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

TL;DR

This study models tungsten erosion and helium transport in the scrape-off layer of L-mode plasmas in ASDEX Upgrade, combining analytical, fluid, and kinetic approaches to understand impurity behavior and material erosion.

Contribution

It introduces a comprehensive comparison of multi-fluid and kinetic models for tungsten erosion and migration in helium plasmas, integrating experimental data and advanced simulations.

Findings

He ion fractions vary near strike-points and in the SOL.

He2+ significantly impacts tungsten erosion at typical plasma temperatures.

W transport differs between models, with ERO2.0 showing more inward migration.

Abstract

Due to its unavoidable presence in thermonuclear DT plasmas and to its peculiar effects on materials, investigating 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 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 (SOL). The role of He ion fraction on AUG tungsten divertor erosion is first estimated by means of a simple analytical model and, afterwards, by exploiting ERO2.0, showing the major impact of He2+ in common AUG plasma temperatures. ERO2.0 findings are also compared with experimental erosion data in the strike-point region, showing the possible impact of extrinsic impurities on divertor erosion. Finally, the multi-fluid and kinetic approaches employed in this work to simulate W erosion and migration are compared, including W also in SOLPS-ITER modelling. The impact of target boundary conditions on the W source in SOLPS-ITER is investigated, in order to find a good agreement with ERO2.0 estimation. Then, W migration in the two codes is compared, showing a stronger W transport towards the X-point in ERO2.0 compared to present SOLPS-ITER simulations with no drifts. Similar W influx in core could be achieved by reducing anomalous diffusivity in ERO2.0.