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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.

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

A modified Boltzmann equation using the corona model was used to study the plasma characteristics of a pulse plasma etching process.

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 WO3 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/cm2 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 WO3 photoelectrodes.

From pulsed plasma experiments focusing on neutral pressure dependence, the impacts of a transition from a low to a high recycling target on the particle load were investigated and discussed in the linear plasma device, Magnum-PSI. Time traces of the target ion flux were mitigated in high neutral pressure cases because of a plasma-neutral interaction. On the other hand, in low neutral-pressure cases, the target ion flux indicated partial suppression in the last part of the pulse. The Langmuir probe, located 200 mm upstream from the target plate, did not exhibit such a suppression. Pulse suppression can be expected from the localized interaction between recycled neutral flux and pulsed plasma in front of the target. The mean-free paths of recycled neutral particles regarding the charge exchange with pulse ions and elastic scattering with background neutral particles were compared. Modeling using a fluid code coupled with a neutral transport code was performed, and it was concluded that dynamic pressure induced by the transient recycled neutral flux caused sufficient momentum loss to stagnate the pulsed plasma toward the target plate.

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

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., ファイル公開：2021-12-01

We clarify a mechanism for longer lifetime of the fast atom beam (FAB) source for surface activated bonding proposed in our previous study. The new FAB source achieves highly efficient FAB irradiation and suppression of generation of fine carbon (C) particles. In this study, the distribution of sputtering and deposition on the inner wall of the C electrodes have been evaluated by measuring the thickness of the electrodes before and after long-time use. In the new source, the C erosion area due to sputtering is localized near the irradiation port and the sputtered C is deposited on other electrode surface, whereas in the conventional one, the erosion occurs on all surfaces of C electrodes. The reduction of electrode erosion due to sputtering contributes significantly to the extension of the lifetime of the new FAB source.

Field emission current accompanied by visualization of emission sites distribution was measured for nanostructured tendril bundles (NTB) on a tungsten surface. These structures are formed in linear simulators of plasma-surface interaction under helium plasma irradiation with Ne or N 2 mixture, and unipolar arcing is often observed for such surfaces. It was shown that field emission current from NTB could be stable for a long period under constant electric field making these structures promising to use as flat cold cathodes. A numerical model was developed for calculating the thermo-field emission current and for checking the possibility of self-heating of the structures depending on their shape. Critical electric field values that lead to self- heating were estimated for different NTB shapes and compared with experimental results.

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

Field emission from tungsten surfaces with nano-tendril bundles formed after exposure to helium plasma with nitrogen or neon impurities was measured using a vacuum diode device. Field enhancement factor and effective emission area were calculated using the Fowler–Nordheim formula and compared for different types of nanostructures. The field enhancement factor for these structures can reach several thousands and the effective emission area can be more than 1000 nm2. Anode luminescence was detected using long-exposure photography and allowed for estimation of current density near the anode. Unipolar arc tracks were observed on the samples with the highest field emission intensity. We show that field emission from nano-tendril bundles is much higher than that from tungsten fuzz or pure tungsten, and fusion-relevant plasma conditions may lead to arc initiation.

Laser-induced arc was initiated on a nanostructured “fuzz” tungsten cathode with simultaneous exposure to a stationary helium (He) plasma. Arc spots moved randomly and eroded almost all the surface area. The diameter of each arc spot was measured to be 5– $10~\mu \text{m}$ . A linear relationship was observed in the arc current–voltage characteristic, which was affected by the presence of resistance through a He plasma column. The specific resistance of He plasmas was estimated from the current–voltage characteristic, which showed a good agreement with theoretical calculations. A greater potential gap should be maintained to keep a higher arc current, indicating a good sign for preventing a high-current arc in fusion devices. The condition of arc ignition was investigated in detail by changing the He plasma parameters and the sheath potential drop. The ignition and sustainment of arcs were very sensitive to both the sheath potential drop and the sheath electric field. Arcs ignited when the sheath potential drop was negatively deeper than −100 V and the sheath electric field was above 2 MV/m. The relatively low electric field threshold would result from the thermofield emission aided by heating of spots from adjacent former spots. This result indicates that the electron density and the electron temperature should be reduced in order to lower the potential drop and the sheath electric field in front of the plasma-facing material surface, which can prevent arcs from igniting in fusion devices.

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.

Divertor particle flux signals on divertor plates were analyzed with the multivariable analysis technique, namely, proper orthogonal decomposition (POD), to characterize the indefinite divertor footprint in the Large Helical Device. To verify whether isotope effects exist in relation to the divertor plasmas, the POD outputs in hydrogen (H) and deuterium (D) plasmas were first compared by analyzing a number of discharges. It was found that the dominant components of the divertor particle flux profiles in H and D plasmas were similar, indicating no isotope effect. In addition, total reconstructed divertor particle fluxes in H plasmas were higher than those in D plasmas, suggesting an isotope effect. Furthermore, the footprint-profile modification with from the SOL-side to the private-side peak due to the edge electron pressure gradient in H and D plasmas were slightly different, relating to an isotope effect. Keywords: Divertor footprint, Isotope effect, Langmuir probe, Proper orthogonal decomposition, LHD

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. Keywords: Nanostructures, Helium, Tungsten, Transmission electron microscope

Surface morphology development and sputtering behavior of Cr, as a test material, have been explored under He plasma exposure at a low incident ion energy of ∼80 eV in multiple linear plasma devices: PISCES-A, PSI-2, and NAGDIS-II. From comparison of the experiments in these devices, deposition of a small amount of heavier impurities (Mo in NAGDIS-II and Ta in PISCES-A) onto Cr is found to result in the formation of cone structures on the Cr surface due to preferential sputtering, resulting in a significant reduction (up to ∼10 times) in the sputtering yield of Cr due to line-of-sight redeposition onto the neighboring cones. The heavier impurities are thought to originate from a sample holding cap/cover, which can be sputtered by a trace amount of intrinsic impurities (C, O, etc) as well as by Cr ionized in the plasma. It can be concluded from the Cr experiments, as well as additional Be data collected in PISCES-B, that heavier impurity deposition plays a major role in the cone structure formation, while other mechanisms (e.g. surface irregularity and oxide) also exist. Keywords: Impurity deposition, Sputtering, Nanostructure

Helium (He) plasma irradiation to tungsten leads to formation of He bubbles and fiberform nanostructures called fuzz on the surface. Although the fuzz growth condition and growth rate have been well investigated, deposition of W together with He ions can change the He effect significantly. For example, the deposition leads to large-scale structures such as nano-tendril bundles and mm-thick large-scale fiberform nanostructures. In this review, current understanding of deposition effects on He irradiation are summarized, and the influence to fusion devices is discussed mainly from the view point of arcing.

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” in the layer, the spot cells group together, and the trajectory of the spot motion becomes “entangled.” 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/ ${r} ^{2}$ , 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.

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.

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 (>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 Nb2 O5 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 Nb2 O5 substrate with Pt deposition.

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.

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., ファイル公開：2020-04-25