Gross and net erosion balance of plasma-facing components in full-W tokamaks

Gross and net erosion of W and other plasma-facing materials have been investigated in the divertor region in D and He plasmas during dedicated experiments on ASDEX Upgrade in L- and H-mode and after full-length experimental campaigns on WEST. Net erosion was determined via post exposure analyses of plasma-exposed samples and components, and we conclude that the same approach is applicable to gross erosion if marker structures with sub-millimeter dimensions are used to eliminate the contribution of prompt re-deposition. In H-mode plasmas, gross erosion during ELMs may exceed the situation in inter-ELM conditions by 1-2 orders of magnitude while net erosion is typically higher by a factor of 2-3. The largest impact on net erosion comes from the electron temperature while the role of impurities is much weaker. Impurities, on the other hand, will lead to the formation of thick co-deposited layers. We have also noted that the rougher the surface, the more will net erosion be suppressed and the thicker the co-deposits will be. In He plasmas, gross erosion is increased compared to the situation in D due to the higher mass and charge states of the plasma particles, but strong impurity fluxes can easily result in apparent net deposition in the divertor. Our results from ASDEX Upgrade and WEST are comparable and indicate that the net-erosion rates are typically <0.1 nm/s excluding the vicinity of the strike-point regions.