Faculty Profiles - OHNO, Noriyasu

Papers - OHNO, Noriyasu

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Control of detached plasma through neutral particle residence time and its impact on metastable atom production Reviewed Open Access

Hayashi Y., Ohno N., Natsume H., Tanaka H., Kajita S., Aramaki M., Mano R., Koyama R., Sawada K.

Physics of Plasmas Vol. 33 ( 1 ) page： 012504 2026.1

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

The dependence of the metastable state (2<sup>3</sup>S) helium (He) density and temperature on the neutral gas pressure, P<inf>n</inf>, in detached plasmas, was investigated in the NAGDIS-II linear plasma device using laser absorption spectroscopy and supporting diagnostics. Particular attention was paid to two operation conditions with different neutral gas residence times: (i) a low puffing and low pumping rates condition (long residence time) and (ii) a high puffing and high pumping rates condition (short residence time). Under condition (i), the metastable density exhibited a gradual increase, and the temperature remained nearly constant with increasing P<inf>n</inf>. In contrast, under condition (ii), the metastable density increased by nearly an order of magnitude with increasing P<inf>n</inf>, accompanied by an increase in the metastable temperature. This difference indicates that strong electron–ion volume recombination occurs under the short-residence condition (ii), leading to the generation of high-density and high-temperature metastable atoms. Optical emission and target ion flux measurements support the enhanced contribution of volume recombination processes under condition (ii). Simulations with the DISCOVER code reproduced these behaviors, showing that metastable atoms are mainly supplied by direct excitation in the upstream region and by recombination in the downstream region, with their temperature reflecting the local ion temperature. These findings highlight that the neutral gas residence time, in addition to P<inf>n</inf>, provides an independent and effective control parameter for plasma detachment and metastable atom dynamics.

DOI： 10.1063/5.0303470

Open Access

Scopus

Recollections for the 50th anniversary of the plasma surface interactions (PSI) in controlled fusion devices conference Reviewed International coauthorship International journal Open Access

Grosman A., Roth J., Winter J., Li J., Ohno N., Maingi R.

Nuclear Materials and Energy Vol. 45 page： 102037 2025.11

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

The Plasma Surface Interactions in Controlled Fusion Devices (PSI) conference reached an important milestone in 2024 with its 50th anniversary. It was celebrated at its venue in Marseille by a special round table discussion gathering 6 former chairmen of its Programme Committees, who gave some highlights presented at the conference during the five decades. The article provides a summary of this overview.

DOI： 10.1016/j.nme.2025.102037

Open Access

Scopus

Reduction of the temperature threshold for tungsten fuzz growth by helium pre-irradiation Reviewed Open Access

Q.Shi, S.Kajita, N.Ohno

Journal of Nuclear Materials Vol. 617 page： 156134 2025.9

Dynamic response of atomic processes in detached helium plasma induced by high-density transient pulse in Magnum-PSI Reviewed International coauthorship International journal

Hayashi Y., Tanaka H., Kajita S., Ohno N., Hwangbo D., Vernimmen J., Scholten J., van der Meiden H., Classen I., Morgan T.

Nuclear Fusion Vol. 65 ( 8 ) page： 086013 2025.7

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

Pulsed plasma experiments simulating the transient divertor heat loads caused by edge-localized modes (ELMs) were conducted in Magnum-PSI, a linear plasma device capable of generating ITER-relevant conditions. The dynamic responses of detached recombining helium (He) plasmas to the pulsed plasma were observed through time-resolved laser Thomson scattering measurements and optical emission spectroscopy. Experimental results indicate that He atom depletion occurs in high-density pulsed plasmas due to a reduced mean-free-path and increased plasma pressure. This limits plasma-neutral interactions, suppressing energy dissipation via direct excitation and electron-ion recombination processes. The He II and He I emissions were found to peak sequentially, suggesting that doubly-ionized He ions recombine first, followed by the recombination of singly-ionized He ions. These findings suggest that the neutral penetration and recombination efficiency are significantly reduced in pulsed plasmas, challenging heat flux mitigation in fusion divertors. The results provide critical insights into the behavior of He ash transported by ELMs and plasma detachment under high-density pulsed conditions, highlighting the need for further studies on neutral fueling strategies in ITER-relevant environments.

DOI： 10.1088/1741-4326/adeac4

Scopus

Discharge characteristics of a hybrid plasma source using hot cathode arc discharge and helicon wave discharge Reviewed International journal

Kinashi Y., Ezumi N., Seto T., Miyauchi R., Bhattarai H., Yamazaki T., Hirata M., Kohagura J., Togo S., Sakamoto M., Ohno N.

Japanese Journal of Applied Physics Part 1 Regular Papers and Short Notes and Review Papers Vol. 64 ( 6 ) page： 060901 2025.5

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

High-density and large-diameter plasma generation is expected to contribute to semiconductor manufacturing, space propulsion, and nuclear fusion. We develop a plasma generation method for high-density, large-diameter plasma by combining the advantages of both hot cathode arc discharge and helicon wave discharge characteristics. The experiment confirmed that hybrid discharges produce a higher-density plasma compared to standalone arc discharges or helicon wave discharges across a wide range of gas pressures. Additionally, the diameter of the plasma was found to be twice as large as that of arc discharges alone. The mechanisms behind hybrid discharge and potential performance improvements are discussed.

DOI： 10.35848/1347-4065/add9f5

Scopus

Behavior of Sputtered Tungsten in the Divertor Plasma Simulator NAGDIS-II Reviewed International journal

TANAKA Hirohiko, KAJITA Shin, OHNO Noriyasu

Plasma and Fusion Research Vol. 20 ( 0 ) page： n/a - 1302020 2025.4

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

<p>The core-degradation effect due to tungsten (W) - a proposed plasma-facing material for the operation of future nuclear fusion reactors such as ITER and DEMO - calls for the study of sputtered W. In this study, sputtered W from a point source in the linear divertor plasma simulator, NAGDIS-II, was investigated. A hyperspectral imaging (HSI) camera was used to image the spatial profile of the sputtered W in helium and argon mixture plasma with different incident ion energies. By applying the Abel transform and fittings with several functions and comparing with the theoretical value, it is found that the ionization effect is not obvious but the geometrical spreading effect is dominant for the axial decay of the local W emission profile.</p>

DOI： 10.1585/pfr.20.1302020

Scopus

CiNii Research

Nanoporous Helium–Silicon Co-Deposition Thin Film via Plasma-Assisted Process for Lithium-Ion-Battery Anodes Reviewed International journal Open Access

Kajita S., Uchida G., Tanaka H., Tabata K., Yamamoto Y., Ohno N.

Advanced Energy and Sustainability Research Vol. 6 ( 3 ) 2025.2

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

In this study, silicon (Si) deposition is performed in a high-density (1018 m−3) helium (He) plasma environment, and He–Si co-deposition layers, where He atoms are implanted into the Si thin film, are formed. The He-containing thin film, which has a porosity of ≈0.5, is composed of smaller clusters with the size of 100–200 nm including many pores with different sizes, which is advantageous for lithium-ion-battery (LIB) negative electrode. It is also shown that substrate copper (Cu) atoms are diffused into the deposition layer and Cu doping occurred naturally. The LIB performance of the He–Si co-deposited thin film (>1 μm in thickness) is evaluated. When the substrate temperature is at 523 K during the deposition, the Si layer has amorphous structure, and the LIB discharge capacity remains 1800 mAh g−1 after 100 cycles. In the results, it is shown that the Si–He co-deposition method can be a novel method to fabricate Cu-doped porous amorphous Si thin film for high-performance LIB application.

DOI： 10.1002/aesr.202400300

Open Access

Scopus

Tungsten fuzz growth at low temperatures (<900 K) on a surface with nanocones Reviewed International journal Open Access

Shi Q., Kajita S., Ohno N.

Nuclear Materials and Energy Vol. 39 page： 101668 2024.5

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

The tungsten (W) fuzzy structure was widely observed in linear plasma devices under the fusion-relevant condition. This type of structure can be used in many application fields benefiting from its larger surface area and high optical absorbance. However, W fuzz can be barely formed when the sample temperature is lower than 900 K. This study decreased the threshold to ∼800 K using W co-deposition on a Si nanocones substrate. It suggests that the conventional threshold temperature is only necessary for He bubble rupture to create a rough surface that induces the uneven trapping of the adatom diffusion.

DOI： 10.1016/j.nme.2024.101668

Open Access

Scopus

Surface modification of ZrC dispersion-strengthened W under low energy He plasma irradiation Reviewed International coauthorship

Li L., Liu Z., Chen Z., Yin C., Mao S., Wu X., Ohno N., Ye M.

Nuclear Fusion Vol. 64 ( 5 ) page： 56008 2024.3

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

ZrC dispersion-strengthened W exhibits high strength/ductility, low ductile-to-brittle transition temperature, and excellent thermal shock resistance, making it a promising candidate plasma-facing material for future fusion devices. In this study, surface modification of 0.5 wt.% ZrC dispersion-strengthened W (WZrC) under low energy and high fluence He plasma irradiation at high temperature was presented. Under the energy of 90 eV and fluence ranging from 6 × 1024 He·m−2-2 × 1026 He·m−2 He irradiation at 920 °C, a typical fuzz nanostructure appeared on the W matrix of WZrC. The thickness of fuzz layer is proportional to the square root of He irradiation fluence. The fuzz showed comparable thickness and structural features to pure W, indicating limited effects of the particle’s addition on resistance to high fluence He irradiation at high temperatures. Under continuous He injection, the fuzz would grow extending onto the particle area, making the particle obscured. Besides, the erosion behavior of particles under He plasma irradiation has been investigated, which is thought to be dominated by a sputtering process. Under the He influence of 6 × 1024 He·m−2, only nanopores were observed in the surface region. With fluence increasing to 5 × 1025 He·m−2, the surface became relatively uneven with larger holes. W aggregated in spots and distributed on the surface of the particle, which might be the result of subthreshold sputtering and deposition. When fluence further increased to 2 × 1026 He·m−2, the particles were eroded completely and covered by the extended fuzz, forming cavities. In addition, distinctive layered nanotendrils were observed above the cavities, which were characterized to consist of inner W-riched skeletons and outer Zr-riched layers. It indicates that the layered nanotendrils should be the result of fuzz extension combined with particle sputtering/deposition.

DOI： 10.1088/1741-4326/ad2f4c

Scopus

Nanopatterning of Si surfaces by normal incident He plasma irradiation Reviewed

Liu Z., Li L., Gao Z., Chen Z., Yin C., Mao S., Kajita S., Ohno N., Ye M.

Applied Physics Letters Vol. 124 ( 10 ) 2024.3

Publisher：Applied Physics Letters

This study reports on the formation of self-organized silicon (Si) nanostructures by 75 eV helium (He) plasma irradiation at normal incidence without the presence of impurities. In contrast to the featureless surface after normal incidence argon (Ar) ion beam irradiation without the co-deposition of impurities, the Si surface exhibits the development of faceted nanostructures under 75 eV He plasma irradiation. The faceted structures are interspersed with valleys that extend in two orthogonal directions, imparting a mountain-like morphology to the surface. Our investigation verifies that the He bubbles align themselves along the direction perpendicular to the surface underneath these valleys. Furthermore, the presence of He bubbles induces distortion in the surface layer and leads to the formation of an amorphous Si layer. The underlying mechanism driving this surface evolution could be attributed to the instability induced by the presence of He bubbles.

DOI： 10.1063/5.0186756

Scopus

Cone array formation on Si surfaces by low-energy He plasma irradiation with magnetron sputtering pre-deposited Ta Reviewed International coauthorship

Liu Z., Li L., Gao Z., Chen Z., Yin C., Mao S., Kajita S., Ohno N., Ye M.

Journal of Applied Physics Vol. 135 ( 9 ) page： 93302 2024.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Applied Physics

Low-energy ion beam irradiation, combined with the introduction of impurities, presents a promising approach for nanopatterning silicon (Si) surfaces. In this research, we investigate the surface evolution of Si (100) surfaces irradiated by 75 eV helium (He) plasma, in the presence of tantalum (Ta), traditionally regarded as an impurity incapable of initiating pattern formation, as evidenced in prior studies. The Ta impurities are pre-deposited onto the Si surfaces using the magnetron sputtering method, which offers a more controlled and quantifiable approach compared with the conventional co-deposition route. After irradiation at 800 K, dense cone arrays are produced on the Si surface. The growth of the cones is explored for fluence spanning (1-10) × 1025 m−2. The cross-sectional scanning electron microscope images indicate that the cone lengths and base width are well characterized by t1/2 dependence. The kinetics of the cone growth follow Fick’s law, characterized by an effective diffusive mechanism with the coefficients of diffusion: D = 6.49 ± 0.83 × 10−16 m−2 s−1. Transmission electron microscope observations reveal that the cone has distinct inner and outer parts with different microstructures and a clear interface. The growth mechanism of these cones is elucidated as a composite process involving preferential sputtering, the destabilizing influence of He bubbles, and the migration of adatoms.

DOI： 10.1063/5.0187265

Scopus

Plasma-Driven Sciences: Exploring Complex Interactions at Plasma Boundaries Reviewed International journal Open Access

Ishikawa K., Koga K., Ohno N.

Plasma Vol. 7 ( 1 ) page： 160 - 177 2024.2

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

Plasma-driven science is defined as the artificial control of physical plasma-driven phenomena based on complex interactions between nonequilibrium open systems. Recently, peculiar phenomena related to physical plasma have been discovered in plasma boundary regions, either naturally or artificially. Because laboratory plasma can be produced under nominal conditions around atmospheric pressure and room temperature, phenomena related to the interaction of plasma with liquid solutions and living organisms at the plasma boundaries are emerging. Currently, the relationships between these complex interactions should be solved using science-based data-driven approaches; these approaches require a reliable and comprehensive database of dynamic changes in the chemical networks of elementary reactions. Consequently, the elucidation of the mechanisms governing plasma-driven phenomena and the discovery of the latent actions behind these plasma-driven phenomena will be realized through plasma-driven science.

DOI： 10.3390/plasma7010011

Open Access

Scopus

Thermal Conductivity Measurement of Fuzzy W Using a Thermoreflectance Method Reviewed

Kajita S., Yagi T., Ohno N.

IEEE Transactions on Plasma Science Vol. Early Access page： 1 - 6 2024.1

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Transactions on Plasma Science

Fuzzy nanostructured tungsten (W) is formed by helium (He) plasma irradiation. In fusion devices, the thermophysical properties are important to consider the response to pulsed heat load from the plasma. In this work, the thermal conductivity of the W fuzz is measured using a pulsed light heating thermoreflectance method. A detailed model was developed to explain the entire temporal evolution of the thermoreflectance signal. It was found that the thermal conductivity of the nanostructured layer was deduced to be in the range of 0.18%–0.21% of the pure (ideal) W and lower than the previously reported highly porous metallic materials. In addition to the porosity, the specific nonuniform nanofiber structure is likely to contribute to the low effective thermal conductivity.

DOI： 10.1109/TPS.2023.3348465

Scopus

Thermal response of nano-tendril bundles to steady heat load Reviewed International coauthorship Open Access

Xue Qi, Tanaka Hirohiko, Zhang Rongshi, Kajita Shin, Hori Kenta, Shi Quan, Kuwabara Tatsuya, Ohno Noriyasu

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 62 ( SN ) 2023.11

Transition from MAR to EIR of deuterium plasma detachment in NAGDIS-II Reviewed International coauthorship Open Access

Shi J., Kaizawa H., Tanaka H., Kajita S., Ohno N., Ding H.

Physica Scripta Vol. 98 ( 11 ) page： 115605 2023.10

Language：English Publishing type：Research paper (scientific journal) Publisher：Physica Scripta

Volumetric plasma recombination in a detached deuterium (D) plasma was experimentally studied in the linear plasma device NAGDIS-II. The radial distributions of the electron temperature (Te) and the electron density (ne) were measured by upstream and downstream laser Thomson scattering (LTS) systems and D-atom excited states of the Balmer series (n = 3-7) were measured by optical emission spectroscopy (OES). The results indicated that the electron static pressure, particle flux and heat flux present a two-step reduction with an increasing neutral pressure, suggesting that molecule activated recombination (MAR) and electron-ion recombination (EIR) were activated, respectively. The difference in the distributions of the atomic state population densities was observed to be a feature of the transition from the MAR-dominant phase to the EIR-dominant phase. The transition between the two phases had a distinct boundary at Te ∼0.7 eV, which was considered a transition point between the phases. The ratios of the atomic state population densities showed different features in the two phases, and are expected to be useful in characterizing the phase and degree of plasma detachment.

DOI： 10.1088/1402-4896/acfeb4

Open Access

Scopus

Application of machine learning for optical emission spectroscopy data in NAGDIS-II Reviewed Open Access

Kajita S., Sakakibara T., Kaizawa H., Natsume H., Tanaka H., Fujii K., Ohno N.

Fusion Engineering and Design Vol. 196 page： 114012 2023.9

Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Fusion Engineering and Design

In this study, we applied machine learning to optical emission spectroscopy (OES) data and device parameters from the linear plasma device NAGDIS-II to explore the potential application of machine learning for predicting electron density, ne, and temperature, Te. The covered ranges of ne and Te, which were measured by an electrostatic probe, are 3.6 × 1017–2.4×1019 m−3 and 0.3–7.1eV, respectively. A three hidden layer neural network (NN) is introduced to model the relationship between ne/Te and the combination of line intensities, radial position, and device parameters. It is shown that the errors in ne and Te become 18.0 and 18.8%, respectively, which were almost the same level for the electrostatic probe, using all available data. Lasso regression and greedy algorithm are used to select the necessary line emissions. It is shown that four or five line intensities are sufficient to obtain almost the same quality as the one with all the other lines.

DOI： 10.1016/j.fusengdes.2023.114012

Scopus

Fabrication of tungsten-based optical diffuser using fiberform nanostructure via efficient plasma route Reviewed International coauthorship Open Access

Feng Shuangyuan, Natsume Hiroki, Kajita Shin, Li Enhao, Yasuhara Ryo, Tokitani Masayuki, Tanaka Hirohiko, Ohno Noriyasu

OPTICS EXPRESS Vol. 31 ( 16 ) page： 25438 - 25445 2023.7

Field emission properties of nano-tendril bundles formed via helium plasma exposure with various additional impurity gases Reviewed International coauthorship Open Access

Zhang Rongshi, Kajita Shin, Hwangbo Dogyun, Tanaka Hirohiko, Feng Shuangyuan, Ohno Noriyasu

MATERIALS RESEARCH EXPRESS Vol. 10 ( 5 ) page： 054002 2023.5

Growth origin of large-scale fiberform nanostructures in He-W co-deposition environment Reviewed Open Access

Hori K, Kajita S, Zhang R, Tanaka H, Ohno N

Scientific reports Vol. 13 ( 1 ) page： 5450 2023.4

Rapid growth of nanostructure on tungsten thin film by exposure to helium plasma Reviewed

Feng Shuangyuan, Kajita Shin, Tokitani Masayuki, Nagata Daisuke, Ohno Noriyasu

PLASMA SCIENCE & TECHNOLOGY Vol. 25 ( 4 ) page： 042001 2023.4

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Papers - OHNO, Noriyasu