Modifying the Properties of Tungsten Based Plasma Facing Materials with Single-Wall Carbon Nanotubes

Tungsten is one of the best candidates for plasma-facing components in fusion reactors owing to its unique properties. But disadvantages such as its brittleness and high ductile-to-brittle transition temperature have restricted its fusion energy application. Single-walled carbon nanotubes （SWCNTs） have the potential to be used as reinforcements due to their excellent mechanical properties. A new method of modifying the properties of tungsten by doping with SWCNTs was introduced. An efficient way of dispersing SWCNTs into the tungsten matrix with strong interfaces by heterocoagulation and ultrasonication was employed, and hot explosive compaction （HEC） technology was selected to compact and sinter the composite powders. The sintering properties, microstructure, densification effect, thermal conductivity, hardness and fracture toughness of the obtained SWCNTs/W bulk samples were tested, and compared with pure tungsten. The influences of SWCNTs on these properties and the main toughening mechanism of SWCNTs in a tungsten matrix were discussed.

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.

Main Directions and Recent Test Modeling Results of Lithium Capillary-Pore Systems as Plasma Facing Components

At present the most promising principal solution of the divertor problemappears to be the use of liquid metals and primarily of lithium Capillary-Pore Systems (CPS) as ofplasma facing materials. A solid CPS filled with liquid lithium will have a high resistance tosurface and volume damage because of neutron radiation effects, melting, splashing and thermalstress-induced cracking in steady state and during plasma transitions to provide the normaloperation of divertor target plates and first-wall protecting elements. These materials will not bethe sources of impurities inducing an increase of Z_(eff) and they will not be collected as dust inthe divertor area and in ducts. Experiments with lithium CPS under simulating conditions of plasmadisruption on a hydrogen plasma accelerator MK-200 [~ (10 - 15) MJ/m^2, ~ 50 μs] have beenperformed. The formation of a shielding layer of lithium plasma and the high stability of thesesystems have been shown. The new lithium limiter tests on an up-graded T-11M tokamak (plasma currentup to 100 kA, pulse length ~0.3 s) have been performed. Sorption and desorption of plasma-forminggas, lithium emission into discharge, lithium erosion, deposited power of the limiter areinvestigated in these experiments. The first results of experiments are presented.

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.

W fuzz layers:very high resistance to sputtering under fusion-relevant He+irradiations

In this study,we have modeled the sputtering process of energetic He;ions colliding with W nano-fuzz materials,based on the physical processes,such as the collision and diffusion of energetic particles,sputtering and redeposition.Our modeling shows that the fuzzy nanomaterials with a large surface-to-volume ratio exhibit very high resistance to sputtering under fusion-relevant He;irradiations,and their sputtering yields are mainly determined by the thickness of fuzzy nano0materials,the reflection coefficients and mean free paths of energetic particles,surface sputtering yields of a flat base material,and the geometry of nano-fuzz.Our measurements have confirmed that the surface sputtering yield of a W nano-fuzz layer with the columnar geometry of nano-fuzz in cross-section is about one magnitude of order lower than the one of smooth W substrates.This work provides a complete model for energetic particles colliding with the nano-fuzz layer and clarifies the fundamental sputtering process occurring in the nano-fuzz layer.

Effect of helium implantation on SiC and graphite

Effects of helium implantation on silicon carbide（SiC） and graphite were studied to reveal the possibility of SiC replacing graphite as plasma facing materials. Pressureless sintered SiC and graphite SMF-800 were implanted with He＋ions of 20 ke V and 100 ke V at different temperatures and different fluences. The He^＋ irradiation induced microstructure changes were studied by field-emission scanning electron microscopy（FESEM）, atomic force microscopy（AFM）, and transmission electron microscopy（TEM）.

Crack Characteristic Measurement in Beryllium after Thermal Shock

he numbers and distribution of cracks in bulk beryllium which was loaded by intense electron beam were observed by optical image analysis, where the crack characteristic measurement and analysis were performed. The results show that: 1) The crack numbers in TR30 of beryllium with fine grain size and relatively high BeO content (no more than 3%) are about 2～4 times more than that in DShG200 of beryllium, whose grain size is larger but BeO content is less than 0.8%, under the same electron beam loading conditions, the ceramic phase of BeO in beryllium makes beryllium more brittle. 2) The successive electron pulse irradiation results in the wider cracks since the cracks formed by the former shot might be enlarged by following shots. The reason might be that there was a stress concentration at original cracks when the second shot came and so that thermal stress raised. DShG200 exhibit the better thermal shock resistance than that of TR30.

Hardening effect of multi-energyW2+-ion irradiation on tungsten–potassium alloy

Tungsten is one of the most promising plasma-facing materials (PFMs) to be used in the nuclear fusion reactor as divertor material in the future. In this work, W2+-ions bombardment is used to simulate the neutron irradiation damage to commercial pure tungsten (W) and rolled tungsten–potassium (W–K). The 7 MeV of 3 × 10^15 W2+-ions/cm2, 3 MeV of 4.5 × 10^14 W2+, and 2 MeV of 3 × 10^14 W2+-ions/cm2 are applied at 923 K in sequence to produce a uniform region of 100 nm–400 nm beneath the sample surface with the maximum damage value of 11.5 dpa. Nanoindentation is used to inspect the changes in hardness and elastic modulus after self-ion irradiation. Irradiation hardening occurred in both materials. The irradiation hardening of rolled W–K is affected by two factors: one is the absorption of vacancies and interstitial atoms by potassium bubbles, and the other is the interaction between potassium bubbles and dislocations. Under the condition of 11.5 dpa, the capability of defect absorption can reach a threshold. As a result, dislocations finally dominate the hardening of rolled W–K. Specific features of dislocation loops in W–K are further observed by transmission electron microscopy (TEM) to explain the hardening effect. This work might provide valuable enlightenment for W–K alloy as a promising plasma facing material candidate.

Theoretical Simulations of Irradiation-Induced Sputtering at Tungsten Surface

The irradiation-induced sputtering and the structural damage at tungsten surface are investigated by using molecular dynamics simulations at the level of quantum mechanics. Our simulations indicate that the sputtered atoms appear when the energy of incident primary knock-on atom （PKA） is more than 200 eV and the incident angle of the PKA is larger than 65°. Meanwhile, the irradiation-induced vacancies are less when the incident angle of PKA is in the range of 45°-65°. So, the optimum incident angles of PKA are suggested to reduce the irradiation-induced damage of the W surface. Furthermore, we find that the interstitials contained in the systems accelerate the sputtering whereas the intrinsic vacancies suppress the sputtering when the PKA is near the defects.