研究者詳細 - 大野　哲靖

論文 - 大野　哲靖

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Nanoporous Helium–Silicon Co-Deposition Thin Film via Plasma-Assisted Process for Lithium-Ion-Battery Anodes 査読有り国際誌 Open Access

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

Advanced Energy and Sustainability Research 6 巻 ( 3 ) 2025年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Tungsten fuzz growth at low temperatures (<900 K) on a surface with nanocones 査読有り国際誌

Shi Q., Kajita S., Ohno N.

Nuclear Materials and Energy 39 巻頁： 101668 2024年5月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Surface modification of ZrC dispersion-strengthened W under low energy He plasma irradiation 査読有り国際共著

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

Nuclear Fusion 64 巻 ( 5 ) 頁： 56008 2024年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Nanopatterning of Si surfaces by normal incident He plasma irradiation 査読有り

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

Applied Physics Letters 124 巻 ( 10 ) 2024年3月

出版者・発行元：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

Cone array formation on Si surfaces by low-energy He plasma irradiation with magnetron sputtering pre-deposited Ta 査読有り国際共著

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

Journal of Applied Physics 135 巻 ( 9 ) 頁： 93302 2024年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Plasma-Driven Sciences: Exploring Complex Interactions at Plasma Boundaries 査読有り国際誌

Ishikawa K., Koga K., Ohno N.

Plasma 7 巻 ( 1 ) 頁： 160 - 177 2024年3月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Thermal Conductivity Measurement of Fuzzy W Using a Thermoreflectance Method 査読有り

Kajita S., Yagi T., Ohno N.

IEEE Transactions on Plasma Science Early Access 巻頁： 1 - 6 2024年1月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Thermal response of nano-tendril bundles to steady heat load 査読有り国際共著

Xue Q., Tanaka H., Zhang R., Kajita S., Hori K., Shi Q., Kuwabara T., Ohno N.

Japanese Journal of Applied Physics 62 巻 ( SN ) 2023年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Japanese Journal of Applied Physics

Nano-tendril bundles (NTBs) are nanostructures formed on the surface of tungsten under helium plasma irradiation with impurities. Upon annealing, NTBs of different sizes exhibit distinct morphological changes. This study aims to elucidate the underlying reasons for the different morphological changes observed in NTBs through the analysis of their thermal responses. A simulation approach was employed to investigate the temperature distribution of NTBs with different heights, aspect ratios, and porosities during annealing experiments. The simulation results revealed varying degrees of impact of height, aspect ratio, and porosity on the temperature distribution of NTBs during annealing. This finding contributes to a better understanding of the thermal properties and reactions of NTBs.

DOI： 10.35848/1347-4065/ace6ac

Transition from MAR to EIR of deuterium plasma detachment in NAGDIS-II 査読有り国際共著

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

Physica Scripta 98 巻 ( 11 ) 頁： 115605 2023年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Application of machine learning for optical emission spectroscopy data in NAGDIS-II 査読有り

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

Fusion Engineering and Design 196 巻頁： 114012 2023年9月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Fabrication of tungsten-based optical diffuser using fiberform nanostructure via efficient plasma route 査読有り国際共著

Feng S., Natsume H., Kajita S., Li E., Yasuhara R., Tokitani M., Tanaka H., Ohno N.

Optics Express 31 巻 ( 16 ) 頁： 25438 - 25445 2023年7月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Optics Express

Optical diffusion is an essential process used to manage photons in a wide range of photoelectric systems. This work proposes an approach to fabricate novel optical diffusers by a plasma-processing technique, using fiberform nanostructures formed by helium plasma irradiation and subsequent annealing. After an annealing procedure in the air for oxidation, the optical properties and the light-diffusing abilities of these nanostructured thin films were studied. In addition to the morphology analysis and total transmittance measurement, the diffusion efficiency of the optical diffusers was analyzed using a transmitted scatter distribution function (TDF). It was revealed that the diffusion efficiency of a device with an irradiation time of 30 minutes could reach 97%. The results demonstrate the potential of these nanostructured optical diffusers for various photoelectric applications.

DOI： 10.1364/OE.493993

Field emission properties of nano-tendril bundles formed via helium plasma exposure with various additional impurity gases 査読有り国際共著

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

Materials Research Express 10 巻 ( 5 ) 頁： 054002 2023年5月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Research Express

Nano-tendril bundles (NTBs) were formed on tungsten via helium (He) plasma exposure with various additional impurity gases, such as neon (Ne), nitrogen (N2), and argon (Ar). The sizes of the NTBs showed different distributions with different additional impurity gases. The field emission property of the NTBs formed with various additional impurity gases was measured. The field-emission property was significantly affected by the morphology of the NTBs, especially the tips of the fibers. In the Ne- and Ar-seeded cases, the NTBs were formed with sharp tips, and the onset electric field for field emission was ∼1 kV mm−1 for all the NTB samples. The Ne-seeded samples showed the most rapid increase in the emission current. In the N2-seeded case, two types of NTBs were formed. The NTBs were formed with sharp tips when the ratio of N2 impurity gas was 2.1%. With an increase in the ratio to 3.0% or higher, the fibers of the NTBs became thicker and the tips became rounder. In the Fowler-Nordheim (F-N) plot analysis, the field enhancement factors were approximately 6000-7000 without significant differences, for all NTBs with sharp tips. NTBs with round tips showed totally different field emission property, as the emission current was only several μA, which is one-tenth of that for the other samples. This suggests that the local morphology of NTBs, especially the geometrical shape of the tips, and not the general size of NTBs, is the main factor in determining the field emission property of NTBs.

DOI： 10.1088/2053-1591/acd1d4

Growth origin of large-scale fiberform nanostructures in He-W co-deposition environment 査読有り

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

Scientific reports 13 巻 ( 1 ) 頁： 5450 2023年4月

Rapid growth of nanostructure on tungsten thin film by exposure to helium plasma 査読有り

Feng S., Kajita S., Tokitani M., Nagata D., Ohno N.

Plasma Science and Technology 25 巻 ( 4 ) 頁： 042001 2023年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Plasma Science and Technology

A fiberform nanostructure was synthesized by exposing high-density helium plasma to a 100 nm thick tungsten thin film in the linear plasma device NAGDIS-II. After helium plasma exposure, the cross-section of samples was observed by a scanning electron microscope, transmission electron microscope, and focused ion beam scanning electron microscope. It is shown that the thickness of the nanostructured layer increases significantly for only a short irradiation time. The optical absorptivity remains high, even though it is exposed to helium plasma for a short time. The usage of the thin film can shorten the processing time for nanostructure growth, which will be beneficial for commercial production.

DOI： 10.1088/2058-6272/ac9f2d

Erratum: Morphologies of co-depositing W layer formed during He plasma irradiation (International Atomic Energy Agency (2018) 58 (106002) DOI: 10.1088/1741-4326/aad24e) 査読有り

Kajita S., Kawaguchi S., Yoshida N., Ohno N., Tanaka H.

Nuclear Fusion 63 巻 ( 4 ) 2023年4月

Correlation of the Orthogonal Basis of the Core Plasma Distribution to the Divertor Footprint Distribution in LHD

Tanaka H., Masuzaki S., Kawamura G., Hayashi Y., Kobayashi M., Suzuki Y., Mukai K., Kajita S., Ohno N.

Plasma and Fusion Research 18 巻 ( 0 ) 頁： 2402021 - 2402021 2023年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：一般社団法人プラズマ・核融合学会

We have applied the multivariable analysis technique called the proper orthogonal decomposition (POD) to both the divertor particle flux distribution and the electron pressure distribution in the core region of LHD. The cross-correlation analysis indicates that 3rd, 4th, and 5th POD modes of the electron pressure distribution are highly correlated with the divertor footprint index which is a measure of where the peak position of the particle flux distribution is located on the inner divertor plate. Both the 3rd and 4th modes seem to correspond to the shift of the electron pressure peak position from the magnetic-axis radius. In contrast, 5th mode has a strong influence on the peripheral gradient of the electron pressure distribution. Their relationships with the divertor footprint could be explained by the finite β and the Pfirsch-Schlüter current effects.

DOI： 10.1585/pfr.18.2402021

Study of the intermittent plasma structure around the divertor simulation experimental module in GAMMA 10/PDX 査読有り

Tanaka H., Ezumi N., Sugiyama T., Gamo H., Shigematsu N., Yoshikawa M., Kohagura J., Hirata M., Togo S., Ohno N., Sakamoto M.

Physics of Plasmas 30 巻 ( 3 ) 頁： 32501 2023年3月

掲載種別：研究論文（学術雑誌）出版者・発行元：Physics of Plasmas

We have investigated the generation region of intermittent plasma structures, which could be due to the blob-like cross field transport around the divertor simulation experimental module (D-module) in the tandem mirror device GAMMA 10/PDX. A positive skewness of the ion saturation current was clearly seen when the electrode of the movable probe was located at the radial edge and just in front of the entrance limiter of the D-module. Fourier analysis and conditional averaging clarified that positive spikes intermittently appeared in the same region. This is the first indication that the phenomenon producing the high-density isolated plasma structures occurred in the upstream of the D-module. A negative skewness was also found, and large amplitude fluctuation was detected between regions showing positive and negative skewness. Furthermore, this and light emission fluctuations become stronger during the transient state from attached to detached state on the V-shaped target.

DOI： 10.1063/5.0129886

Structural Correlation of Random Lasing Performance in Plasma-Induced Surface-Modified Gallium Nitride 査読有り

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

ACS Applied Optical Materials 1 巻 ( 1 ) 頁： 412 - 420 2023年1月

出版者・発行元：ACS Applied Optical Materials

The surface of GaN has been modified by a simple method that irradiates the substrate with Ar plasma and impurity codeposition, which is termed CoDE (Co-Deposition Etching). Unique structures were formed by the assistance of a small amount of Mo deposition without degradation of the crystal structure. The size of protrusions on the structure is influenced by the amount of deposited Mo particles. The mechanism for the structure formation is discussed based on transmission electron microscopy (TEM) observations and energy dispersive X-ray spectroscopy (EDS) measurements. Based on the GaN surface roughness, we have demonstrated random laser action in the UV region under photoexcitation. The lowest threshold (0.06 J/cm2) for random lasing was observed where the size of the structure is larger than 0.05 μm2, which was formed under conditions with the least amount of deposition. The correlation between the submicro/nanosize structures generated by plasma irradiation and lasing was evaluated in detail. The critical parameter to optimize the random lasing was determined to be the size of structures that can be modified by the Mo deposition rate. The results confirmed that the emission properties can be controlled by the processing conditions. This random laser fabrication technique is expected to be widely applicable to various direct-bandgap compound semiconductors.

DOI： 10.1021/acsaom.2c00085

Motion analysis of arc spots on tungsten fuzz by means of self-avoiding random walk model 査読有り

Hwangbo D., Nishijima D., Kajita S., Ohno N.

Japanese Journal of Applied Physics 62 巻 ( SA ) 頁： SA1007 2023年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Japanese Journal of Applied Physics

Arc spot motions on nanostructrued “fuzz” tungsten surfaces are investigated on the basis of a two-dimensional Monte Carlo random walk simulation. To compare with previous experimental results, the effects of self-avoiding, bifurcation and extinction of arc spots, as well as the fuzz thickness dependence, are included in the simulation. Under a given temporal arc current profile which is set as close as possible to the experiment, the results derived from the simulation reveal that the motion and the number of arc spots depend significantly on the fuzz thickness.

DOI： 10.35848/1347-4065/ac79fe

Field emission current from protrusion structures formed by helium plasma with various impurity gases 査読有り

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

Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV 2023-June 巻頁： 529 - 532 2023年

出版者・発行元：Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV

Rod-like protrusion structures were formed on tungsten by helium plasma exposure with various impurity gases seeded as nitrogen (N2) and argon (Ar). Surface morphology observations and field emission current measurements were done for two samples. Cross-section observation shows that the inside of the protrusions was solid but not porous like conventional fuzz. Therefore, a new formation mechanism which is different from fuzz can be considered. The field emission current from N2-seeded case was slightly higher than that from fuzz, but much lower than that from NTBs, while the field emission current from Ar-seeded case was similar to that from NTBs. The difference in field emission current could be due to the different morphology of the tips between two samples.

DOI： 10.23919/ISDEIV55268.2023.10200357

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