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

Scopus

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

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

DOI： 10.1364/OE.493993

Web of Science

Scopus

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

DOI： 10.1088/2053-1591/acd1d4

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Scopus

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

DOI： 10.1038/s41598-023-32621-5

PubMed

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

DOI： 10.1088/2058-6272/ac9f2d

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Scopus

Publisher：Nuclear Fusion  

There are no changes to the fitting values in the original article.

DOI： 10.1088/1741-4326/acbbde

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Scopus

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

<p>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 βand the Pfirsch-Schlüter current effects.</p>

DOI： 10.1585/pfr.18.2402021

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CiNii Research

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0129886

Web of Science

Scopus

Publisher：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

Scopus

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

DOI： 10.35848/1347-4065/ac79fe

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Scopus

Publisher：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

Scopus

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

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

DOI： 10.1016/j.nme.2022.101281

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Scopus

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

Unique surface nanostructures were formed on semiconductors over a large area by Ar plasma irradiation with RF sample bias. As an application of these wavelength ordered size structure, we have demonstrated a random laser emission at UV region.

DOI： 10.1109/CLEO-PR62338.2022.10432590

Scopus

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

DOI： 10.13182/FST13-A16911

Web of Science

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

DOI： 10.1016/j.jnucmat.2010.12.237

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

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Language：English   Publishing type：Research paper (international conference proceedings)  

Web of Science

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

Web of Science

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

DOI： 10.1002/ctpp.2150320312

Web of Science

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

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

DOI： DOI: 10.1063/5.0129886

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

DOI： 10.1002/aesr.202200141

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

DOI： doi.org/10.1021/acsaom.2c00085

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

DOI： DOI: 10.1016/j.apsusc.2022.153668

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

DOI： 10.1088/1361-6587/ac8acb

Scopus

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

DOI： 10.1016/j.nme.2022.101243

Scopus

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

<p>Helium (He)-Tungsten (W) co-deposition experiments were conducted in the linear plasma device Co-NAGDIS at a temperature of 1223 K to observe the characteristics of fuzz growth on the W surface with auxiliary W deposition. The dependence on the deposition rate of W was investigated and a clear difference in fuzz thickness was found between He-only and co-deposition experiments. In addition, the fuzz structures on a single sample exhibited a spatially nonuniform distribution, which was probably caused by the nonuniformity in the deposition rate.</p>

DOI： 10.1585/pfr.17.1302094

Scopus

CiNii Research

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

DOI： DOI: 10.1585/pfr.17.2405041

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

<p>To investigate the isotope effect on the plasma detachment in helium (He) and hydrogen (H)/deuterium (D) mixture plasmas, we performed H₂, D₂or He gas puffing into He plasma in the linear plasma device NAGDIS-II. Axial distributions of electron density (<i>n</i>_e) and electron temperature (<i>T</i>_e) were obtained using a movable Langmuir probe. Additionally, optical emission spectroscopy (OES) was applied to measure axial distributions of Balmer lines and He I lines. When the neutral gas pressure was high (Δ<i>P_n</i>= 7 ∼10 mTorr), <i>n</i>_edistribution in He-D₂mixture plasma was similar to that in pure He plasma, showing a sharp decrease at the downstream region where <i>T</i>_e< 1 eV. In contrast, in He-H₂mixture plasma, a decrease in <i>n</i>_ewas confirmed from upstream region where <i>T</i>_e> 1 eV. The upstream H_α/H_γin He-H₂plasma was significantly larger than the D_α/D_γin He-D₂plasma at the same <i>T</i>_e. This result indicated that molecular activated recombination (MAR) processes significantly occurred in He-H₂plasma, while electron-ion recombination (EIR) processes were dominant in He-D₂and pure He plasmas.</p>

DOI： 10.1585/pfr.17.2402027

Scopus

CiNii Research

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

DOI： doi.org/10.1039/D2CP00289B

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

DOI： 10.1016/j.nme.2022.101178

Scopus

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

DOI： 10.1063/5.0079524

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

DOI： 10.1088/1741-4326/ac10e7

Scopus

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

<p>For the development of a high nitrogen atom density source, we produced nitrogen plasmas and measured the nitrogen atom density using vacuum ultraviolet absorption spectroscopy (VUVAS) at high neutral gas pressure (>1.5 mTorr) and discharge power (>500 W) in the NAGDIS-T, which could generate spiral-shaped dissociative recombining plasmas. It was seen that the nitrogen atom density increases with increasing neutral gas pressure and discharge power, and it reached 6.2 × 10¹⁷m⁻³. In the low gas pressure case, the estimated atom density was confirmed to be consistent with that measured by the actinometry method.</p>

DOI： 10.1585/pfr.17.1201004

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CiNii Research

Publisher：Optics InfoBase Conference Papers  

Unique surface nanostructures were formed on semiconductors over a large area by Ar plasma irradiation with RF sample bias. As an application of these wavelength ordered size structure, we have demonstrated a random laser emission at UV region.

DOI： 10.1364/CLEOPR.2022.CFP8J_02

Scopus

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

DOI： 10.1016/j.apsusc.2021.151979

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：一般社団法人 プラズマ・核融合学会  

<p>We performed helium (He) plasma irradiation to molybdenum (Mo) substrate with auxiliary Mo deposition. Different from W cases, large-scale fiberform nanostructure (LFN) growth did not occur easily. The experiments suggested that the experimental condition of Mo LFN growth has a much narrower conditional window than that of fuzz growth or another hidden control factor exists for LFN growth.</p>

DOI： 10.1585/pfr.16.1206105

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

DOI： 10.1088/1402-4896/ac2c20

Scopus

Other Link： https://iopscience.iop.org/article/10.1088/1402-4896/ac2c20/pdf

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

DOI： 10.1088/1361-6463/ac1190

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

DOI： 10.1016/j.jphotochem.2021.113420

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

DOI： 10.1088/1402-4896/ac0866

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

DOI： 10.1585/pfr.16.2405069

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

DOI： 10.1088/2058-6272/abea71

Scopus

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

DOI： 10.1585/pfr.16.2402041

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

DOI： 10.1585/pfr.16.2405019

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

DOI： 10.35848/1347-4065/abe683

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

DOI： 10.1585/pfr.16.1202013

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

DOI： 10.35848/1347-4065/abe3a5

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

DOI： 10.1016/j.jnucmat.2021.152844

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

DOI： 10.1585/pfr.16.1206001

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

DOI： 10.1016/j.nme.2020.100848

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

DOI： doi.org/10.1016/j.nme.2020.100848

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

DOI： 10.1088/1361-6595/abc817

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

DOI： 10.1585/pfr.15.1505081

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

DOI： 10.1585/pfr.15.1301082

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

DOI： 10.1016/j.nme.2020.100843

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

DOI： 10.1016/j.nme.2020.100828

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

DOI： 10.1088/1361-6587/abb88f

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

DOI： 10.1063/5.0015912

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

DOI： 10.1016/j.nme.2020.100812

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

DOI： 10.1016/j.vacuum.2020.109799

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

DOI： 10.1585/pfr.15.1205061

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

DOI： 10.35846/1347-4065/aba456

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

DOI： 10.1002/ppap.202000126

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

DOI： 10.1016/j.jnucmat.2020.152350

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

DOI： 10.1088/2053-1591/aba394

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

DOI： 10.1063/5.0010416

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

DOI： 10.1088/1361-6587/ab8e19

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

DOI： 10.1063/1.5134869

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

DOI： 10.1016/j.surfcoat.2020.125798

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

DOI： 10.1585/pfr.15.2402012

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

DOI： 10.1088/1402-4896/ab52c6

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

DOI： 10.1585/pfr.15.1201004

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

DOI： 10.7567/1347-4065/ab5bbf

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

DOI： 10.1002/ctpp.201900114

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

DOI： 10.1088/1741-4326/ab73c3

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

DOI： 10.7567/1347-4065/ab4601

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

DOI： 10.1109/TPS.2019.2942725

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

DOI： 10.1585/pfr.14.1202135

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

DOI： 10.1088/1361-6595/ab318d

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

DOI： 10.1585/pfr.14.3401069

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

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

DOI： 10.1016/j.nme.2019.03.014

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

© 2019 WILEY-VCH Verlag GmbH &amp; 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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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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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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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1585/pfr.14.2402036

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

DOI： 10.1063/1.5081929

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

DOI： 10.1016/j.nme.2018.12.008

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

DOI： 10.1585/pfr.14.2402036

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

DOI： 10.1007/978-3-319-99834-3_12

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

© 2018 Elsevier B.V. Tungsten fuzz is a nanostructure formed on tungsten surface under helium plasma impact. In this work, we investigate this structure using the scattering of keV-energy protons. Energy spectra of scattered protons have double-peak shape that corresponds to the scattering from fuzz and underlying pure tungsten. The energy losses of protons in fuzz are estimated as the difference between primary beam energy and the energy corresponding to the local minimum between two peaks. Computer simulations with SRIM code also demonstrate double-peak shape of energy spectra of protons scattered from fuzzy target. Fuzz and pure tungsten stopping powers for protons are compared and fuzz density relative to pure tungsten is estimated and discussed. Evolution of H+energy spectra during fuzz irradiation by Ar+beam is measured and discussed.

DOI： 10.1016/j.nimb.2018.07.038

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

DOI： 10.1016/j.fusengdes.2018.04.104

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

DOI： 10.1088/1361-6587/aae2d7

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

DOI： 10.1088/1741-4326/aad24e

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

© 2018 Wiley-VCH Verlag GmbH &amp; Co. KGaA, Weinheim Fiberform nanostructured niobium (Nb) was fabricated by one step helium (He) plasma irradiation. He ion implantation formed He nano-bubbles on a Nb plate and led to formation of protrusions while migrating in Nb matrix; fiberform nanostructures (FN) were grown when the fluence became high (&gt;1026 m−2). The necessary conditions for the formation of Nb FN were revealed to be the surface temperature range of 900–1100 K and the incident ion energy higher than 70 eV. The sample was oxidized at 573–773 K in an air atmosphere, and Pt nanoparticles were photo-deposited on the Nb2O5 samples. The surface was analyzed by scanning electron microscope, transmission electron microscope, x-ray photoelectron spectroscopy, and ultraviolet-visible spectrophotometry. Photocatalytic activity of the fabricated materials was studied using methylene blue (MB) decolorization process. An enhanced photocatalytic performance was identified on FN Nb2O5 substrate with Pt deposition.

DOI： 10.1002/cphc.201800769

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

DOI： 10.1088/1741-4326/aacd1f

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

DOI： 10.1585/pfr.13.1201099

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

IEEE Experimental data on the effect of the thickness of a nanostructured layer present on the surface of a tungsten cathode on its arcing properties have been analyzed. It has been shown that if the nanostructured layer is not completely destroyed as a result of the operation of a cathode spot cell, the spot ``sinks&#039;&#039; in the layer, the spot cells group together, and the trajectory of the spot motion becomes ``entangled.&#039;&#039; To interpret the experimental results, the grouping of arc spot cells was simulated using a random walk model based on the Monte Carlo method into which an attractive force was introduced and assuming that the cell plasma density, i.e., the cell initiation probability, is proportional to 1/r&amp;#x00B2;, where r is the distance between two spot cells. It has been demonstrated that spot cells are entangled, forming a group and moving together when the attractive force between them is strong enough. When this force decreases and the directionality of the cells increases as the cathode spot shifts to a lower thickness region of the nanostructured layer, the velocity of motion of the spot increases and its trace left on the surface becomes narrower.

DOI： 10.1109/TPS.2018.2859953

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

DOI： 10.1063/1.5040800

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

DOI： 10.1088/1361-6587/aac0d8

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

DOI： 10.1002/ctpp.201700157

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

Line emissions from helium (He) plasmas have been used for measurement of electron density and temperature in various situations. We performed measurements of He line intensities in recombining plasmas in a divertor simulator, followed by discussion of some potential difficulties of using the line intensity ratio (LIR) method in a divertor environment under detached conditions. When using four line emissions (501.6, 667.8, 706.5, and 728.1 nm), the discrepancy of the results compared to results from the laser Thomson scattering (LTS) method increased along with increasing gas pressure. However, the LIR method agreed well with LTS when a recombination sensitive line emission (447.1 nm) was added. The influence of the transport of metastable atoms is also discussed. Additionally, anomalous characteristics of the LIR method were revealed in hydrogen helium mixture plasmas when the temperature was in the range of 1-4 eV. These anomalies were likely attributable to the dissociative recombination of HeH+.

DOI： 10.1063/1.5029414

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

DOI： 10.1585/pfr.13.3406065

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

DOI： 10.1585/pfr.13.1402033

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

Plasma irradiation was used to create nanostructured vanadium oxide with potential commercial and industrial applications. Morphology changes were induced at the nano- and micro-meter scale, accompanied by the growth of helium nanobubbles. Micrometer-sized pillars, cube-shaped nanostructures, and fuzzy fiberform nanostructures were grown on the surface
the necessary conditions in terms of the incident ion energy and the surface temperature for those morphology changes were revealed. Hydrogen production experiments using a photocatalytic reaction with aqueous methanol solution were conducted on the fabricated samples. Enhanced H2 production was confirmed with the plasma irradiated nanostructured sample that had been oxidized in air atmosphere. Photocatalytically inactive vanadium oxide exhibited a high photocatalytic activity after nanostructurization of the surface by helium plasma irradiation.

DOI： 10.1088/1361-6463/aabe44

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier B.V.  

Growth of helium bubbles near the surface of metals leads to various morphology changes when sufficient helium ions are continuously implanted on the surface. In this study, low energy (&lt
100 eV) helium plasma irradiations to various noble metals were conducted, and consequent surface morphology changes were analyzed. It was found that fiberform nanostructures were formed by the helium plasma irradiation on rhodium and ruthenium thin films, which were deposited by magnetron sputtering, and on platinum and iridium wires. Growth of fiberform structures were not identified on gold, silver, and palladium samples exposed to helium plasmas even though the irradiations were conducted at various surface temperatures, though pinholes were observed on palladium surface. We discussed the relation between the morphology changes and material properties.

DOI： 10.1016/j.surfcoat.2018.02.026

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

DOI： 10.1585/pfr.13.1205001

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

DOI： 10.1063/1.4997840

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Language：Japanese   Publisher：Plasma Physics and Controlled Fusion  

The stability with respect to a peelingballooning mode (PBM) was investigated numerically with extended MHD simulation codes in JET, JT-60U and future JT-60SA plasmas. The MINERVA-DI code was used to analyze the linear stability, including the effects of rotation and ion diamagnetic drift (w∗i), in JET-ILW and JT-60SA plasmas, and the JOREK code was used to simulate nonlinear dynamics with rotation, viscosity and resistivity in JT-60U plasmas. It was validated quantitatively that the ELM trigger condition in JET-ILW plasmas can be reasonably explained by taking into account both the rotation and w∗i effects in the numerical analysis. When deuterium poloidal rotation is evaluated based on neoclassical theory, an increase in the effective charge of plasma destabilizes the PBM because of an acceleration of rotation and a decrease in w∗i. The difference in the amount of ELM energy loss in JT-60U plasmas rotating in opposite directions was reproduced qualitatively with JOREK. By comparing the ELM affected areas with linear eigenfunctions, it was confirmed that the difference in the linear stability property, due not to the rotation direction but to the plasma density profile, is thought to be responsible for changing the ELM energy loss just after the ELM crash. A predictive study to determine the pedestal profiles in JT-60SA was performed by updating the EPED1 model to include the rotation and w∗i effects in the PBM stability analysis. It was shown that the plasma rotation predicted with the neoclassical toroidal viscosity degrades the pedestal performance by about 10% by destabilizing the PBM, but the pressure pedestal height will be high enough to achieve the target parameters required for the ITER-like shape inductive scenario in JT-60SA.

DOI： 10.1088/1361-6587/aa8bec

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Nature Publishing Group  

Helium plasma irradiation on metal surfaces leads to the formation of metallic fuzzy nanostructures accompanied by the growth of helium bubbles in metals. The mechanism of the growth process, its impact for fusion devices, and potential application have been explored. Here we show enhanced growth of large-scale fuzz by precipitating additional metallic particles during helium plasma irradiation. The growth rate of the fuzzy structures became orders of magnitude greater than conventional fuzz growth
in an hour of irradiation, 1 mm-thick visible tungsten and molybdenum fuzzy fur structures covered a tungsten metal substrate. Additional precipitation of metallic ions breaks the bottleneck diffusion process
moreover, further acceleration in the growth rate could have occurred if the electric sheath shape was influenced by the grown structure and the electric field that formed around the structure started collecting ions.

DOI： 10.1038/s41598-017-18476-7

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

DOI： 10.1585/pfr.13.3406074

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

© 2017 IAEA, Vienna. This paper describes the recent progress in divertor simulation research using the GAMMA 10/PDX tandem mirror towards the development of divertors in fusion reactors. During a plasma flow generation experiment in the end cell of the GAMMA 10/PDX, ICRF heating in the anchor cell successfully extended the particle flux up to 3.3 1023 m2 s-1. Superimposing the short pulse of the ECH also attained a maximum heat flux of ∼30 MW m-2. We have succeeded in achieving and characterizing the detachment of the high-temperature plasma, which is equivalent to the SOL plasma of tokamaks, by using the divertor simulation experimental module (D-module) in the GAMMA 10/PDX end cell, in spite of using a linear device with a short magnetic field line connection length. Various gases (Ar, Xe, Ne and N2) are examined to evaluate the effect of radiation cooling against the plasma flow at the MW m-2 level in the divertor simulation region and the following results are obtained: (i) Xe gas was most effective in the reduction of heat and particle fluxes (1%, 3%, respectively) and has a stronger effect on electron cooling (down to ∼1.6 eV) in the used gas species. (ii) Ne gas was less effective. On the other hand, (iii) N2 gas showed more favorable effects than Ar in the lower pressure range. These results will contribute to the progress in detached plasma operation and in clarifying the radiation cooling mechanism towards the development of future divertors.

DOI： 10.1088/1741-4326/aa7cb4

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Authorship：Lead author   Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1585/pfr.12.1405040

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

DOI： 10.1088/1741-4326/aa867a

Language：English   Publishing type：Research paper (international conference proceedings)  

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

DOI： 10.1088/1741-4326/aa7fc2

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

Mass loss of nanostructured tungsten, which was formed by helium plasma irradiation, due to laser ablation, sputtering, and arcing was investigated. Below the helium sputtering energy threshold (&lt
117 eV), sputtering by impurity gases such as nitrogen and oxygen was dominant. Sputtering yield became closer to theoretical value with increasing the incident ion energy, and sputtering by helium ions became the major contribution when the energy was &gt
200 eV. Reduction in sputtering on nanostructured surface was observed. Arcing was initiated using laser pulses, and the erosion rate by arcing was measured. The erosion rate increased with arc current, while the erosion per Coulomb was not affected by arc current.

DOI： 10.1016/j.nme.2017.03.004

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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

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

DOI： 10.1016/j.nme.2016.12.017

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

DOI： 10.1016/j.nme.2016.12.020

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

DOI： 10.1016/j.nme.2017.05.001

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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

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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

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

DOI： 10.1016/j.nme.2017.02.025

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

DOI： 10.1063/1.4990077

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

DOI： 10.1063/1.4986100

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

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

DOI： 10.1585/pfr.12.1202027

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

DOI： 10.1088/1741-4326/aa6dce

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

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

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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

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

DOI： 10.1088/1361-6587/aa5394

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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

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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

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

DOI： 10.1585/pfr.11.1206125

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

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

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

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

DOI： 10.1038/srep30380

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

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

DOI： 10.7567/JJAP.55.056203

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The resonant magnetic perturbation (RMP) island introduced in the stochastic edge of the large helical device (LHD) plasma plays an important role in the stabilization of the plasma detachment (Kobayashi et al 2013 Nucl. Fusion 53 093032). The plasma enters in the sustained detachment phase in the presence of an RMP once the line averaged density exceeds a critical value with a given input power. During detachment the enhanced radiation from the stochastic edge of the LHD undergoes several spatiotemporal changes which are studied quantitatively by an infrared imaging video bolometer (IRVB) diagnostic. The experimental results are compared qualitatively and quantitatively with the radiation predicted by the 3D transport simulation with fluid model, EMC3-EIRENE. A fair amount of qualitative agreement, before and after the detachment, is reported. The issue of overestimated radiation from the model is addressed by changing the free parameters in the EMC3-EIRENE code till the total radiation and the radiation profiles match closely, within a factor of two with the experimental observations. A better quantitative match between the model and the experiment is achieved at higher cross-field impurity diffusion coefficient and lower sputtering coefficient after the detachment. In this article a comparison, the first of its kind, is established between the quantified radiation from the experiments and the synthetic image obtained from the simulation code. This exercise is aimed towards validating the model assumptions against the experimentally measured radiation.

DOI： 10.1088/0029-5515/56/4/046002

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

DOI： 10.1585/pfr.11.1202005

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

DOI： 10.1063/1.4940310

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

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

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DOI： 10.1585/pfr.10.1402083

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms  

DOI： 10.1016/j.nimb.2015.07.035

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

Language：English  

© 2015 IAEA, Vienna. This paper assesses the three-dimensional (3D) effects of the edge magnetic field structure on divertor/scrape-off layer transport, based on an inter-machine comparison of experimental data and on the recent progress of 3D edge transport simulation. The 3D effects are elucidated as a consequence of competition between transports parallel () and perpendicular () to the magnetic field, in open field lines cut by divertor plates, or in magnetic islands. The competition has strong impacts on divertor functions, such as determination of the divertor density regime, impurity screening and detachment control. The effects of magnetic perturbation on the edge electric field and turbulent transport are also discussed. Parameterization to measure the 3D effects on the edge transport is attempted for the individual divertor functions. Based on the suggested key parameters, an operation domain of the 3D divertor configuration is discussed for future devices.

DOI： 10.1088/0029-5515/55/10/104021

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

DOI： 10.1088/0031-8949/90/9/095604

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

DOI： 10.1088/0029-5515/55/8/083016

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

DOI： 10.1088/0029-5515/55/10/104018

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

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

DOI： 10.1088/0029-5515/55/7/073013

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

DOI： 10.1016/j/jnucmat.2015.01.036

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

DOI： 10.1038/ncomms6816

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

DOI： 10.1016/j.jnucmat.2015.01.018

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

DOI： 10.1016/j.jnucmat.2015.01.068

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DOI： 10.1016/j.jnucmat.2014.12.018

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

DOI： 10.1016/j.jnucmat.2014.12.011

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

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

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

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DOI： 10.1063/1.4904917

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

DOI： 10.1585/pfr.9.3405136

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

DOI： 10.1063/1.4895718

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

DOI： 10.1380/ejssnt.2014.343

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

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

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Contributions to Plasma Physics  

DOI： 10.1002/ctpp.201410074

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

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

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

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

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Helium plasma irradiation leads to the formation of nanostructures on the surface when certain conditions are satisfied. The surface morphology changes due to helium plasma irradiation were investigated on single crystal tungsten samples with different crystal orientations. Differences in surface morphology changes were observed in (100) and (110) single crystal tungsten samples when the helium fluence was less than 10(25) m(-2). However, the differences disappeared as growth proceeded. Tungsten alloys were prepared in order to investigate the influence of the additional undersized atomic species on nanostructure growth. The influence of the doping of undersized atoms, i.e. 2 at% Fe doped and 2 at% Cr doped atoms, is presented.

DOI： 10.1088/0031-8949/89/02/025602

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

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

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

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

Behavior of arc spots was investigated in detail using a nanostructured tungsten specimen with different thicknesses of the nanostructured layer. From the observation using a fast framing camera, it was found that the velocity of the arc spots significantly altered as passing the boundary of the two layers. The changes in spot velocity and spot width were discussed theoretically using the ecton model. The fractal dimension of the arc trail evaluated by using a box-counting method was significantly changed. Also, the width of arc trail was increased with the nanostructured layer thickness. From the SEM analysis of the specimen, the amount of tungsten eroded by arcing for two different thickness cases was estimated, and the erosion rates were discussed. (C) 2014 The Authors. Published by Elsevier B.V.

DOI： 10.1016/j.rinp.2014.03.001

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

DOI： 10.7567/JJAP.52.11NC02

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

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

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

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

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Field emission property of nanostructured tungsten formed by the exposure to a helium plasma is measured to investigate the initiation process of arcing in nuclear fusion devices. Field emission current from the nanostructured tungsten and tungsten with helium bubbles was significantly higher than that from the polished tungsten specimen. However, field enhancement factor was on the order of 100, which was almost comparable to that of bulk tungsten; the increase in the field emission current is attributed to primarily an increase in the effective emission surface area. From the calculation of thermo-field emission current from a heated tungsten, it is thought that a significant current emission can be still initiated even when the field enhancement factor is not so high when tungsten is exposed to high density plasmas such as the one accompanied with ELMs. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.fusengdes.2013.05.014

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Scopus

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

The JT-60SA project implemented by Japan and Europe is progressing on schedule towards the first plasma in March 2019. After careful R&D, procurements of the major components have entered their manufacturing stages. In parallel, disassembly of JT-60U has been completed on time, and the JT-60SA tokamak assembly is expected to start in January 2013. The JT-60SA device, a highly shaped large superconducting tokamak with a variety of plasma control actuators, has been designed in order to contribute to ITER and to complement ITER in all the major areas of fusion plasma development necessary to decide DEMO reactor construction. Detailed assessments and prediction studies of the JT-60SA plasma regimes have confirmed these capabilities: using ITER- and DEMO-relevant plasma regimes, heating conditions, and its sufficiently long discharge duration, JT-60SA enables studies on magnetohydrodynamic stability at high beta, heat/particle/momentum transport, high-energy ion physics, pedestal physics including edge localized mode control, and divertor physics. By integrating these studies, the project provides 'simultaneous and steady-state sustainment of the key performance characteristics required for DEMO' with integrated control scenario development.

DOI： 10.1088/0029-5515/53/10/104010

Scopus

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

It is found that resonant magnetic perturbation (RMP) fields have a stabilizing effect on the radiating edge plasma, realizing stable sustainment of radiative divertor (RD) operation in the Large Helical Device (LHD). Without RMP, thermal instability leads to radiative collapse. Divertor power load is reduced by a factor of 3-10 during the RMP-assisted RD phase, while maintaining relatively good core plasma confinement with confinement enhancement factor . It has also been demonstrated that the RMP field itself can initiate transition to RD operation by increasing perturbation strength, while keeping constant density and injection power. The results show a possibility of a new control knob for divertor power load in a 3D magnetic field configuration. It is also found that after the transition to RD, the energy confinement enhancement factor based on ISS04 scaling increases by a factor of 1.4 compared with the attached phase. The operation range of the RMP-assisted RD is identified in terms of RMP strength and radial location of the resonance layer of the RMP. A 3D edge radiation structure is analysed using the edge transport code EMC3-EIRENE and the results are compared with experiments. The comparison indicates that the application of RMP modulates the 3D edge radiation structure such that an intense radiation appears around the X-point of the m/n = 1/1 island in the case with RMP, while it is located at the inboard side without RMP. © 2013 IAEA, Vienna.

DOI： 10.1088/0029-5515/53/9/093032

Scopus

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

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

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

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

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

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

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

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

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

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

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

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

Be-seeded, high-flux, deuterium/helium mixture plasma exposure experiments on tungsten target materials have been performed to simulate ITER all tungsten divertor erosion/modification and deposition phenomena. The exposure conditions are kept fixed at a typical low-ion-energy of 60 eV and a flux of 3-6 × 1022/m2/s. Sample temperature is 1123 K and plasma exposure times spanning 1050-10,100 s are explored. The typical ratio of He/D ions is 0.2 and Be content is 0.2%. A He-induced nanostructure layer is formed on the exposure surfaces of tungsten materials and the surface of the nanostructure is covered by a thin layer of Be and O. A fraction of the re-eroded Be from the target is deposited on a glassy carbon plate with line of sight to the tungsten target. Rutherford backscattering spectrometry analyses show that the Be redeposit layer is in the form of laminae. Small amounts of Mo, W and C are also found in the redeposited Be layer. Elastic recoil detection analyses show that D, He and H are also included in the redeposited Be layer. © 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2013.05.006

Scopus

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

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

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

DOI： 10.1088/0029-5515/53/5/053013

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

DOI： doi.org/10.1063/1.4798597

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

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

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

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

In this study, in order to clarify the influence of a probe electrode during probe measurements of an atmospheric pressure microwave plasma, simultaneous observations of probe current and the change of plasma shape using a high-speed camera have been done. Fast and random repetitions of attachment and detachment of the plasma torch to the probe electrode have been observed. It is found that the response time is related to ion sound speed. Different behaviors of the plasma torch for a floating metal electrode or an insulator have been observed. Probe current-voltage characteristics show asymmetric double probe shape © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/ctpp.201310014

Scopus

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

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

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：プラズマ・核融合学会  

Sustaining a detached plasma is important for the reduction of the heat load on a divertor. The externally controlled methods such as gas puffing in the divertor region, etc. have been studied for maintaining detached plasmas. In the recent LHD experiments, a resonant magnetic perturbation (RMP) is utilized to establish the detached plasma, which is one of the candidates to reduce the divertor heat load. The finite plasma response field has been detected when the RMP is imposed, and its behavior is investigated from the viewpoint of the magnetic island dynamics. A parameter of the phase difference, Δ<i>θ</i> (defined as the phase difference between the plasma response field and the RMP), changes from antiphase to in-phase while the plasma transits from the attached to the detached states. Finally, when the Δ<i>θ</i> reaches at certain critical value, Δ<i>θ</i>_crit, the plasma enters the detached state. Since this critical value does not have a dependency on plasma beta and collisionality, Δ<i>θ</i>_critseems to be a threshold to establish the detached plasma. From the viewpoint of magnetic island dynamics, the behavior of Δ<i>θ</i> approaching in-phase implies the "trend" of island growth. The detached plasma might be established in the case of the tendency for the plasma response field to reinforce the island width.

DOI： 10.1585/pfr.8.1402058

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

The physical properties of tungsten with nanostructure on its surface are investigated focusing on the thermal radiation and cooling characteristics. First, direct evidence of substantial W surface cooling has been clearly shown with use of a very thin thermocouple inserted into W target, which solves an uncertainty associated with a radiation thermometer. Second, the above measurements of W surface temperature make it possible to estimate quantitatively the total emissivity from which we may evaluate the radiative power through the Stefan-Boltzmann equation, which is very important for mitigation evaluation of a serious plasma heat load to the plasma-facing component. © 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2013.01.176

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：39th EPS Conference on Plasma Physics 2012, EPS 2012 and the 16th International Congress on Plasma Physics  

In summary, we proposed saddle shaped stellarator system for vertical mode in toka-mak. These preliminary results demonstrate the feasibility of vertical stability, even if only two selected coils which are localized at one side of the torus. However as noted earlier, the experiment were conducted with the circular cross-section tokamak which is stable vertically. Further work is needed to confirm the improvement in the vertical stability in elongated tokamaks. In addition, very large coil current comparable to the plasma current are needed in our system. In order to reduce the current, future work also should focus on optimal configuration of the coils.

Scopus

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

DOI： 10.1585/pfr.7.1402152

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

DOI： 10.1585/pfr.7.2405121

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

DOI： 10.1088/0741-3335/54/10/105015

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

DOI： 10.1585/pfr.7.1405100

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

DOI： 10.1002/ctpp.201210027

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

DOI： 10.1585/pfr.7.1202049

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

DOI： 10.1016/j.cap.2012.04.024

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

DOI： 10.1585/pfr.7.1201033

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Arcing is a long standing plasma-surface interaction issue in nuclear fusion research, and the issue has been revived recently from new points of view. In this study, arcing under fusion relevant conditions is demonstrated in a linear plasma device using a pulsed laser to mimic the transient heat load. Ignition conditions of arcing were investigated systematically by changing the laser power, plasma conditions, surface nature and so on. Moreover, the cross-sectional view of arc trail was observed by transmission electron microscope, and the erosion of nanostructured tungsten due to arcing was estimated. It was found that more than half of the nanostructured tungsten still remained after an arc spot run on the nanostructure.

DOI： 10.1088/0741-3335/54/3/035009

Web of Science

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

Helium plasma irradiation and electron heating experiments were conducted using tungsten in the divertor simulator NAGDIS-II. Helium plasma irradiation to tungsten led to the formation of nanostructures on the surface, while the nanostructures were annihilated after the potential of the specimen was changed to positive for several 10 min so that electrons irradiated the sample without ion irradiation. The specimens were analyzed in detail by transmission electron microscope with the help of focused ion beam technique. It is revealed that the helium nano-bubbles still remained even after the nanostructures were disappeared from the surface. Porosity of the nanostructured tungsten was measured from the TEM images. (c) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2011.11.044

Web of Science

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

DOI： 10.1143/JJAP.51.01AB03

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

DOI： 10.1143/JJAP.51.01AJ03

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

We have measured ion temperature as well as electron temperature in plasma blobs observed in a linear plasma device by using an improved ion sensitive probe. Current-voltage characteristics of the ion sensitive probe inside and outside plasma blobs were re-constructed with a conditional sampling method. It is clearly found that both ion and electron temperatures in plasma blobs decrease more slowly in a cross-field direction than those in a bulk plasma without plasma blobs. © 2012 American Institute of Physics.

DOI： 10.1063/1.3681778

Scopus

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

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

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

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：38th EPS Conference on Plasma Physics 2011, EPS 2011 - Europhysics Conference Abstracts  

The validation of current decay model during the current quench was investigated in high (3p disruptive discharges on JT-60U. It is experimentally confirmed that the electron temperature does not play an important role in the determination of the current decay time in the initial phase of current quench and that values obtained from L/R model are two orders of magnitude larger than the experimental current decay time. The current decay time predicted by the modified L/R model, in which the time derivative of plasma inductance is taken into account, is in fairly good agreement with the experimental data. In future work, we need to perform the MHD equilibrium code for free-boundary condition such as DINA [5] and TSC [6] code under the axisymmetric assumption in order to clear up the determination mechanism of time derivative of plasma inductance.

Scopus

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

The status of ongoing research projects of tungsten R&D in Japan is summarized in this paper. For tungsten material development, a new improved fabrication technique, the so-called superplasticity-based microstructural modification, is described. This technique successfully improved fracture strength and ductility at room temperature. Recent results on vacuum plasma spray W coating and W brazing on ferritic steels and vanadium alloys are explained. Feasibility of these techniques for the manufacture of the blanket is successfully demonstrated. The latest findings on the effect of neutron damage in tungsten on T retention and on the change in mechanical and electrical properties are described. Retention characteristics for neutron-damaged W were different compared to those for ion-damaged W. Upon neutron irradiation, tungsten alloys containing transmutation elements of W (Re and Os) show changes in properties that are different compared with those shown by pure W. The effects of mixed plasma exposure (D/He/C) are described. Both D/He and D/C mixed ion irradiations significantly affect ion-driven permeation in W. He bubble dynamics play a key role in nano-structure formation on the W surface. © 2011 The Royal Swedish Academy of Sciences.

DOI： 10.1088/0031-8949/2011/T145/014029

Scopus

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

Stainless steel (SUS) is used as the structure material in nuclear fusion devices. Heating and O 2 plasma discharge are considered for the removal of tritium. Oxide layer growth is anticipated on heating SUS at a high temperature in low-pressure residual O 2 gas. When O 2 plasma discharge is performed, oxygen plasma reaches the structure material and iron oxides such as Fe 2O 3 are formed. Thus, a knowledge of the retention behavior of hydrogen isotopes or deuterium (D) in the oxide layers is important in designing the devices. We investigated D retention in Fe 2O 3 films and the influence of Fe 2O 3 crystal structure on D retention and found that D retention in γ-Fe 2O 3 (cubic-maghemite) is larger than that in α-Fe 2O 3 (hexagonal-hematite). The results for Fe 2O 3 are compared with those for SUS and Fe. © 2011 The Royal Swedish Academy of Sciences.

DOI： 10.1088/0031-8949/2011/T145/014042

Scopus

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

DOI： 10.1016/j.jnucmat.2011.06.026

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

DOI： 10.1016/j.jnucmat.2011.02.007

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

DOI： 10.1016/j.jnucmat.2011.01.078

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DOI： 10.1088/0029-5515/51/10/102001

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

DOI： 10.1143/JJAP.50.08JG01

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

DOI： 10.1016/j.jnucmat.2010.12.021

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

DOI： 10.1016/j.jnucmat.2010.08.019

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

Divertor simulation study has been started as a new research plan, by making best use of a large linear plasma device. The experiment of generating the plasma flow with high heat and particle flux was successfully performed at an end-mirror exit of the GAMMA 10 tandem mirror. In typical hot-ion-mode plasmas, the heat-flux density of 0.6 MW/m2 and the particle-flux density of 1022 particles/s m2 were simultaneously achieved in the case of only ICRF heating and superimposing the 300 kW ECH pulse attained the peak value of the net heat-flux up to 8 MW/m2 on axis. The above experimental results and the simulation analysis of ICRF heating using the Fokker-Planck code give a clear prospect of generating the required performance for divertor studies by building up the plasma heating systems to the end-mirror cell. Detailed behavior of the plasma flow and the future research plan are also described. © 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jnucmat.2010.11.081

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

DOI： 10.1088/0741-3335/53/7/074002

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

DOI： 10.1143/JJAP.50.01AB04

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

DOI： 10.1143/JJAP.50.01AB01

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

DOI： 10.1143/JJAP.50.01AH02

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