Hydrogen Retention in a Tungsten Re-Deposition Layer Formed by Hydrogen RF Plasma

Tungsten is a candidate material for plasma facing components for a fusion reactor. Although many studies on hydrogen behavior in tungsten have been carried out, there is insufficient database for a tungsten re-deposition layer. We have made a tungsten re-deposition layer by a sputtering method using a hydrogen and deuterium RF plasma and have investigated hydrogen retention in the layer and the distribution of the layer in the vacuum chamber. The amount of deposited tungsten increased 2.4 times with varying RF power from 100 W to 250 W. It was found from the SEM observation on the cross section that the formed layer has a columnar structure. At high energy (RF power: 250W), a lot of blisters were observed on the surface. The ratio of hydrogen atoms to tungsten atoms (H/W) in the layer was observed to be 0.1 ~ 0.4 with varying RF power. These values of hydrogen retention were much larger than that for absorption into tungsten. Tritium inventory in a D-T fusion reactor may become larger than expected by the formation of tungsten redeposition layer.