Helium Plasma Irradiation Research Articles

Application of Nanostructured Tungsten Fabricated by Helium Plasma Irradiation for Photoinduced Decolorization of Methylene Blue

Application of Nanostructured Tungsten Fabricated by Helium Plasma Irradiation for Photoinduced Decolorization of Methylene Blue

Field emission property of nanostructured tungsten formed by helium plasma irradiation

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.

Growth annealing equilibrium of tungsten nanostructures by helium plasma irradiation in non-eroding regimes

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 1300–1600 K, 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 10 21 –10 22 m −2 .

Impact of arcing on carbon and tungsten: from the observations in JT-60U, LHD and NAGDIS-II

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 shown that 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. −500 V. Frequent initiation of arcing annihilates the nanostructure growth due to helium plasma irradiation on the surface.

Helium plasma implantation on metals: Nanostructure formation and visible-light photocatalytic response

It has been found recently that low-energy helium (He) plasma irradiation to tungsten (W) leads to the growth of W nanostructures on the surface. The process to grow the nanostructure is identified as a self-growth process of He bubbles and has a potential to open up a new plasma processing method. Here, we show that the metallic nanostructure formation process by the exposure to He plasma can occur in various metals such as, titanium, nickel, iron, and so on. When the irradiation conditions alter, the metallic cone arrays including nanobubbles inside are formed on the surface. Different from W cases, other processes than growth of fiberform structure, i.e., physical sputtering and the growth of large He bubbles, can be dominant on other metals during irradiation; various surface morphology changes can occur. The nanostructured W, part of which was oxidized, has revealed a significant photocatalytic activity under visible light (wavelength >700 nm) in decolorization of methylene blue without any co-catalyst.

Recovery of Tungsten Surface with Fiber-Form Nanostructure by Plasmas Exposures

One of the serious concerns for tungsten materials in fusion devices is the radiation defects caused by helium plasma irradiation since helium is a fusion product. The fiber-formed nanostructure is thought to have a possible weakness against the plasma heat flux on the plasma-facing component and also may destroy the reflectivity of optical mirrors. In this paper an interesting method for the recovery of such tungsten surfaces is shown. The recovery process depends on the grade and manufacturing process of tungsten materials.

Adhesion Strength of Electroless Copper Plated Layer on Fluoropolymer Surface Modified by Medium Pressure Plasma

We demonstrated high adhesive fluoropolymer/copper interface through combination of atmospheric pressure plasma with liquid phase self-assembly. However, there are some disadvantages in atmospheric pressure plasma technique, such as small processing area due to the localized plasma and high gas temperature. Medium pressure plasma process has some advantages over atmospheric pressure plasma systems. A large plasma volume and low gas temperature, available for surface treatment of polymer material, can be easier obtained at medium pressure than at atmospheric pressure, which can result in a higher overall productivity. In this paper, we investigated the adhesion strength of electroless copper plated layer formed on poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA) surface modified by combination of medium pressure helium plasma irradiation and aminated acrylic polymer grafting. The 90° peel test result for copper plating film formed on the treated PFA films showed the adhesion strength of 0.44 N/mm without increasing the surface roughness.

Near Infrared Radiation from Heated Nanostructured Tungsten

Helium plasma irradiation to metals leads to morphology changes in nanometer scales by the formation of helium bubbles. The nanostructure significantly alters the optical property of the metal. In this study, near infrared radiations from a heated nanostructured tungsten are measured with a spectrometer. Energy balance between the influx of plasma and radiation from the sample is discussed in terms of the optical emittance. It is shown that the optical emittance is increased considerably by the morphology change in the range from visible to near infrared, and the efficiency of radiation is increased by two to three times.

TEM analysis of high temperature annealed W nanostructure surfaces

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.

Recovery of Tungsten Surface with Fiber-Form Nanostructure by the Argon Plasma Irradiation at a High Surface Temperature

One of the serious concerns for tungsten materials in fusion devices is the radiation defects caused by helium plasma irradiation, while the helium is one of fusion products. Fiber-formed nanostructure is worried to have a possible weakness against the plasma heat flux and may destroy the reflectivity as an optical mirror. In this communication an interesting method for a recovery of such a tungsten surface is shown.

Formation of Metallic Nanostructure by Helium Plasma Irradiation

Morphology changes on various metals by helium irradiation are shown experimentally. It has been revealed that a whisker-like nanostructure was formed on tungsten and molybdenum surfaces by the exposure to a helium plasma. After helium irradiation to titanium and stainless steel, many dots in several hundreds of nanometers cover the surface. The morphology change may be caused by the growing process of helium bubbles in the sub-surface region.

Formation of Metallic Nanostructure by Helium Plasma Irradiation

Formation of Metallic Nanostructure by Helium Plasma Irradiation

Surface modification of tungsten mirrors due to low-energy helium plasma irradiation in the compact PWI simulator APSEDAS

Surface modification of a polycrystalline tungsten (W) mirror due to low-energy helium plasma irradiation has been studied in the compact plasma–wall interaction (PWI) simulator APSEDAS. Many fine irregularities of a few μm are generated on the W surface at Ts∼1400 K due to He bubble formation. On the other hand, a distinctive change in the surface morphology cannot be seen from the SEM micrograph on a W mirror exposed at Ts<710 K. In both samples, the optical reflectivity with the incident angle θ=5° decreases more at shorter wavelength. From in situ measurement of the optical reflectivity with θ=75°, it is found that the optical reflectivity at λ∼555 nm of the low Ts exposure increased by ∼6% and decreased to the original level at a fluence of ∼3×1024 He+ m−2, and then increased again by ∼6% with the fluence up to ∼1.6×1026 He+ m−2. Calculations using a model of the multiple reflection in the modification region suggest that the thickness of the modification layer of the low Ts sample exceeds 70 nm.

Formation and mitigation of fiberform nanostructured tungsten by helium and sub-ms laser pulse irradiations

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.

Plasma-assisted laser ablation of tungsten: Reduction in ablation power threshold due to bursting of holes/bubbles

Nanosecond laser ablation of tungsten (W) exposed to helium plasma is investigated using optical emission spectroscopy. Submicrometer-sized holes/bubbles are formed on the surface of W when it was exposed to the helium plasma at a sufficiently high temperature (≳1500–1600K). The emissions from a virgin W (before the helium plasma irradiation) cannot be detected when the fluence is &amp;lt;1J∕cm2; however, the threshold fluence for the detection of neutral W emission after it was exposed to the helium plasma is ∼0.2J∕cm2. The physical mechanism of laser-induced bursting of holes/bubbles is proposed for achieving a significant reduction in ablation power threshold.

Sub-ms laser pulse irradiation on tungsten target damaged by exposure to helium plasma

The effects of a transient heat load on tungsten damaged by helium plasma irradiation have been investigated using a ruby laser with long pulse duration in the divertor simulator NAGDIS-II (Takamura et al 2002 Plasma Sources Sci. Technol. 11 A42). The pulse width of the ruby laser was ∼0.6 ms, which is close to that of the expected heat load accompanied by type-I edge localized modes (ELMs) in ITER operation. Helium holes/bubbles, which were formed in the surface region of powder metallurgy tungsten due to the exposure to the helium plasma, disappeared after the laser pulse irradiation to the tungsten surface with sufficient pulse energy. The results indicated that the transient heat loads similar to those expected by ELMs will mitigate damages such as bubbles and holes produced by helium irradiation. When a vacuum plasma sprayed tungsten coating on graphite was exposed to the helium plasma, the surface was covered with arborescent nanostructured tungsten containing many helium bubbles inside the structure. Melting traces were found on the surface after the laser pulses irradiated the surface even though the pulse energy was lower than that for melting bulk tungsten. A numerical temperature calculation of the sample suggested that the effective thermal conductivity near the surface dramatically decreased by several orders of magnitude due to the formation of nanostructured tungsten.

Formation of Nanostructured Tungsten with Arborescent Shape due to Helium Plasma Irradiation

Deeply nanostructured tungsten with an arborescent shape was found for the first time to be formed on tungsten-coated graphite by a high-flux helium plasma irradiation at surface temperatures of 1250 and 1600 K, an incident ion energy of 12 eV (well below the physical sputtering threshold) and a helium ion fluence of 3.5 × 1027 m-2.

Dynamic Simulation of Multiplier Effects of Helium Plasma and Neutron Irradiation on Microstructural Evolution in Tungsten

Plasma-facing and high heat flux materials in a fusion reactor suffer two types of damage: displacement damage caused mainly by high- energy neutrons and surface damage, such as erosion, sputtering, and blistering, caused by hydrogen and helium plasma. Usually, these two kinds of damage are investigated separately. In the present study, multiplier effects of helium plasma and neutron irradiation on microstructural evolution in tungsten were investigated using computer simulations based on a rate theory. Neutron irradiation with 10 � 6 dpas � 1 at 873 K and a helium flux of 10 18 m � 2 � s � 1 with 10-keV energy were used as typical irradiation conditions. The effects of irradiation temperature and defect production rate on the interaction between helium and defects were also investigated. The simulation results revealed the rapid diffusion of helium in tungsten. The helium concentration was saturated in tungsten 0.067 mm thick within 0.01 s at 873 K, and the formation of helium- vacancy clusters was nearly uniform in the matrix except at the surfaces facing towards and away from the irradiation after 0.01 s. This meant that the 10-keV helium plasma could enhance formation of helium-vacancy clusters in the region without damage produced by helium. The concentration of 6He-V clusters reached a very high level (10 � 6 ) even after a 1-s irradiation with a defect production rate of 10 � 6 dpas � 1 at 873 K. The concentration of helium-vacancy clusters decreased with increasing irradiation temperature and decreasing defect production rate. The accumulation of helium and the formation of helium-vacancy clusters depended on not only the vacancy concentration, which was determined by the irradiation temperature and defect production rate, and helium concentrations, but also irradiation time.

Formation mechanism of bubbles and holes on tungsten surface with low-energy and high-flux helium plasma irradiation in NAGDIS-II

A systematic study on the formation mechanism of micron-sized He bubbles and holes in powder metallurgy tungsten due to helium ion irradiation with an ion energy below 30 eV and a particle flux above 10 22 m −2 s −1 has been performed in the linear divertor plasma simulator NAGDIS-II. Holes are formed with incident helium ion energy above 5 eV, which could be related to the surface barrier potential energy for He penetrating into tungsten. Tungsten surface temperature strongly influences the number and size of hole. Above 1600 K, bubbles and/or holes with several hundreds nano-meter diameter appear on the tungsten surface. Single crystal tungsten, which has much fewer intrinsic defects than powder metallurgy tungsten, was also irradiated by He plasmas. There is no qualitative difference in the hole formation between the two grades of tungsten. Bubble and hole formation mechanisms are discussed based on the experimental results.

Incident ion energy dependence of bubble formation on tungsten surface with low energy and high flux helium plasma irradiation

Tungsten (W) specimens were irradiated by low energy (<65 eV) and high flux (∼ 10 22–23 m −2 s −1 ) helium plasma to investigate the incident ion energy dependence of helium bubble formation. Experimental results indicate the existence of the incident ion energy threshold for the bubble formation. The threshold energy around 15 eV could associate with the surface potential for He ions entering to the W surface.