Abstract:Growth of tungsten nano-tendrils ("fuzz") has been observed for the first time in the divertor region of a high-power density tokamak experiment. After 14 consecutive helium L-mode discharges in Alcator C-Mod, the tip of a tungsten Langmuir probe at the outer strike point was fully covered with a layer of nano-tendrils. The depth of the W fuzz layer (600 ± 150 nm) is consistent with an empirical growth formula from the PISCES experiment. Re-creating the C-Mod exposures as closely as possible in Pilot-PSI experiment can produce nearly-identical nano-tendril morphology and layer thickness at surface temperatures that agree with uncertainties with the C-Mod W probe temperature data. Helium concentrations in W fuzz layers are measured at 1-4 at.%, which is lower than expected for the observed sub-surface voids to be filled with several GPa of helium pressure. This possibly indicates that the void formation is not pressure driven.

Nanotendril “fuzz” will grow under He bombardment under tokamak-relevant conditions on tungsten plasma-facing materials in a magnetic fusion energy device. We have grown tungsten nanotendrils at low (50 eV) and high (12 keV) He bombardment energy, in the range 900–1000 °C, and characterized them using electron microscopy. Low energy tendrils are finer (~22 nm diameter) than high-energy tendrils (~176 nm diameter), and low-energy tendrils have a smoother surface than high-energy tendrils. Cavities were omnipresent and typically ~5–10 nm in size. Oxygen was present at tendril surfaces, but tendrils were all BCC tungsten metal. Electron diffraction measured tendril growth axes and grain boundary angle/axis pairs; no preferential growth axes or angle/axis pairs were observed, and low-energy fuzz grain boundaries tended to be high angle; high energy tendril grain boundaries were not observed. We speculate that the strong tendency to high-angle grain boundaries in the low-energy tendrils implies that as the tendrils twist or bend, strain must accumulate until nucleation of a grain boundary is favorable compared to further lattice rotation. The high-energy tendrils consisted of very large (>100 nm) grains compared to the tendril size, so the nature of the high energy irradiation must enable faster growth with less lattice rotation.

Measurements are conducted to identify the motion of tungsten and helium atoms during the formation of tungsten fuzz. In a first series of experiments the mobility of helium within the growing fuzz was measured by adding 3 He to the different stages of plasma exposure under conditions that promoted tungsten fuzz growth. Ion beam analysis was used to quantify the amount of 3 He remaining in the samples following the plasma exposure. The results indicate that the retention of helium in bubbles within tungsten is a dynamic process with direct implantation rather than diffusion into the bubbles, best describing the motion of the helium atoms. In the second experiment, an isotopically enriched layer of tungsten (~92.99% 182 W) is deposited on the surface of a bulk tungsten sample with the natural abundance of the isotopes. This sample is then exposed to helium plasma at the conditions necessary to support the formation of tungsten 'fuzz'. Depth profiles of the concentration of each of the tungsten isotopes are obtained using secondary ion mass spectrometry (SIMS) before and after the plasma exposure. The depth profiles clearly show mixing of tungsten atoms from the bulk sample toward the surface of the fuzz. This supports a physical picture of the dynamic behavior of helium bubbles which, also, causes an enhanced mixing of tungsten atoms.

Tungsten targets are exposed to controlled sequences of D 2 and He, and He and D 2 plasma in the Pisces-A linear plasma device, with a view to studying the outward and inward transport of D across a He implanted surface, using thermal desorption mass spectrometry. Differences in transport are interpreted from changes in peak desorption temperature and amplitude for D 2 release, compared against that of control targets exposed to just D 2 plasma. Desorption data are modeled with Tmap-7 to infer the nature by which He leads to the 'reduced inventory' effect for H isotope uptake. A dual segment (surface-30 nm, bulk) W Tmap-7 model is developed, that simulates both plasma exposure and thermal desorption. Good agreement between desorption data and model is found for D 2 release from control targets provided that the implanted flux is reduced, similar to that reported by others. For He affected release, the H isotope transport properties of the surface segment are adjusted away from control target bulk values during the computation. Modeling that examines outward D transport through the He implanted layer suggests that a permeation barrier is active, but bubble induced porosity is insufficient to fully explain the barrier strength. Moderately increased diffusional migration energy in the model over the He affected region, however, gives a barrier strength consistent with experiment. The same model, applied to inward transport, predicts the reduced inventory effect, but a further reduction in the implanted D flux is necessary for precise agreement.

FeW binary mixed layers were prepared as a model system for reduced-activation ferriticmartesitic (RAFM) steel for studying their dynamic erosion behaviour resulting from energetic deuterium (D) irradiation. This investigation aims towards an assessment of RAFM steels as plasmafacing material. The surface composition of the model layers is modified by D irradiation. W is enriched at the surface due to the preferential sputtering of Fe. The W surface concentration increases with D fluence. It depends on the D impinging energy as well as the initial W fraction of the FeW layer. No significant development of surface topography was observed within the examined conditions. The erosion yield of a FeW layer is comparable to that of pure Fe in the low-fluence range and decreases with increasing D fluence. These results indicate that the dynamic change of erosion yield is significantly correlated with the surface W enrichment.