Open Questions and Data Needs

The review of different erosion processes reveals considerable progress, both in experimental investigation and understanding, of physical sputtering and chemical erosion of plasma facing materials. The review leads to the identification of the following open questions and data needs  

Main uncertainties remain in the treatment of compounds and intermixed materials. The main parameter governing sputtering is the surface binding energy which is insufficiently known for mixed materials. 

In the material modification due to non-recycling ions in a hydrogen plasma the mobility of the constituents will modify the surface concentrations and influence the erosion of the substrate. These effects can only be investigated in a controlled way using dual ion beam experiments. Such a system is being commissioned at IPP and results for the interaction of different plasma facing materials are expected in the near future. 

Temperature effects in physical sputtering are still under discussion. For the case of graphitic materials, it is known that displaced atoms throughout the collision cascade can diffuse to the surface and sublimate already at temperatures below 1000 K. It has been documented that similar effects exits for metals with low melting point. A model indicates a similar process as for graphite and a thorough investigation of this effects is necessary. 

In the field of chemical erosion the composition of hydrocarbons and radicals is not fully investigated, especially at the low ion energies expected in divertor application. The nature of the eroded species is coupled to their transport in the plasma and their sticking and re-erosion properties. 

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