Long-term erosion of plasma-facing materials with different surface roughness in ASDEX Upgrade

The effect of surface roughness on the long-term erosion patterns of tungsten coatings was investigated in the outer strike-point region of ASDEX Upgrade during its 2010-11 plasma operations. The net erosion rates of rough coatings (R_a = 5-6 μm) were three to seven times smaller than those of smooth coatings (R_a = 0.4-0.8 μm). This is because rough surfaces are largely modified and damaged in the microscopic scale but the material is re-deposited together with boron, deuterium and carbon on the shadowed sides of the most prominent surface features. In addition, we observed that W coatings were eroded on average at a rate of 0.03 nm s^-1, which was three to four times smaller than the value for Cr, simulating here steel.