawada, Ichimura, M.Fukumoto, N.Ohno, S.Masuzaki, S.Togo, Kohagura, M.Yoshikawa

Nuclear Materials and Energy Vol. 12 page： 1004-1009 2017.5

Increase in the energy absorption of pulsed plasma by the formation of tungsten nanostructure Reviewed

D. Sato, N. Ohno, F. Domon, S. Kajita, Y. Kikuchi, I. Sakuma

Nuclear Fusion Vol. 57 ( 6 ) page： 066028(8 pp) 2017.4

Language：English Publishing type：Research paper (scientific journal)

© 2017 IAEA, Vienna. The synergistic effects of steady-state and pulsed plasma irradiation to material have been investigated in the device NAGDIS-PG (Nagoya DIvertor Simulator with Plasma Gun). The duration of the pulsed plasma was ∼0.25 ms. To investigate the pulsed plasma heat load on the materials, we developed a temperature measurement system using radiation from the sample in a high time resolution. The heat deposited in response to the transient plasma on a tungsten surface was revealed by using this system. When the nanostructures were formed by helium plasma irradiation, the temperature increase on the bulk sample was enhanced. The result suggested that the amount of absorbed energy on the surface was increased by the formation of nanostructures. The possible mechanisms causing the phenomena are discussed with the calculation of a sample temperature in response to the transient heat load.

DOI： 10.1088/1741-4326/aa67ad

Web of Science

Scopus

Tailoring of fuzzy nanostructures on porous tungsten skeleton by helium plasma irradiation Reviewed

Shin Kajita, Hirohiko Tanaka, Noriyasu Ohno

Japanese Journal of Applied Physics Vol. 56 ( 3 ) page： 030303(4 pp) 2017.3

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

Porous tungsten skeleton, which was fabricated by sintering of tungsten powder, was exposed to helium plasmas, and the fuzzy nanostructures were tailored on the surface. The hemispherical optical reflectance of the samples was measured at the wavelength of 633 nm. It was shown that the optical reflectance of the porous tungsten skeleton was lower than that of flat tungsten samples. The minimum reflectance was &gt
0.4%, suggesting that the darkest metallic material was fabricated. The advantage of the porous tungsten skeleton with nanostructures for optical application is discussed.

DOI： 10.7567/JJAP.56.030303

Web of Science

Scopus

Plasma detachment in linear devices Reviewed

Ohno N.

PLASMA PHYSICS AND CONTROLLED FUSION Vol. 59 ( 3 ) page： 034007(9 pp) 2017.3

Localized density fluctuation in the downstream of detached plasma Reviewed

Kohei Takeyama, Noriyasu Ohno, Masayuki Yoshikawa, Hirohiko Tanaka, Shin Kajita

Plasma and Fusion Research Vol. 12 ( 1 ) page： 1202007 2017.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Japan Society of Plasma Science and Nuclear Fusion Research

The density fluctuation was measured with a 70-GHz microwave interferometry system in the linear divertor plasma simulator NAGDIS-II while changing from the attached to the detached states. In the detached plasma, a large amplitude fluctuation of ≤ 10 kHz appeared in the peripheral region. The fluctuation has a maximum value at the neutral gas pressure of 30 - 34 mTorr. Further, the measurement of the light emission associated with the plasma recombination indicates that the large amplitude fluctuation is located at just downstream of the recombination front, in which the recombination process is the most enhanced.

DOI： 10.1585/pfr.12.1202007

Web of Science

Scopus

Transverse motion of a plasma column in a sheet plasma Reviewed

Takuya Onda, Shin Kajita, Takaaki Iijima, Akira Tonegawa, Noriyasu Ohno, Hirohiko Tanaka

CONTRIBUTIONS TO PLASMA PHYSICS Vol. 57 ( 2 ) page： 87 - 93 2017.2

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-V C H VERLAG GMBH

In this paper, we show the dynamic behaviors of a sheet plasma and the corresponding release of blob-like plasmas to the periphery of the sheet based on observation with a high-speed camera. We found that the plasma column moves in a transverse direction reciprocally when injecting a small amount of Ar gas to the He plasma from the plasma source region. The motion continued periodically at a frequency of similar to 5 kHz. The frequency of the plasma column motion decreased with increasing Ar gas flow rate and increasing magnetic field. The mechanism of the transverse motion is discussed. Furthermore, in response to the reciprocating motion, release of a blob-like plasma was observed. The differences in the behavior of the released plasma compared with the other linear devices are discussed.

DOI： 10.1002/ctpp.201500142

Web of Science

In Situ TEM Observation of Helium Bubbles Collapsing on Nanostructured Tungsten during Annealing Reviewed

M. Yajima, N. Yoshida, S. Kajita, and N. Ohno

Plasma and Fusion Research Vol. 11 page： 1206125 2016.12

Measurement of heat diffusion across fuzzy tungsten layer Reviewed Open Access

S.Kajita, T.Yagi, K.Kobayashi, M.Tokitani, N.Ohno

Results in Physics Vol. 6 page： 877-878 2016.11

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Papers - OHNO, Noriyasu