Erosion enhancement by impurity entrainment in the highly collisional plasmas of Magnum-PSI

Abstract

Extrinsic impurity seeding is planned for ITER to avoid high heat fluxes to the tungsten divertor targets. Nevertheless, excessive sputtering by impurities would lead to a reduction in fusion power output and divertor lifetime. Unlike today's fusion devices, the entrainment of impurities is expected to play an increasing role in ITER's highly collisional near-surface plasma. Impurity entrainment refers to the acceleration of impurities with the plasma flow by collisional drag, leading to elevated impact energies. To investigate impurity entrainment under ITER-like plasma conditions ($n_e > 2 \cdot 10^{20}$ m$^{-3}$, $T_e <$ 5 eV), argon-seeded hydrogen plasmas were formed with the linear plasma generator Magnum-PSI. Doppler shifts in spatially-resolved emission profiles of argon (Ar II - 480.6 nm) and hydrogen (H$_\mathrm{\beta}$ - 486.1 nm) reveal the entrainment of seeded argon towards the upstream hydrogen velocity ($\sim$9 cm from surface), corresponding to $\sim$0.39 of the common-system sound speed. In addition, the sputtering yield of tungsten by argon bombardment for the argon-seeded hydrogen plasma was compared to a pure argon plasma by monitoring tungsten (W I - 400.9 nm) emission. These sputtering experiments indicate further entrainment of the argon impurities to $\sim$0.65 of the common-system sound speed at the sheath edge, leading to a large increase in their impact energy. Extrapolation of the Magnum-PSI results to ITER, using existing SOLPS-ITER simulations to determine the divertor plasma conditions, indicates more than an order of magnitude increase in gross erosion of the divertor targets due to impurity entrainment. However, the net erosion rate could still be kept under control if high re-deposition rates are achieved, which is treated in the companion paper \cite{Cornelissen2023}. The combination of a low sputtering threshold energy and high impact energy of impurities constrains the fine balance between radiative cooling with impurity seeding and avoiding extensive divertor erosion.