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Detached plasma formation is a way to reduce the heat load on the wall in magnetic fusion devices. This study proposes a novel analysis technique consisting of the conditional averaging, sliding window, and tomography to reveal the spatiotemporal behavior of the rotating radial ejection event of detached plasma, which further contributes to local heat load reduction. The used equipment is a high-speed camera and an electrostatic probe located at the periphery of the linear plasma device NAGDIS-II. By applying this method, four-dimensional (4D) behavior of the emission structure along time (1D) and space perpendicular and parallel to the magnetic field (3D) was clarified; a rotating distorted structure appears as a precursor, which is then scraped and transported radially and axially. The proposed method is widely applicable to short-term rigid-body rotating structures, especially in linear plasmas. [ABSTRACT FROM AUTHOR]

Helium plasma irradiation to various metals leads to peculiar morphology changes as forming fiberform nanostructures called fuzz when a certain set of conditions is satisfied. In this Tutorial, we overview experimental observations about the fuzz growth process, conditions, growth rate, etc. on tungsten (W), and fuzz growth observed on other various metals. The fuzz growth mechanism, open questions about fuzz growth, and current understandings on these issues are discussed based on simulation and theoretical works. Several applications (gas sensors and photocatalytic application) with fuzz are shown. We hope that this Tutorial will help new researchers in this field to perform experiments and simulations to reveal new perspectives of fuzz. [ABSTRACT FROM AUTHOR]

In this study, we investigated the effects of helium (He) plasma irradiation and thermal treatment on the morphology, the surface characteristics, and the photoelectrochemical performance of bulk iron (Fe) sheets. After processing, all samples consisted mainly of an α -Fe 2 O 3 surface. Helium plasma irradiation resulted in a cathodic shift in the onset potential of the photocurrent from 1.16 V RHE (no pretreatment) to 0.57 V RHE (plasma exposure at 660 ° C), where V RHE represents the potential versus reversible hydrogen electrode (RHE). An increase in the surface roughness and a smaller amount of hydroxyl or oxygen vacancies at the surface after photoelectrochemical measurement are related to the decrease of the onset potential. The results show that He plasma irradiation does not only modify the surface morphology, but also affects the functional properties of the materials. • The photoelectrochemical performance of α -Fe 2 O 3 is investigated. • Helium plasma irradiation resulted in a cathodic shift in the onset potential. • The onset potential shifted from 1.16 to 0.57 V RHE. • A small amount of hydroxyl or oxygen vacancies are likely to be the most influential. [ABSTRACT FROM AUTHOR]

When tungsten (W) is deposited with helium (He) plasma (He–W co-deposition) on W surface, enhanced growth of fiberform nanostructure (fuzz) occurs, and sometimes it grows into large-scale fuzzy nanostructures (LFNs) thicker than 0.1 mm. In this study, different numbers of mesh opening (apertures) and W plates with nanotendril bundles (NTBs), which are tens of micrometers high nanofiber bundles, were used to investigate the condition for the origin of the LFN growth. It was found that the larger the mesh opening, the larger the area where LFNs are formed and the faster the formation tends to be. On NTB samples, it was found that NTBs grew significantly when exposed to He plasma with W deposition, especially when the size of the NTB reached ∼ 0.1 mm. The concentration of the He flux due to the distortion of the shape of the ion sheath is proposed as one of the reasons to explain the experimental results. [ABSTRACT FROM AUTHOR]

Forming nanocone structures on a silicon (Si) surface by low (<100 eV) energy helium plasma has been proposed in recent years as a simple method for fabricating black Si, which is an attractive material for photon absorption through the solar spectrum. In this study, different appearances of the Si surface were observed and analyzed with a scanning electron microscope. By introducing impurities of molybdenum and tungsten during plasma irradiation, it was revealed that the formation and the distribution of nanocones have a clear dependence on the amount of impurities on the surface. [ABSTRACT FROM AUTHOR]

Reducing the optical reflection of silicon (Si) material is a crucial issue on the surface of solar cells and other photonic applications. Fabrication of nanocone structures on the Si surface (black Si) by plasma (helium or argon) irradiation is a novel technique in recent decades with advantages, such as simple, economical, and harmless to the Si substrate. However, the uniformity and controllability of the surface remain problematic. In this study, uniform black silicon was obtained by low energy (<50 eV) helium ion irradiation with auxiliary Mo co-deposition. The characteristics of the nanocone show a clear relation with the Mo ratio. Deposited Mo was found to accumulate on the cone body, especially on the tip, and it worked as a nano mask to induce the formation of the nanocone and protect it from sputtering. The surface with high and large diameter nanocones possesses low reflectances of roughly 2%. The study of the relation between Mo ratio, characteristics of the nanostructure, and the reflectance of the surface raises a potential method of surface tailoring. [ABSTRACT FROM AUTHOR]

Helium line intensities have been utilized to measure the electron density, ne, and temperature, Te, by comparing measured line intensities to a collisional-radiative model (CRM). In this study, we use multiple regression analysis to train a model of the helium line intensities and ne/Te obtained from a Thomson scattering system in the linear plasma device Magnum-PSI; based on the trained model, we predict ne and Te from line intensities. We show that this method can also obtain radial profiles of ne and Te. We discuss appropriate selections of line pairs for the prediction based on the multiple regression analysis. A big advantage of this method against the standard technique using CRM is that modeling of atomic population distributions is not required, which sometimes needs to take into account various effects such as radiation trapping, transport of helium atoms in metastable states, etc. [ABSTRACT FROM AUTHOR]

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, n e , and temperature, T e. The covered ranges of n e and T e , which were measured by an electrostatic probe, are 3.6 × 10 17 – 2. 4 × 1 0 19 m−3 and 0. 3 – 7. 1 eV , respectively. A three hidden layer neural network (NN) is introduced to model the relationship between n e / T e and the combination of line intensities, radial position, and device parameters. It is shown that the errors in n e and T e 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. • Machine learning was used for plasma measurement. • Optical emission spectroscopy (OES) data was used for the input. • The electron density, n e , and temperature, T e are deduced. • A three hidden layer neural network (NN) is introduced. • The errors in n e and T e are reduced to 18.0 and 18.8%, respectively. • 4-5 lines are enough to keep the measurement quality. [ABSTRACT FROM AUTHOR]

Three-dimensional spatiotemporal dynamics of detached helium plasma parameters along time, radius, and magnetic field were revealed in the linear device NAGDIS-II. To measure plasma parameters before and after the radial plasma ejection that was enhanced around the volume-recombining region, the conditional averaging technique was applied. The radial ejection was found to correlate with low-frequency changes of plasma-column parameters, which seemed to suppress the axial movement of the recombining region. Moreover, an azimuthal charge separation inside the ejected structure was observed, similar to the typical edge transport phenomenon: blobby plasma transport. The neutral flow effect was suggested as a candidate of the driving force. [ABSTRACT FROM AUTHOR]

Helium (He) line emissions have been utilized to measure the electron density (ne) and temperature (Te), and validity checks have been conducted in various linear devices. In this study, we performed optical emission spectroscopy (OES) of He line emissions in the linear plasma device Magnum-PSI, where the used density range was 1–8 × 1020 m−3, which was much higher than those used until now. We observed nine line emissions in the wavelength range of 388–728 nm and deduced ne and Te based on comparisons with a collisional radiative model. From the variation of the difference between the experiments and calculations, the joint probability distribution of ne and Te was deduced. We will discuss the effect of radiation trapping, in particular, based on comparisons between OES measurement results and Thomson scattering measurements. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Elucidation and control of the low temperature recombining plasmas under fusion relevant conditions are crucial issues. In this study, we applied a conditional average for laser Thomson scattering technique to measure the temporal evolution of the plasma instability in detached recombining plasmas in the divertor simulator NAGDIS-II. In recombining plasmas, it is shown that the electron energy distribution function has a non-Maxwellian component, and the fraction of the low electron temperature component increases with the gas pressure. It is found that one of the major reasons to cause the non-Maxwellian distribution function is in the temporal variations of the electron temperature. By using the conditional averaging method, the temporal evolutions of the spatial (vertical and radial) profiles of the electron density and temperature are revealed for different frequency ranges. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Interactions between nanosecond laser pulses and a tungsten substrate having submicron holes near the surface formed by exposure to helium plasmas are investigated experimentally and numerically. After tungsten surface having helium holes was irradiated by nanosecond laser pulses in helium plasmas, scanning electron microscope micrographs of the tungsten surface show that the roughness of the surface is significantly enhanced under certain experimental conditions. For an understanding of the physical mechanisms to arise the phenomena, heat conduction in the substrate having holes is modeled by solving a three-dimensional heat conduction equation. The model calculations show that the surface reaches a melting point locally even if the pulse energy is low enough to lead to the melting of a virgin substrate. On the basis of surface temperature calculations and from an evaluation of the tensile stress put on the lid of the hole, repetitive explosions of the helium holes caused by heating the lids are considered to be the mechanism enhancing the surface roughness. Simultaneous irradiation of laser pulses and the helium ions may have a drilling effect on tungsten with repetitive formation and explosions of the subsurface helium holes. [ABSTRACT FROM AUTHOR]

This study investigated the characteristics of arcing initiated on nanostructured fuzzy tungsten (W) surfaces in helium (He) plasmas. Under the same He plasma conditions, the relationship between arc voltage and current was characterized as linear due to plasma resistance across the He plasma from the target surface to the plasma source. Properties of the erosion caused by arcing were investigated by focusing on two parameters: arc current and fuzzy layer thickness. The erosion per charge was significantly dependent on the thickness of the fuzzy layer but did not depend on the arc current. Sample surface analysis revealed that drastic changes in arc spot grouping features were accompanied by the fuzzy layer thickness variation. The ecton model indicated that the erosion as ions does not play a main role, but mechanical destruction by momentum transfer from the explosive electron emission centre to neighbouring nanowires seems to enhance the erosion greatly. [ABSTRACT FROM AUTHOR]

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+. [ABSTRACT FROM AUTHOR]

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 (< 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 ∼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. [ABSTRACT FROM AUTHOR]

The influence of stray light in the divertor spectroscopy system in ITER is quantitatively investigated using a ray tracing simulation. Simulation results show that the stray light is negligible at positions in the divertor where the plasma emission is strong. However, it is also shown that the stray light can be significantly greater than the real signal if the plasma intensity is low. Deuterium and beryllium emissions are used for the assessment; for beryllium cases in particular, since the emission profile may be non-uniform in the divertor region, the influence of stray light can be non-negligible at some positions, e.g., above the divertor dome. [ABSTRACT FROM AUTHOR]

Helium (He)-induced nanostructured tungsten sheets were synthesized by He plasma irradiation under different plasma exposure durations. After calcination, nanostructured tungsten oxide samples were used as photoelectrodes to test photoelectrochemical (PEC) performance. The results showed that nanostructured WO 3 photoanodes have higher PEC performance compared to the sample without nanostructures. The 15 min irradiated sample had the highest photocurrent density of 3.5 mA/cm 2 under the thermodynamic potential of water oxidation (1.23 V vs. RHE). It was found that the oxide layer thickness and exposed crystal facet have a significant impact on PEC performance. The plasma synthesis technique has proved to be an effective method for preparing nanostructured WO 3 photoelectrodes. [Display omitted] • Nanostructured WO 3 synthesized by helium plasma irradiation and calcination. • The 15min-WO 3 -773K photoelectrode exhibited the highest photocurrent density. • The reasons for the excellent photoelectrochemical performance have been proved. • It can be ascribed to the proper oxide layer thickness and exposed crystal facets. [ABSTRACT FROM AUTHOR]

Highlights: [•] Systematic helium plasma irradiations to titanium were conducted. [•] The ion energy was <100eV and the high fluence was >1025 m−2. [•] Various types of morphology changes occurred on titanium surface. [•] They can be categorized nanocone, pinhole, porous structure, and rough surfaces. [•] He bubble growth, surface diffusion, and physical sputtering are important. [Copyright &y& Elsevier]

The thermal response of nanostructured tungsten, which was fabricated in the linear divertor simulator NAGDIS-II, was investigated using pulsed plasma in the MAGNUM-PSI device and by using high powered laser pulses. The temperature evolution in response to the pulses was measured with an infrared fast framing camera. The temperature increase in response to the pulses on the nanostructured sample was significantly greater than that of the pristine sample both for plasma and laser pulses. After the pulsed plasma/laser irradiation, the nanostructured surface was observed to have melted although the measured surface temperature was much less than the melting temperature. The mechanism to account for the difference in the temporal evolution of temperature between the nanostructured and pristine samples is discussed. It is suggested that there are thermally isolated part on the nanostructures and anomalous temperature increase and melting have occurred locally on them. The amount of vapourized tungsten in response to type-I edge localized mode in ITER is discussed based on experimental observation. [ABSTRACT FROM AUTHOR]

Highlights: [•] A detailed designing for collection optical system of divertor Thomson scattering system in JT-60SA is conducted. [•] The assessment of the density and temperature errors of the measurement system is conducted. [•] It is shown that the measurement could be done with the temperature error of 50% when the density was 1020 m−3. [•] The availability of the laser transmission mirrors for the measurement system is discussed. [•] Several guidelines to improve the measurement system are discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] The field emission property from helium irradiated nanostructured tungsten was measured. [•] The field enhancement factor was approximately 200, which was not so high, but the current density was significantly increased. [•] The increase in the field emission indicates the increase in the ignition probability. [ABSTRACT FROM AUTHOR]

Characterization of a pulsed plasma formed with a plasma gun device was performed. The averaged electron density was measured with an interferometer system using a Zeeman laser at 633 nm. It was found that the plasma with the density of \sim 10^22~m^-3 was successfully formed. The temporal evolution of the plasma was observed with a fast framing camera. It was found that the shape of the plasma was significantly varied temporally. From the intensity profile, the vertical emission profile was obtained using Abel inversion. The emission profile had a flat top shape with the radius of \sim20~mm, and gradually decreased with the radius. The penetration behavior of the gun plasma across the magnetic field was observed. [ABSTRACT FROM AUTHOR]

Abstract: Helium plasma irradiation to tungsten leads to the formation of nanostructures on the surface. On the other hand, when the surface temperature is raised without helium irradiation, the nanostructure is annealed out. In this study, the boundary between the formation and annealing of tungsten nanostructures are investigated in the divertor simulator NAGDIS-II. When the helium ion flux to the surface was gradually decreased at the surface temperature of 1300–1600K, the nanostructure growth was terminated at some point, and the nanostructures started to be annealed out. The helium ion flux threshold that determines the boundary between the growth and the annealing was varied by the irradiation history, and the flux threshold was in the range of 1021–1022 m−2. [Copyright &y& Elsevier]

When tungsten is exposed to helium plasmas, it was found that nanostructures are grown on the surface. Arcing can be easily initiated in response to a transient heat load with a pulsed laser, which demonstrates transient heat loads in fusion devices. In this paper, we show the behavior of arcing initiated on the nanostructured tungsten. From the measurement of field-emission current, it is thought that the increase in the field emission by helium irradiation may be a key process for the initiation. The behavior of the free running arc spots are observed with fast framing camera. Interesting behavior of initiated arc spots including retrograde motion, bifurcation, and random motion are shown. The spectroscopic study of the emission from the arc spot revealed that the emission contained a continuum emission in visible range in addition to many line emissions. [ABSTRACT FROM AUTHOR]

Arcing is a long-standing plasma-surface interaction issue, and the issue is currently being revived. This paper assesses the impact of arcing in fusion devices based on the observations in JT-60U, the Large Helical Device (LHD) and the linear divertor simulator NAGDIS-II. To investigate the first initiation process of arcing, field emission currents from several tungsten samples are measured. It is shownthat the field emission current increases significantly after tungsten is exposed to helium plasmas. A postmortem analysis of JT-60U tiles reveals that arcing phenomena occurred on carbon baffle plates inside the vacuum vessel in JT-60U. From the observation of the arc trails recorded on the baffle plate, the amount of eroded materials is discussed. The arcing seems to occur frequently on the inner baffles rather than the outer baffles. From LHD, it is shown that the arcing can be initiated on nanostructured tungsten even without transient events. The erosion of tungsten by arcing will become an important issue in a fusion reactor, where helium fluence is significantly increased. From the experiments in NAGDIS-II, it is shown that arcing can be initiated even without transient heat load when the target voltage is low enough, e.g. -500V. Frequent initiation of arcing annihilates the nanostructure growth due to helium plasma irradiation on the surface. [ABSTRACT FROM AUTHOR]

Deuterium atomic density in the divertor region of fusion reactors is an important parameter. In this study, atomic density is estimated in the divertor simulator NAGDIS-T by use of an absorption effect of Lyman- β, of which the wavelength is in the vacuum ultraviolet (VUV). The calibration of the spectrometer in the VUV wavelength was performed by using a branching ratio method. The intensity ratio of Lyman- β to Balmer- α significantly decreased in detached plasmas because of radiation trapping. The atomic density was estimated by use of self-absorption effect. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

Abstract: 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 10min 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. [Copyright &y& Elsevier]

Abstract: Pulsed laser irradiation (∼0.1–4MJ/m2 at 1–10ms) to a beryllium (Be) target has been performed under steady-state deuterium (D) plasma exposure in the linear divertor plasma simulator PISCES-B to investigate the response of Be to transient heat loads such as edge localized modes. Emission intensities from Be atoms and beryllium deuteride (BeD) molecules are observed in front of the Be target by using two fast framing cameras simultaneously; those exhibit similar time evolution to each other, but spatial profiles are different. While the Be I light emission peaks just in front of the target, the BeD emission peaks away from the target. This indicates that Be atoms are directly ejected from the surface, and BeD molecules are volumetrically formed in the plasma. It is also found that the time evolution of light emission can be qualitatively well reproduced by the evaporation flux of Be atoms, estimated from the calculated surface temperature. The Be surface is thought to be eroded as atoms due mainly to evaporation during laser irradiation. [Copyright &y& Elsevier]

Abstract: In future fusion devices, helium irradiation to tungsten could lead to the formation of tungsten nanostructure, which has both pros and cons as a plasma facing component. Although the nanostructure may have considerable influences in fusion devices, there still exist open questions with regards to the formation mechanism and its characteristics. Here we show from a detailed TEM investigation the roles of helium nanobubbles on the formation of the nanostructured tungsten. The result also indicates that the growth of helium bubbles beneath the nanostructured layer could lead to the formation of tungsten dusts, which might have fatal influence to the tokamak operation, when the surface temperature is significantly high, say higher than 1800K. [Copyright &y& Elsevier]

Abstract: The effect of helium irradiation on in-vessel mirror materials, i.e. molybdenum and rhodium, are investigated experimentally. By the exposure to helium plasmas with low incident ion energy (∼50eV) at different surface temperatures, the optical reflectivity of molybdenum and rhodium decreases significantly. From the surface analysis, it is shown that fiberlike nanostructure is formed on molybdenum surface when the surface temperature is high (at 1500K), while rough surface is observed when the surface temperature is low (<1000K). The decrease in the optical reflectivity is significant particularly for short wavelength ranges, typically, less than 300nm. The results indicate that the helium irradiation should be taken into account for in-vessel mirror materials for the optical diagnostics in ITER. [Copyright &y& Elsevier]

Abstract: In order to investigate the morphology change of tungsten due to helium plasma exposure, tungsten (W) materials fabricated in different processes, ITER grade W, ultra-fine grained W (W–0.8TiC), and powder metallurgy W after helium plasma exposure were observed by scanning electron microscope. In all the three cases, pin-holes are observed on the surface; moreover, the non-uniformity of the formation of helium bubbles and fiberform nanostructure was observed on ITER grade W. From the detailed analysis of the crystal orientation with the help of focused ion beam method (FIB) indicates that the initial growth of the nanobubbles may be slow on the (110) face of tungsten. [Copyright &y& Elsevier]

A combination of helium line intensities and a collisional radiative model has been used to measure electron density and temperature. However, radiation trapping of resonance lines may disturb the measurements due to disturbances in the population distribution of helium atoms. In this study, we show that the principal contribution of radiation trapping in helium plasma can be evaluated by additionally measuring one or two specific line intensities from the singlet state. The inclusion of the effects of radiation trapping sufficiently compensates for anomalous increases in the electron density and temperature, and consequently yields proper values. An experiment was performed in the divertor simulator NAGDIS-II, and the method's validity was confirmed by comparing the spectroscopically obtained results and the values from the electrostatic probe method. [ABSTRACT FROM AUTHOR]

Abstract: In the ITER edge Thomson scattering measurement system, polychromators with optical band-pass filters and avalanche photodiodes are planned to be used. In this paper, we optimized the transmission wavelength ranges of the optical filters by a numerical method. Since measurements in a high electron temperature range are required for the measurement system, the Thomson spectrum could overlap a strong line emission of when the electron temperature is rather high. It is shown that a filter whose transmission wavelength range is shorter than becomes important to decrease the measurement error in the high temperature range. Moreover, it is found that a filter whose transmission wavelength range is above 1064nm (laser wavelength) is useful to improve the measurement accuracy, in particular, when the number of filters is more than six. [Copyright &y& Elsevier]

Abstract: Clear footprint of an arc spot, which is formed after arc spot runs on a fiberform nanostructured tungsten, reveals the detailed structure and motion of arc spot in a magnetic field. Fine inner structure of an arc trail is observed, exhibiting that many sub-arc spots existed inside the arc spot, with forming a group. The sub-arc spots move randomly but they globally move to some direction, which is affected by the axial and parallel magnetic field with respect to the specimen. The trails are analyzed by using box-counting method and fractality of the arc trails in magnetic field is discussed. It is shown that the nanostructured metal can clearly record the footprint of the arcs, similar as the man''s footprints in the snow, and be a powerful tool for basic arc experiments. [Copyright &y& Elsevier]

A ray tracing code combined with a He I collisional-radiative (CR) model has been developed to investigate effects of radiation trapping on the population distribution of He atoms in a weakly ionized plasma. The code accurately reproduces experimentally observed vertical profiles of emission intensities in a linear plasma device. On the other hand, the CR code and that modified with the optical escape factor method to include the radiation trapping effects show that vertical profiles significantly deviate from measured ones at the periphery of the plasma column. This work therefore demonstrates that accurate matching of line intensity profiles over the entire plasma column requires a ray tracing code, not just an optical escape factor approximation. [ABSTRACT FROM AUTHOR]

Fiberform nanostructured tungsten is formed on tungsten surface by helium plasma irradiation under a specific condition. It is revealed that the optical reflectivity of tungsten is significantly decreased by the formation of the nanostructure. We present the experimentally obtained necessary condition to form the structure in the linear plasma device NAGDIS-II. We investigate the effects of laser irradiation to the helium-irradiated surface on the optical reflectivity. It is revealed that the sub-ms laser pulse irradiation alleviates the surface roughness and recovers the optical reflectivity. However, when the structure is finer, the recovery rate becomes low, typically less than half; moreover, it decreases with the number of pulses. [ABSTRACT FROM AUTHOR]

In this study, we investigate designs for polychromators of the ITER edge Thomson scattering system by means of numerical calculations. For this purpose, we discuss the optimum transmission wavelength ranges of the optical filters and the optimum number of filters. Five different filter arrangements, including those with an optical notch filter to eliminate Dα emission, are compared with each other. The measurement accuracy when using a polychromator is evaluated for midplane port configuration. It is found that the optimum filter number keeping in mind the cost performance cost is 5 or 6. Moreover, the use of an optical notch filter increases the measurement accuracy by approximately 5%. [ABSTRACT FROM AUTHOR]

In order to realize high-performance titanium dioxide (TiO 2) photocatalysts for ethylene decomposition, titanium (Ti) thin film samples with fiberform nanostructures were prepared by helium (He) plasma treatment. Then, the oxidation condition for ethylene decomposition were optimized in terms of the oxidation temperature and duration. It is shown that He plasma irradiation with Ti deposition followed by oxidation in the air formed anatase-rich TiO 2. The rutile anatase transition temperature increased to higher than 1000 K on the He treated samples and the best photocatalytic performance is twice higher than that on TiO 2 sample formed in a magnetron sputtering device. Helium plasma treatment induces vacancies, He clusters, and He bubbles together with formation of nanostructures. The results suggested that those morphology changes lead to the formation of stable anatase structure, which is likely the key factor for the enhancement of photocatalytic activity. Based on the obtained ethylene decomposition rate, we will discuss the necessary light source and amount of photocatalysts to keep vegetables fresh in a long-distance transport cargo. [Display omitted] • Helium plasma treatment with titanium deposition formed nanostructured thin film. • After oxidation, He plasma treated thin film formed anatase rich TiO 2. • He treated thin film had twice higher performance in ethylene decomposition. • Necessary equipment was assessed to decompose ethylene in a container. [ABSTRACT FROM AUTHOR]

The radial density profile of negative hydrogen ions in magnetized plasma is investigated in the divertor simulator MAP (Material and plasma)-II [S. Kado et al., J. Plasma Fusion Res. 71, 810 (2005)] by measurement and numerical simulation. The laser photodetachment method is used to evaluate the H- density by considering the influence of the magnetic field. The density of H- has a hollow profile and exhibits a peak in the peripheral region, though the electron density and temperature exhibit a peak at the center of the plasma column. The density profile of H- does not agree with the calculation result obtained from the rate equation, in which the local production and extinction rates are balanced, under the present experimental condition. To understand the behavior of negative ions, their trajectories are calculated by numerically solving the equation of motion by considering the effect of collisions. The negative ion density profile calculated from the particle simulation agrees well with the measured negative ion density profile. It is shown that the cross-field H- transport due to the radial electric field with the assistance of the elastic collisions plays an important role in enhancing the negative ion density in the peripheral region. [ABSTRACT FROM AUTHOR]

The electron density and temperature obtained from the line intensity ratio method of He I (λ=667.8, 706.5, and 728.1 nm) are compared to the probe method in a divertor simulator. When a collisional radiative model that does not include the effect of the radiation transport was used for the analysis, ne obtained from the spectroscopic method was significantly higher than that from the electrostatic probe method. The discrepancy between the two methods increases with the gas pressure; in other words, it increases with the optical thickness. In the case that the effect of the radiation trapping is taken into consideration using optical escape factor, the discrepancy becomes moderate. And then, the parameters obtained from the line intensity ratio method agree with the probe method within a factor of 2 in the case that the radiation trapping was introduced with R=0.05 m, which corresponds to the column radius of the spatial profile of the excited population density. In recombining plasmas, however, it was shown that the line intensity ratios might not be appropriate because the recombining component broke the monotonic increase/decrease dependences of the line intensity ratios on ne and Te. A measurement of another He I line intensity of 447.1 nm (2 3P←4 3D) is proposed for solving the problem. [ABSTRACT FROM AUTHOR]

From the helium (He) plasma irradiations to tungsten performed in the Magnum-PSI device, the effects of deposition of metals on the helium-plasma induced fiberform nanostructures (fuzz) are discussed. It was found that fuzz was not formed at the center of the plasma cylinder if there were significant metallic impurities from the source. Deposition of metallic impurities (mainly molybdenum and copper) counteracted the growth of fuzz. In addition to the effects of metals from the source, we installed a sputtering source near the sample to replicate the deposition environment in fusion devices. The thickness of fuzzy layer was ∼ 7 µm, which was about five times greater than that without deposition, at the He flux of 1.3 × 10 26 m − 2 , suggesting that the growth rate of fuzz layer was significantly accelerated due to the deposition of tungsten. [ABSTRACT FROM AUTHOR]

• Feasibility study of the in-vessel lighting system was performed using a ray tracing simulation. • In general, the wall is uniformly illuminated when the wall has a diffuse reflectance while the specular case has more brightness. • Some field of views requires couple of minutes to obtain enough signal levels. • In dark areas, concurrent integration for longer integration time will be helpful to decrease the total inspection time. In-vessel lighting system is being developed for ITER. The diagnostic system is for inspecting and detecting eventual damages and defects of in-vessel components between discharges by illuminating the vacuum vessel with light sources. In this work, feasibility study of the in-vessel lighting system is performed using a ray tracing simulation. We installed sources and detectors in a ray tracing model using the field of views of wide-angle viewing systems and calculated the radiance profiles at detectors for different wall reflection scenarios. It was found that the wall is uniformly illuminated when the wall has a diffuse reflectance while the specular case has more brightness. We will discuss feasibility of the diagnostics system based on expected signal and noise levels. [ABSTRACT FROM AUTHOR]

Co-deposition of wall material and ions from plasmas occurs on material surfaces in fusion devices. In this study, we focus on tungsten (W) deposition layer in helium (He) plasmas (He-W co-deposition) formed at surface temperatures of 473–773 K. Usually, He irradiation at the surface temperature below 1000 K does not lead to significant morphology changes; we will show from transmission electron microscope observations that the roughness of the co-deposition layer has a significant temperature dependence even when the temperature is less than 800 K. We conducted deuterium plasma irradiation to the He-W co-deposition layer. It was found that the deuterium retention on the He-W co-deposition layer increased significantly with increasing the formation temperature of the co-deposition layer. [ABSTRACT FROM AUTHOR]