Hydrogen permeation through disordered nanostructured tungsten films

We report results on long-term hydrogen outgassing and permeation studies of structurally highly disordered tungsten films, deposited on 40 mm diameter highly permeable Eurofer substrates, using the Pulsed Laser Deposition technique. Hydrogen interaction with tungsten is becoming a highly relevant topic since tungsten was recognized as the most promising candidate for the first wall of future fusion reactors. Prediction of hydrogen isotopes migration and their abundance after plasma exposure is uncertain due to a great role played by structural disorder that is formed on the W surface. Vacancy sites are theoretically predicted to trap multiple H atoms exothermically, but their density and their potential influence on permeability has not been experimentally investigated yet. In our work, permeability of W films having different thicknesses (1 and 10 mu m) was initially extremely low, and was gradually increasing over a several-day campaign. The final values at 400 degrees C, lying between P=1.46 x 10(-15) mol H-2/(m s Pa-0.5) and P=4.8 x 10(-15) mol H-2/(m s Pa-0.5), were substantially lower than those known for well ordered films. Surprisingly, the 10 mu m thick W film initially contained a very high amount of hydrogen, similar to 0.1 H/W, which was gradually releasing during the twenty-day campaign. (C) 2012 Elsevier B.V. All rights reserved.