Sensitivity of WallDYN material migration modeling to uncertainties in mixed-material surface binding energies

The WallDYN package has recently been applied to a number of tokamaks to self-consistently modelthe evolution of mixed-material plasma facing surfaces. A key component of the WallDYN model is theconcentration-dependent surface sputtering rate, calculated using SDTRIM.SP. This modeled sputteringrate is strongly influenced by the surface binding energies (SBEs) of the constituent materials, whichare well known for pure elements but often are poorly constrained for mixed-materials. This work examinesthe sensitivity of WallDYN surface evolution calculations to different models for mixed-materialSBEs, focusing on the carbon/lithium/oxygen/deuterium system present in NSTX. A realistic plasma background is reconstructed from a high density, H-mode NSTX discharge, featuring an attached outer strikepoint with local density and temperature of 4e20 m^-3 and 4 eV, respectively. It is found that variousmixed-material SBE models lead to significant qualitative and quantitative changes in the surface evolutionprofile at the outer divertor, with the highest leverage parameter being the C-Li binding model.Uncertainties of order 50%, appearing on time scales relevant to tokamak experiments, highlight the importance of choosing an appropriate mixed-material sputtering representation when modeling the surfaceevolution of plasma facing components. These results are generalized to other fusion-relevant materialswith different ranges of SBEs.