Effect of oxygen on hydrogen retention in W deposition layers formed by hydrogen RF plasma

In the vessel of a fusion reactor, co-deposition layers are formed by plasma-wall interactions. It has been reported by the present authors that a large amount of hydrogen can be trapped in tungsten deposition layers formed by hydrogen plasma sputtering depending on the experimental condition. In the present study, the influence of oxygen on hydrogen behavior in tungsten deposition layers was investigated. Overall, hydrogen retention increased with increasing oxygen concentration. The tungsten deposition layer containing oxygen of 1.69 as O/W retained hydrogen of 0.70 as H/W, which is larger than the value of H/W (around 0.1) in the deposition layer containing no oxygen. It was observed that some amounts of hydrogen were released as water vapor from the deposition layer in which a large amount of oxygen was detected. A tungsten deposition layer containing a large amount of oxygen was spontaneously flaked off after plasma was turned off.