We report observations from the DIII-D tokamak indicating that boron (B) powder injection in tokamak plasmas improves wall conditions similarly to glow discharge boronization (GDB). Isotopically enriched B powder (B11 > 95%) was introduced gravitationally in a sequence of H-mode plasma discharges at rates up to ∼160 mg s−1 for durations up to 3 s. Boron injection to cumulative amounts ≤0.1 g appeared to improve wall conditions similarly to boronization, with indications of reduced wall fueling, reduced recycling at the outer strike point and reduced impurity content at breakdown. Post-mortem analysis of graphite samples exposed to far scrape-off layer plasma fluxes during boron injection confirm the formation of a B-C layer, with average surface composition B:C ∼ 1. The results suggest that injecting boron-rich powders in tokamak plasmas can effectively replenish boron films on carbon plasma facing components to improve wall conditions and extend the duration of the beneficial effects of GDB.

No abstract

No abstract

Nanocrystalline tungsten oxide thin films (about 75 nm in thickness) produced by thermal oxidation of tungsten substrates were exposed to low energy He plasma (≈ 20 eV/He) with a flux of 2.5×10 18 m-2 s-1 at two temperatures: room temperature and 673 K. The structure and morphology modifications which occur after this He bombardment and annealing treatments was studied using Raman spectroscopy and transmission electron microscopy. Due to the low fluence (4×10 21 m-2) and low ion energy, we have observed only few morphology modifications after He plasma exposure at room temperature. On the contrary, at 673 K, a change in the layer color is observed associated to an important erosion. Detailed analyses before/after exposure and before/after annealing allow us to describe the He interaction with the oxide layer, its erosion and structural modification at the atomic and micrometer scale.