Faculty Profiles - OHNO, Noriyasu

Papers - OHNO, Noriyasu

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Helium-plasma–induced straight nanofiber growth on HCP metals Reviewed

S.Kajita, T.Nojima, T.Okuyama, Y.Yamamoto, N.Yoshida, N.Ohno

Acta Materialia Vol. 181 page： 342 - 351 2019.10

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

© 2019 Acta Materialia Inc. Low-energy helium (He) plasma irradiations were conducted on ruthenium (Ru) and rhenium (Re), which have hexagonal close packed (HCP) crystal structures. Growth of linear shaped fiberform nanostructures were identified on the surfaces of the both metals after the He plasma irradiation. We also conducted He plasma irradiation while Re particles were deposited on tungsten substrate; 3-mm-thick large scale fiberform nanostructures were grown on the surface. The crystal orientation was analyzed using diffraction patterns of Re and Ru nanofibers together with detailed transmission electron microscope observations. It was found that the growth of linear nanofibers has a preferential crystal orientation in the growth direction and it is always in the c-direction of the HCP crystals. Potential growth processes and mechanisms are proposed based on the experimental observations.

DOI： 10.1016/j.actamat.2019.09.053

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Field Emission From Nanostructured Tendril Bundles Reviewed International coauthorship

D.Sinelnikov, D.Bulgadaryan, D.Hwangbo, S.Kajita, V.Kumaev, N.Ohno

IEEE TRANSACTIONS ON PLASMA SCIENCE Vol. 47 ( 11 ) page： 5186 - 5190 2019.10

Kinetics of deuterium penetration into neutron-irradiated tungsten under exposure to high flux deuterium plasma Reviewed

M. Yajima, Y. Hatano, N. Ohno, T. Kuwabara, T. Toyama, M. Takagi, K. Suzuki

Nuclear Materials and Energy Vol. 21 page： 100699 2019.8

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

© 2019 The Authors The objective of this study is to investigate the effects of neutron (n) irradiation on hydrogen isotope transport in tungsten (W). W samples were irradiated with neutrons in a fission reactor and then exposed to high flux deuterium (D) plasma at 563 K in a linear plasma device. The fraction of D release increased with increasing exposure time. In addition, the D retention in n-irradiated W increased in proportion to the square root of the plasma exposure time. These observations were explained by increase in the penetration depth of D with filling up displacement damages acting as strong trapping sites.

DOI： 10.1016/j.nme.2019.100699

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Application of Ion Sensitive Probe to High Density Plasmas in Magnum-PSI Reviewed International coauthorship

Y.Hayashi, N.Ohno, H.van der Meiden, J.Scholten, S.Kajita, J.van der Berg, R.Perillo, J.Vernimmen, T.Moragan

PLASMA AND FUSION RESEARCH Vol. 14 page： 1202135(3 pp) 2019.7

Spatial and temporal measurement of recombining detached plasmas by laser Thomson scattering Reviewed

S. Kajita, H. Ohshima, H. Tanaka, M. Seki, H. Takano, N. Ohno

PLASMA SOURCES SCIENCE & TECHNOLOGY Vol. 28 page： 105015 (9pp) 2019.7

Photocatalytic decomposition of ethylene using He plasma induced nano-TiO<inf>2</inf> Reviewed

S.Kajita, Y.Tomita, E.Yasunaga, T.Yoshida, K.Miyaguchi, H.Tanaka, N.Ohno

Japanese Journal of Applied Physics Vol. 58 page： 070903(4 pp) 2019.6

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

© 2019 The Japan Society of Applied Physics. Ethylene decomposition via photocatalytic reaction using ultra-violet light was performed using an oxidized titanium (Ti) sheet and/or thin film samples fabricated by helium (He) plasmas treatments. We prepared nanostructured Ti samples formed by He plasma irradiation and additional thin film deposited samples using radio frequency magnetron sputtering. The samples prepared in this study had a slightly lower photocatalytic activity compared to the thin film deposited on tungsten nanostructure samples previously reported; the anatase formation was solely identified on Ti sheet samples and is hopeful for further improvement of photocatalytic activity of plasma treated titania.

DOI： 10.7567/1347-4065/ab2830

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Fabrication of a nanostructured TiO<inf>2</inf> photocatalyst using He plasma-irradiated tungsten and ethylene gas decomposition Reviewed

Y.Tomita, S.Kajita, E.Yasunaga, T.Yoshida, N.Ohno, H.Tanaka

Japanese Journal of Applied Physics Vol. 58 ( SE ) page： SEEG01(6 pp) 2019.5

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

© 2019 The Japan Society of Applied Physics. Decomposition of ethylene gas by a photocatalyst is an environmentally friendly and sustainable process. We fabricated titanium dioxide thin films deposited on nanostructured tungsten substrates formed by exposure to helium plasma and evaluated their photocatalytic performance. The nanostructures were maintained even after sputtering and annealing, and the proportion of tungsten inside the thin film increased with increasing oxidation temperature. The photocatalytic activity was improved due to formation of the nanostructure.

DOI： 10.7567/1347-4065/ab09c8

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Investigation of recombination front region in detached plasmas in a linear divertor plasma simulator Reviewed International coauthorship

N.Ohno, M.Seki, H.Ohshima, H.Tanaka, S.Kajita, Y.Hayashi, H.Natsume, H.Takano, I.Saeki, M.Yoshikawa, H.van der Meiden

Nuclear Materials and Energy Vol. 19 page： 458 - 462 2019.5

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

We have investigated detailed plasma structure around the recombination front region (RFR), where volume electron-ion recombination (EIR) strongly occurs in detached plasmas, by using two-dimensional (2D) movable probe system as well as laser Thomson scattering diagnostics. The 2D distribution of electron density, electron temperature, and plasma potential around the RFR, has been revealed for the first time. The spatial distribution of the particle loss rate due to EIR estimated from electron temperature and electron density profiles indicated that the three-dimensional structure of the RFR adopts a cone structure. Furthermore, in the peripheral region of the RFR, the radial profile of the electron density was widened locally with increasing cross-field transport caused by plasma fluctuation. From the heat transport analysis of electrons, it was found that the thermal relaxation process between electrons and ions is important for the cooling of electrons, and that the convection of electrons regulates electron heat transport near the RFR.

DOI： 10.1016/j.nme.2019.03.010

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Generation of Spiral Shape Nitrogen Recombining Plasma for Atomic Nitrogen Source Reviewed

K. Asaoka, N. Ohno, Y. Hayashi, S. Kajita, and H. Tanaka

Plasma and Fusion Research Vol. 14 page： 3401069(4 pp) 2019.4

Characterization of He induced nanostructures using SEM image analysis Reviewed

H.Tanaka, S.Kajita, N.Ohno

Plasma and Fusion Research Vol. 14 ( Specialissue2 ) page： 3402049(6 pp) 2019.4

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

© 2019 The Japan Society of Plasma Science and Nuclear Fusion Research. Helium induced nanostructures on tungsten were characterized by applying basic statistical analysis techniques to scanning electron microscope (SEM) images. As the analysis targets, pinhole- and fuzz-dominant surfaces irradiated with different ion fluences were employed. Although two-dimensional Fourier analysis of the normalized brightness of SEM images clarified several characteristic features, their differences are small to distinguish the dominant nanostructures. On the other hand, probability density functions (PDFs) of the normalized brightness indicate that hole- and fuzz-dominant surfaces have long tail and Gaussian-like shape, respectively. By using several indexes with considering the black and white saturation, it was found that kurtosis and Kullback- Leibler divergence could provide reasonable judgement as the characterization index.

DOI： 10.1585/pfr.14.3402049

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Synergistic effect of nitrogen and hydrogen seeding gases on plasma detachment in the GAMMA 10/PDX tandem mirror Reviewed

N.Ezumi, T.Iijima, M.Sakamoto, Y.Nakashima, M.Hirata, M.Ichimura, R.Ikezoe, T.Imai, T.Kariya, I.Katanuma, J.Kohagura, R.Minami, T.Numakura, M. Yoshikawa, S. Togo, MD S.Islam, MD M.Islam, K.Nojiri, A.Terakado, S.Jang, Y.Kinoshita, T.Mikami, S.Yamashita, T.Yoshimoto, T.Hara, A.Hatayama, K.Ichimura, S.Kado, S.Masuzaki, T.Nakano, N.Ohno, K. Sawada, H. Tanaka, A.Tonegawa and R.Perillo

Nuclear Fusion Vol. 59 page： 060030 2019.4

Ignition and Behavior of Arc Spots on Helium Irradiated Tungsten under Fusion Relevant Condition Reviewed

S.Kajita, D.Hwangbo, N.Ohno

IEEE Transactions on Plasma Science Vol. 47 ( 8 ) page： 3909 - 3916 2019.4

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

© 1973-2012 IEEE. In nuclear fusion reactors, unipolar arcing is a longstanding plasma surface interaction issue. In this paper, short summaries of morphology changes by helium plasma irradiation under fusion relevant conditions and ignition of arcing on the He irradiated surfaces are given. Helium plasma irradiation leads to surface morphology changes, and arcing can be ignited on the surface in response to pulsed heat load accompanied by plasma instabilities. Then, we focused on arcing ignited on large-scale fiberform nanostructures (LFNs) formed with deposition of metallic particles together with helium-ion irradiation. Dusts release from the arc spot and the amount of material erosion are discussed based on the experiments.

DOI： 10.1109/TPS.2019.2908193

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Analysis of indefinite divertor footprint with proper orthogonal decomposition in hydrogen/deuterium plasmas in LHD Reviewed

H.Tanaka, S.Masuzaki, G.Kawamura, M.Kobayashi, Y.Suzuki, T.Morisaki, N.Ohno LHD Experimental Group

Nuclear Materials and Energy Vol. 19 page： 378 - 383 2019.3

Increased energy absorption into W due to the metal deposited layer from an ELM-like pulsed plasma Reviewed

T.Nakamori, N.Ohno, H.Tanaka, S.Kajita, Y.Kikuchi, T.Akiyama

Plasma and Fusion Research Vol. 14 page： 1401051(5 pp) 2019.3

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

© 2019 The Japan Society of Plasma Science and Nuclear Fusion Research. It is important in divertor physics research to investigate thermal response process of beryllium deposited tungsten to high transient heat load such as ELMs. We investigated the heat load on aluminum-coated tungsten by comparing the measured back surface temperature and calculation results of the one-dimensional heat conduction equation. When the plasma power was low enough so that the evaporation did not occur sufficiently, the heat load on the aluminum-coated tungsten was higher than the virgin tungsten. It was caused by the reduction in the energy reflection coefficient due to the formation of aluminum layer. The results suggested that the beryllium deposited layer adversely affects the heat load on the divertor plate when the heat load was not high such as Grassy-ELM.

DOI： 10.1585/pfr.14.1401051

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Double-probe measurement in recombining plasma using NAGDIS-II Reviewed

Y. Hayashi, H. Nishikata, N. Ohno, S. Kajita, H. Tanaka, H. Ohshima, M. Seki

Contributions to Plasma Physics Vol. 59 ( 7 ) page： e201800088 2019.2

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

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim We have studied the validity of the double-probe method in recombining plasmas. Electron temperature (Te) measured with a double probe was quantitatively evaluated by taking into account the influences of plasma potential fluctuation, plasma resistivity, and electron density fluctuation on the current–voltage characteristics. Differential potential fluctuation and plasma resistivity between two electrodes have a minor effect on Te especially when the inter-distance is small (typically 1 mm). Scattering of measured Te due to the density fluctuation was sufficiently suppressed by making the data acquisition time long (typically 4 s) and taking the average. There is a good agreement between Te measured with the optimized double-probe method and that with laser Thomson scattering diagnostics.

DOI： 10.1002/ctpp.201800088

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Development of Thomson scattering measurement system for Upstream plasmas in the NAGDIS-II device Reviewed

H.Takano, H.Ohshima, S.Kajita, H.Tanaka, N.Ohno

Plasma and Fusion Research Vol. 14 ( Specialissue1 ) page： 2405031(4 pp) 2019.2

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

© 2019 The Japan Society of Plasma Science and Nuclear Fusion Research. A new laser Thomson scattering (LTS) measurement system in the upstream region of the linear plasma device NAGDIS-II is established in addition to the recently introduced LTS in the downstream. Further, horizontal distributions of the electron temperature and electron density were compared with those obtained from the Langmuir probe, indicating plasma disturbance due to the insertion of the probe head. We discuss the variation of the electron energy distribution function at upstream and downstream in the recombining plasma.

DOI： 10.1585/pfr.14.2405031

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Growth of membrane nanostructures on W co-deposition layer Reviewed

S. Kajita, N. Yoshida, S. Kawaguchi, H. Tanaka, N. Ohno, D. Nagata, M. Tokitani

Nuclear Materials and Energy Vol. 18 page： 339 - 344 2019.2

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

© 2019 The Authors Large-scale mm thick fiberform nanostructures are grown when materials are subjected to high flux helium ions with tungsten particles (ions and neutrals) precipitation. Observations of the large-scale fiberform nanostructures (LFNs) were conducted by scanning electron microscope (SEM) and transmission electron microscope (TEM) analyses. In particular, this study focuses on membrane nanostructures. Membranes that contain He bubbles were always observed when LFNs covered the surface. We discuss the growth process of the membrane structures based on detailed observations.

DOI： 10.1016/j.nme.2019.02.004

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Spatiotemporal Structure of Hα Emission from the Detached Plasma in GAMMA 10/PDX Reviewed

H.Tanaka, M.Sakamoto, N.Ezumi, A.Terakado, K.Nojiri, T.Mikami, Y.Kinoshita, K.Kobayashi, M.Yoshikawa, J. Kohagura, N.Ohno

Plasma and Fusion Research: Regular Articles Vol. 14 page： 2402036(5 pp) 2019.2

Growth process of nano-tendril bundles with sputtered tungsten Reviewed

D. Hwangbo, S. Kajita, H. Tanaka, and N. Ohno

Nuclear Materials and Energy Vol. 18 page： 250 - 257 2019.1

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

© 2019 The Authors Nano-tendril bundles (NTBs) were generated in DC He plasma with impurity gas-seeding (Ne and N 2 ) under sputtering-dominated conditions. NTB morphologies were characterized using the net erosion yield of samples. NTBs’ height and area were analyzed and ion fluence dependence was discussed. For both impurities, the number of NTBs increased steadily with increasing the fluence. It was revealed that NTB can be generated newly and continuously regardless of ion fluence. Surface observations suggested that micro-protrusions can evolve into NTBs. The effect of re-deposition of W was discussed based on SEM observations. Line-of-sight deposition seemed to play a key role of W feeder to enable NTBs to grow greater.

DOI： 10.1016/j.nme.2019.01.008

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Multipoint measurements employing a microwave interferometer and a Langmuir probe in the detached linear plasma Reviewed

H. Natsume, H. Tanaka, S. Kajita, M. Yoshikawa, M. Seki, H. Ohshima, and N. Ohno

AIP ADVANCES Vol. 9 page： 015016 (6pp) 2019.1

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Papers - OHNO, Noriyasu