Observations of hydrocarbon film deposition in the MAST tokamak

Two divertor tiles from the Mega-Ampere Spherical Tokamak (MAST) were analysed using interference fringe analysis, nuclear reaction analysis and Rutherford backscattering analysis. The analysis allowed the quantification of the co-deposited layers thickness of the deuterium and carbon mixture and the detection of the presence of other impurities such as oxygen and iron. A layer thickness varying from 10 to 270 nm was measured with a typical D/(D+C) ratio of ≈0.36. During the same campaign in which the analysed divertor tiles were exposed to the plasma, the ion flux to the divertor plates has been measured by a set of Langmuir probes mounted on the divertor ribs. This paper shows that the sputtering co-efficient that can be inferred from the ion fluences measured by the probes and by the redeposition of C in those amorphous layers is consistent with physical and chemical sputtering yields in conventional tokamaks. This paper reports the beginning and the intended developments of a project undertaken in MAST to study and quantify the phenomena of plasma surface interaction both at the walls and at the divertor of the machine in order to contribute to the material choice and design of the plasma facing components of a next step device such as ITER.