Formation of graphite re-deposition layer by hydrogen RF plasma

Abstract

It has been revealed that a graphite re-deposition layer is formed in a fusion plasma device and can trap fuel atoms when a carbon-based material is applied as a plasma facing component. However, there are not sufficient data for quantitative prediction of hydrogen and carbon behavior with the formation of the re-deposition layer under several plasma conditions. We produced a graphite re-deposition layer on quartz substrates by a sputtering method using hydrogen RF plasma. The re-deposition layer was approximately uniformly formed on the substrate located on the ground electrode, but in some experimental conditions no deposition was observed there. This is due to the effect of re-sputtering by the incident hydrogen ions. It was found that the layer formed on and around the ground electrode had a dense columnar structure and a packed fibrous structure, respectively. With varying RF power or hydrogen pressure, the H/C ratio changed between approximately 0.1 and 0.45, and is considerably larger than that for absorption into graphite bulk. This indicates that the formation of graphite re-deposition layer makes tritium inventory in a D-T fusion reactor to dramatically grow. The plasma parameters in each experimental condition were investigated by the Langmuir probe method.