Plasma exposed surfaces

• Wikipedia links
• Beryllium
• Secondary Ion Mass Spectroscopy

Plasma exposed surfaces are investigated to provide data for understanding and modelling the impurity transport in the plasma edge region (Scrape-Off Layer, SOL), and the material erosion and deposition processes inside the vessel.

The interaction of plasma with the CFC plasma facing tiles is the major source of free carbon in the plasma, while Beryllium Evaporators, used periodically mainly to reduce the amount of oxygen impurities in the plasma and improve plasma conditions, represent the primary source of beryllium in JET. Carbon and beryllium are transported towards the upper tiles of the inner divertor (tiles 1 and 3 in the figure) where beryllium is stacked and carbon, after deposition, is re-eroded through chemical sputtering and transported towards the inner flat tiles (tile 4).

Cross-section of the JET divertor tile set used in 1997-2001

Laser ablation tests have been performed on a CFC plasma-facing component from the TEXTOR tokamak (Jülich, Germany). Both 1-time and 10-fold scanning fully removed the deposited layers without damaging the graphite substrate.

The divertor tiles exposed in JET in the 1998-2001 campaigns have been used to assess the amount of beryllium and carbon deposited at the plasma facing materials. Secondary Ion Mass Spectroscopy (SIMS) depth profiling has been made from a number of samples on inner divertor tiles 1, 3 and 4.

The deposit forms two layers on tiles 1 and 3. The outer layers (~2-6 µm thick on tile 1 and 10-16 µm on tile 3) contain mostly carbon together with deuterium and a smaller amount of beryllium. The films underneath the surface layer are very rich in beryllium (~2-14 µm on tile 1 & 12-21 µm on tile 3). The measurements allowed the estimation of the amount of beryllium on the tiles 1 and 3 and thus the calculation of the total amount of beryllium deposited at the inner divertor: 22 ± 9 g. Unlike tiles 1 and 3, very little beryllium was found in the ~85 μm thick film on tile 4 in the shadowed region, where almost only carbon, with very high deuterium content, and a well-marked interface to the carbon fibre composite substrate has been observed. Similar investigations have been carried out for the tiles of the outer divertor (6, 7 and 8) and, in general, the deposition patterns of fuel atoms, beryllium and carbon showed much less heavy deposition and fuel accumulation in the outer divertor than in the inner. This was not expected from classical modelling of erosion/deposition. The asymmetry in the JET deposition pattern could be explained by increased carbon erosion by the plasma in the main chamber and sputtering at the inner divertor surfaces.