Effects of Low Energy and High Flux Helium/Hydrogen Plasma Irradiation on Tungsten as Plasma Facing Material

The High-Z material tungsten (W) has been considered as a plasma facing material in the divertor region of ITER (International Thermonuclear Experimental Reactor). In ITER, the divertor is expected to operate under high particle fluxes (> 1023 m-2s-1) from the plasma as well as from intrinsic impurities with a very low energy (< 200 eV). During the past dacade, the effects of plasma irradiation on tungsten have been studied extensively as functions of the ion energy, fluence and surface temperature in the burning plasma conditions. In this paper, recent results concerning blister and bubble formations on the tungsten surface under low energy (< 100 eV) and high flux (> 1021 m-2s-1) He/H plasma irradiation are reviewed to gain a better understanding of the performance of tungsten as a plasma facing material under the burning plasma conditions.

Surface Cracking Behaviors of Ultra-Fine Grained Tungsten Under Edge Plasma Loading in the HT-7 Tokamak

Tests of the candidate plasma facing materials（PFMs） used in experimental fusion devices are essential due to the direct influence of in-situ plasma loading.A type of ultrafine grained（UFG） tungsten sintered by resistance sintering under ultra-high pressure（RSUHP） method has been exposed in the edge plasma of the HT-7 tokamak to investigate its performance under plasma loading.Under cychc edge plasma loading,the UFG tungsten develops both macro and micro cracks.The macro cracks are attributed to the low temperature brittleness of the tungsten material itself,while the micro cracks are generated from local intense power flux deposition.

Exposure of Equal-Channel Angular Extruded Tungsten to Deuterium Plasma

Surface morphology and deuterium retention in ultrafine-grained tungsten fabricated by equal-channel angular pressing（ECAP） have been examined after exposure to a low energy,high-flux deuterium（D） plasma at fluences of 3×10^24 D/m^2 and 1×10^25 D/m^2 in a temperature range of 100 ℃-150 ℃.The methods used were scanning electron microscopy（SEM） and thermal desorption spectroscopy（TDS）.Sparse and small blisters（0.1 μm） were observed by SEM after D plasma irradiation on every irradiated surface;yet they did not exhibit significant structure or plasma fluence dependence.Larger blisters or protrusions appeared after subsequent TDS heating up to 1000 ℃.The TDS results showed a single D desorption peak at 220℃ for all samples and the D retention increased with increasing numbers of extrusion passes,i.e.,the decrease of grain sizes.The increased D retention in this low temperature range should be attributed to the faster diffusion of D along the larger volume fraction of grain boundaries introduced by ECAP.

Effect of Treatment Temperature on Iron Nitride Foils Irradiated with Nitrogen Plasma

We investigated the effect of treatment temperature on the magnetic property of iron nitride foils irradiated with nitrogen plasma. The iron nitride foils irradiated with nitrogen plasma were composed of ε-Fe2/3N, γ'-Fe4N and γ nitrogen austenite in α-Fe of the matrix. The saturation magnetization of the iron nitride foils decreased with increasing the surface temperature. The coercive force of the iron nitride foils increased with increasing the surface temperature.

High-Yield High-Efficiency Positron Generation in High-Z Metal Targets Irradiated by Laser Produced Electrons from Near-Critical Density Plasmas

An improved indirect scheme for laser positron generation is proposed. The positron yields in high-ZZ metal targets irradiated by laser produced electrons from near-critical density plasmas and underdense plasma are investigated numerically. It is found that the positron yield is mainly affected by the number of electrons of energies up to several hundreds of MeV. Using near-critical density targets for electron acceleration, the number of high energy electrons can be increased dramatically. Through start-to-end simulations, it is shown that up to 6.78×10106.78×1010 positrons can be generated with state-of-the-art Joule-class femtosecond laser systems.

Conversion of Catalytically Inert 2D Bismuth Oxide Nanosheets for Effective Electrochemical Hydrogen Evolution Reaction Catalysis via Oxygen Vacancy Concentration Modulation

Oxygen vacancies(Vo)in electrocatalysts are closely correlated with the hydrogen evo-lution reaction(HER)activity.The role of vacancy defects and the effect of their concentration,how-ever,yet remains unclear.Herein,Bi2O3,an unfavorable electrocata-lyst for the HER due to a less than ideal hydrogen adsorption Gibbs free energy(ΔGH*),is utilized as a perfect model to explore the func-tion of Vo on HER performance.Through a facile plasma irradia-tion strategy,Bi2O3 nanosheets with different Vo concentrations are fabricated to evaluate the influence of defects on the HER process.Unexpectedly,while the generated oxygen vacancies contribute to the enhanced HER performance,higher Vo concentrations beyond a saturation value result in a significant drop in HER activity.By tunning the Vo concentration in the Bi_(2)O_(3)nanosheets via adjusting the treatment time,the Bi2O3 catalyst with an optimized oxygen vacancy concentration and detectable charge carrier concentration of 1.52×10^(24)cm^(−3)demonstrates enhanced HER performance with an overpotential of 174.2 mV to reach 10 mA cm^(−2),a Tafel slope of 80 mV dec−1,and an exchange current density of 316 mA cm−2 in an alkaline solution,which approaches the top-tier activity among Bi-based HER electrocatalysts.Density-functional theory calculations confirm the preferred adsorption of H*onto Bi2O3 as a function of oxygen chemical potential(ΔμO)and oxygen partial potential(PO2)and reveal that high Vo concentrations result in excessive stability of adsorbed hydrogen and hence the inferior HER activity.This study reveals the oxygen vacancy concentration-HER catalytic activity relationship and provides insights into activating catalytically inert materials into highly efficient electrocatalysts.

Ultralow-weight loading Ni catalyst supported on two-dimensional vermiculite for carbon monoxide methanation

Nickel-based catalysts represent the most commonly used systems for CO methanation.We have successfully prepared a Ni catalyst system supported on two-dimensional plasma-treated vermiculite（2D-PVMT）with a very low Ni loading（0.5 wt%）.The catalyst precursor was subjected to heat treatment via either conventional heat treatment（CHT）or the plasma irradiation method（PIM）.The as-obtained CHT-Ni/PVMT and PIM-Ni/PVMT catalysts were characterized with scanning electron microscopy（SEM）,energy dispersive X-ray（EDX）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,inductively coupled plasma-atomic emission spectroscopy（ICP-AES）and high-angle annular dark field scanning transmission electron microscopy（HAADF-STEM）.Additionally,CHT-NiO/PVMT and PIM-NiO/PVMT catalysts were characterized with hydrogen temperature programmed reduction（H2-TPR）.Compared with CHT-Ni/PVMT,PIM-Ni/PVMT exhibited superior catalytic performance.The plasma treated catalyst PIM-Ni/PVMT achieved a CO conversion of93.5%and a turnover frequency（TOF）of 0.8537 s^-1,at a temperature of 450℃,a gas hourly space velocity of 6000 ml·g^-1·h^-1,a synthesis gas flow rate of 65 ml·min^-1,and a pressure of 1.5 MPa.Plasma irradiation may provide a successful strategy for the preparation of catalysts with very low metal loadings which exhibit excellent properties.

Recent studies of tungsten-based plasma-facing materials in the linear plasma device STEP

This contribution summarized the recent studies of tungsten-based plasma-facing materials in the linear plasma device like the simulator for tokamak edge plasma(STEP),focusing on the examination of newly developed tungsten(W)-based materi-als and plasma-induced defects in pure W.Pure W,W-V,W-Y_(2)O_(3)and W-ZrC samples were exposed to a high-flux plasma of~1021-1022 m^(−2)s^(−1) with a fluence up to 1026 m^(−2) at a surface temperature below 500 K.The investigation of fundamental evolution of plasma-induced defects in pure W indicated a critical role of hydrogen-dislocation interactions.Suppressed surface blistering was observed in all W-based materials,but deuterium desorption behavior and retention were distinct with respect to different materials.The studies showed that the linear plasma device like the STEP was indispensable in the understanding of plasma-material interactions and the qualification of new materials for future fusion reactors.

He bubble-driven growth of W fuzz during the interaction between H_(2)/He plasmas and W materials

The tungsten(W)material as the divertor of fusion reactors is exposed to low-energy and high-flux He/H isotope irradiation,leading to the growth of fuzz layers.The W fuzz growth does not show any dependence on the plasma irradiation devices used;however,it is strongly dependent on He^(+)fluence and energy,irradiation temperature,and impurity level.When the incident He ions collide inside the dense fuzz layers with the extremely high specific area,their mean free path can be up to 690 nm.Up to now,the He bubble-driven W fuzz growth process is not entirely understood;however,it can be closely related to the surface bursting of He bubbles in the W surface layer and W surface erosion by He^(+)implantation.The formation of He bubbles can be attributed to the solute He diffusion into defects or bubbles,which is strongly affected by the temperature and He^(+)fluence.The W fuzz grows over the W surface,where the micro-stress caused by He bubbles in the W surface layer acts as the driving force.The W fuzz layer inhibits He^(+)implantation into W bulk,and provides an entire protection against the He^(+)erosion into W bulk beneath the fuzz layer.In this review article,the current status of experiment and theory are presented,and some of the remaining issues are discussed.