Steel as an option for a plasma-facing material at the main chamber wall of a fusion reactor

At present, the most promising plasma-facing material (PFM) for the main chamber wall of a fusion reactor is tungsten (W). Nevertheless, a few years ago it was suggested to consider RAFM steel in certain areas of the main chamber wall in DEMO where the particle and power flux are not prohibitive.

At the main wall of a fusion device, the wall flux is dominated by charge-exchange neutral hydrogen isotopes. The energy distribution of the impinging species is dominated by particles with energies below about 200 eV but comprises also small fractions of high energy species with energies up to several keV. Simulations show that the erosion yield for the charge-exchange neutral flux impinging on the first wall is for pure iron about one order of magnitude higher than for pure tungsten. However, RAFM steel is a composite material containing amongst other elements small amounts of W (~0.45 at%) and its sputtering behavior will be different from that  of pure iron. Due to preferential sputtering of the lighter elements W is expected to enrich at the surface during the course of operation which would lead to a reduction of the sputter yield. 

However, the existing data base on the sputtering of steel is still sparse and models with sufficient predictive capability have to be developed and benchmarked to allow extrapolation to a future reactor. The presentation will review the basic physical processed that are relevant for sputtering of steels and the currently available experimental data. Modeling results from SDTrim.SP simulations will be presented and compared to experimental data where available. Effects such as, e.g., diffusion, surface morphology development and the impact of plasma impurities and their influence on the interpretation of available experimental data will be critically discussed.