Investigation of helium exhaust dynamics at the ASDEX Upgrade tokamak with full-tungsten wall*

An efficient removal of helium "ash" from burning plasmas is necessary to avoid fuel dilution and degradation of plasma confinement. Extrapolations of helium exhaust towards reactor-grade tokamaks rely on a deep understanding of the underlying physics mechanisms. The investigation presented herein was performed at the ASDEX Upgrade (AUG) tokamak. This is an ideal test environment thanks to the optimized divertor geometry, an extensive diagnostic coverage and the presence of plasma-facing components (PFCs) made of tungsten, which is an increasingly relevant candidate for the PFCs in ITER and DEMO. To interpret the experimentally observed exhaust dynamics in H-mode plasmas we developed a novel multi-reservoir particle balance model. This simulates plasma transport and wall retention in a self-consistent way, and disentangles the contributions of active pumping and wall pumping to the observed helium behavior. We found that the limited performance of the AUG pumping system and the high retention capability of helium in the plasma-exposed tungsten surfaces quantitatively play a similar role in hindering an efficient exhaust. As such, the role of the walls as particle reservoirs for helium should be taken into account also in future devices, and emphasizes the need for efficient active pumping systems.