Deuterium plasma exposure of thin oxide films on tungsten-oxygen removal and deuterium uptake

Recently we have shown that thin (33 to 55 nm) oxide films on tungsten (W) prevent deuterium (D) uptake from a low-temperature plasma (5 eV/D; 370 K; 1.4 x 1024 D/m2) into metallic W. Here we continue this investigation with higher D fluence up to 2.3 x 1025 D/m2 and, additionally, with higher D energies up to 38 eV/D or a higher sample temperature of 500 K. We investigate the reduction mechanism of the oxide and determine under which conditions the permeation barrier effect for D breaks down. We show that during prolonged plasma exposure at 5 eV/D and 370 K, the W-enriched zone that forms at the surface of the oxide stops growing and continues to protect the oxide beneath it against further reduction. For a fluence of 1.6 x 1025 D/m2, an originally 55 nm thick oxide film reduces D uptake into the metal by 99 %. D enters the metal solely by small cracks in the oxide that form during reduction by the plasma. For higher temperature or D energies, the reduction becomes stronger and more inhomogeneous. D uptake into the metal increases proportional to the area of oxide/metal interface on which the oxide is reduced. The new experiments confirm our previous assumption that D uptake into W is blocked due to the difference in enthalpy of solution of D in W oxide and metallic W. D can only enter the metal once the oxide at the interface is sufficiently reduced. Based on the obtained oxide reduction rates, we made a first tentative estimate of the relevance of naturally occurring W oxide films for D uptake in the plasma-facing components of a fusion reactor. We found that the natural oxide will have none or only a very minor effect in this case.