Tungsten erosion and scrape-off layer transport modelling in L-mode helium plasma discharges in ASDEX Upgrade

Due to its unavoidable presence in thermonuclear DT plasmas and its peculiar effects on materials, investigation of the role of helium (He) in plasma–wall interaction (PWI) in current tokamaks is fundamental. In this work, PWI in L-mode He plasma discharges in ASDEX Upgrade (AUG) is modelled by exploiting simplified analytical approaches and two state-of-the-art codes. SOLPS-ITER is employed both to provide a suitable background plasma for erosion simulations and to interpret diagnostics measurements in terms of the He+/2+ fraction. In particular, a 50%:50% concentration of the two He ions is found in the proximity of the strike points, while He2+ represents the dominant population farther in the scrape-off layer. The role of He ion fraction on AUG tungsten (W) divertor erosion is first estimated by means of a simple analytical model, and afterwards by exploiting ERO2.0, showing the major impact of He2+ at common AUG plasma temperatures. The potential influence of extrinsic impurities on divertor erosion is inferred from a preliminary comparison of ERO2.0 simulation results with experimental erosion measurements in the strike-point region. A comparison between the multi-fluid and kinetic approaches for simulating W erosion and migration reveals significant discrepancies in the predicted W source and transport. In particular, the routinely adopted perfect entrainment assumption in SOLPS-ITER, i.e. setting the same velocity at the entrance of the magnetic pre-sheath for both main ions and impurities, is found to substantially overestimate the W source compared with ERO2.0 results. Moreover, ERO2.0 predicts stronger transport of W towards the X-point than current SOLPS-ITER simulations without drifts. Comparable W influx into the core can, however, be reproduced in ERO2.0 by reducing the anomalous diffusivity.