Improved erosion estimates for the STEP divertor

• Improved erosion estimates for the STEP divertor. • Optimised plasma background for inner divertor: small effect on resulting erosion/deposition for case studied. • Boltzmann decay of n e in sheath: moderate effect on outer target erosion/deposition, minor effect at inner target. • Tungsten background flux can significantly change net erosion, depending on influx even change to net deposition. Net tungsten erosion and deposition profiles are simulated for the outer and inner lower divertor of STEP, a spherical prototype fusion reactor targeting ∼ 2040. In this contribution, previous modelling work [1] has been extended by studying the influence of various input parameters. Main aim is to analyse the influence of several modelling improvements on the erosion/deposition characteristics at the divertor targets. This comprises the consideration of an electron density decay according to the Boltzmann relation within the sheath region as well as a certain tungsten influx to the divertor originating from main wall erosion. Also, for the inner divertor improved background plasma parameters are applied taking into account a gradient along the flux surfaces compared to formerly constant electron temperature and density and ion temperature along the flux surfaces. ERO simulations have been performed for one selected plasma case with Ar seeding, both for the outer (with peak target T e ∼ 25 eV) and inner lower divertor (with peak target T e ∼ 3 eV). The simulated tungsten erosion and redeposition profiles do not significantly change for the cases studied if the Boltzmann-related decay of electron density within the sheath is considered. The assumption of tungsten within the background plasma can notably alter the overall erosion/redeposition behaviour at a background tungsten concentration of 1/10 with respect to the Ar divertor concentration. The inner divertor target plate shows net deposition everywhere, while at the outer one the net erosion zone becomes smaller and the maximum net erosion peak reduces by a factor of two. Lastly, the improved background plasma for the inner divertor has moderate effects, for instance the overall tungsten redeposition at the target plate increases from 88.7% to 94.4%.