Damage and deuterium retention of re-solidified tungsten following vertical displacement event-like heat load

AbstractSurface morphology and hydrogen isotope retention of W specimen melted with vertical displacement event-like heat load and subsequent deuterium (D) plasma exposure were studied. Applied heat loads using electron beam without raster scanning were about 190 and 230 MW/m2 in heat flux and 0.08, 0.12 and 0.16s in duration. After the heat load application, specimens showed apparent melting spots with grain growth or dense micrometer scale convex structure. Cracks were observed only in the part with the convex structure. D retention in the melted part of specimens was not significantly larger than in the reference specimen despite large changes of surface characteristics

First principle calculations of energy of agglomerated helium in the period 6 elements

Difference of helium (He) agglomeration energies between period 6 elements, tantalum (Ta), tungsten (W), iridium (Ir) and gold (Au), is illustrated by using first principles calculations based on density functional theory (DFT). It is found that He in W and Ir can agglomerate more easily than Ta and Au. We investigate a relationship between the He agglomeration tendency and the growth of nanostructure by He plasma irradiation. Thus, the four metals are exposed to He plasma irradiation. Each metal has different structures after the He plasma exposure. Surface nanostructures of W and Ir are fuzzy fiber-like while these structures are not observed in Ta and Au. In the meantime, W and Ir have a tendency to agglomerate He atoms at a vacancy or interstitial sites easily. This correlation suggests that the He agglomeration may play a role for understanding the fuzz formation mechanism. Keywords: Helium plasma induced structures, Fuzz, Nanostructures, Density functional theor

Surface morphology changes of silicon carbide by helium plasma irradiation

Silicon carbide (SiC) and its composites are candidate materials for the blanket components and for the first wall in a fusion reactor. If the SiC is used without any armor materials for the first wall, it is exposed by helium (He) plasma as well as hydrogen plasma. Characteristic surface morphology changes are reported for various materials by He plasma exposure. Thus, we exposed SiC specimens to He or simultaneous deuterium (D) and He (D + He) plasma by various conditions and then observed surface morphology changes by SEM. As a result, needle-like structures and whiskers-like structures at the tip were formed in He plasma and D + He irradiation, while only needle-like structures were formed in D plasma. Therefore, it indicated that the effects of He were attributed to form whiskers-like structures. Although the structures are different among He plasma, simultaneous D + He plasma and D plasma irradiations, sputtering is considered to be a dominant process for the formation of the structure formation. However, the effects of He atoms in the structure could also be attributed to form whiskers-like structures

Surface morphology of Tungsten-F82H after high-heat flux testing using plasma-arc lamps

F82H reduced activation steel coated with vacuum plasma sprayed (VPS) tungsten is a candidate as a plasma facing material for main chamber components in future fusion reactors. Due to different coefficients of thermal expansion (CTE), significant thermal stresses are expected in these bimetallic materials. Thus, a major uncertainty in the performance of W/F82H components during the operation under high-heat fluxes is the effect of CTE mismatch. In this study, a high intensity plasma-arc lamp was used for high-heat flux cycling tests of W/F82H specimens. While no surface damage was observed for specimens tested for 100–200 cycles at a heat flux of 1.4 MW/m2 pulse when the backside surface temperature was maintained below 550 °C, significant cracking occurred at higher temperatures. A simple analytical model for bimetallic materials indicated that the stress in the VPS-W layer is likely to exceed its failure stress solely due to the bilayer thermal stress. A finite element analysis of the state of stress and deformation confirmed that a significant stress also would occur at the W surface due to the rigid-body like constraint imposed by the clamp, which can be the main cause of the cracking. Keywords: High-heat flux testing, Thermal stress, Bimetallic, Tungsten, Reduced activation stee

Effect of periodic deuterium ion irradiation on deuterium retention and blistering in Tungsten

The effect of periodic irradiation on Deuterium (D) retention and blistering in Tungsten (W) was investigated. W samples were exposed to D plasma at a fixed fluence while varying the irradiation cycle number (1-shot, 2-shots and 3-shots). Exposure energy and flux were ∼50eV and ∼1 ×1022 D m−2 s−1, respectively. Sample temperatures were 537K and 643K. At 573K, D retention and blister density decreased with increasing number of irradiation cycle. In contrast at 643K, D retention showed no dependence on number of irradiation cycle. Therefore, sample temperature during irradiation is an important parameter in comparing the results of continuous and periodic irradiation, especially in studies involving extremely-high-flux (>1024 D m−2 s−1) irradiation and fluence dependency of D retention. Keywords: Tungsten, Hydrogen isotope retention, Blistering, Divertor, TD

Study of mirror effect on scrape-off layer-divertor plasma based on a generalized fluid model incorporating ion temperature anisotropy

The mirror effect on scrape‐off layer (SOL)‐divertor plasma profiles was studied by relaxing the assumption of homogeneous magnetic fields used in a one‐dimensional (1D) plasma fluid model, which incorporates the anisotropic ion temperature. For collisionless plasmas, the perpendicular ion temperature proportional to the magnetic field strength is observed, which reflects the conservation of the magnetic moment. In diverging field cases, the plasma flow becomes supersonic, while it is kept subsonic in contracting field cases. Parallel momentum gains/losses and energy transfers between parallel and perpendicular components caused by the mirror effect are also observed. In addition, it is indicated that the viscous flux approximation is invalid in inhomogeneous magnetic fields regardless of the plasma collisionality

Observation and particle simulation of vaporized W, Mo, and Be in PISCES-B plasma for vapor-shielding studies

Interactions of Tungsten (W), Molybdenum (Mo), and Beryllium (Be) vapors with a steady-state plasma were studied by the PISCES-B liner plasma experiments as well as Particle-In-Cell (PIC) simulations for the understanding of vapor-shielding phenomena. Effective cooling of the plasma by laser-generated Be vapor was observed in PISCES-B. On the other hand, no apparent cooling was observed for W and Mo vapors. The PIC simulation explains these experimental observations of the difference between low-Z and high-Z vapors. Decrease of electron temperature due to the vapor ejection was observed in case of a simulation of the Be vapor. As for the W vapor, it was found that the plasma cooling is localized only near the wall at a higher electron density plasma (∼1019m−3). On the other hand, the appreciable plasma cooling can be observed in a lower density plasma (∼1018m−3) for the W vapor. Keywords: Vapor shielding, Plasma-vapor interaction, Particle-in-cell simulatio