Characterization of first wall materials in RFX-mod

Plasma density control during experiments is an issue in all the devices where material components face to the plasma and suffer non-negligible thermal loads. In such cases, the particle influxes are found to be proportional to the power load, but the retention properties of the material and its surface temperature under load play a major role. At RFX-mod, a Reversed Field Pinch (RFP) experiment where 0.5s deuterium discharges with plasma current up to 2MA and typical n/nG in the range 0.2-0.4 are performed, the replacement of the actual full graphite wall is under design with the aim of mitigating the effect of the plasma wall interaction, reducing the deuterium and impurities release during discharges. The behaviour of various materials under load has been investigated by calculations based on the model of Pitcher [1], highlighting the importance of the different thermal properties of the wall material in determining the release of particles. A selection of materials has been then exposed to the RFX-mod plasma and tested by power loads up to tens of MW/m2, in order to assess their compatibility with machine operation. During experiments, the materials were inspected by visible cameras and their surface temperature was monitored by IR cameras. The microscopic analysis of the retrieved samples allowed to determine some technological limitations of the materials and to discuss the solutions that can be adopted to overcome them.