SOLPS-ITER modelling of helium transport, recycling and pumping at the ASDEX Upgrade tokamak

Extrapolating the observed behavior of helium exhaust in current tokamaks towards future reactors requires the understanding of the underlying physical mechanisms determining helium transport, recycling and pumping. Helium compression is the main physics-based figure of merit characterizing how efficiently helium is transported towards the divertor and recycled at the target plates. Moreover, helium gas transport in the subdivertor region towards the pumps is strongly influenced by vessel geometry and installed pumps. The SOLPS-ITER code package is used to model H-mode He-seeded deuterium plasmas at the ASDEX Upgrade tokamak, and compared to recent experiments. The simulations generally indicate a poor recycling of helium in the divertor, compared to that of deuterium, in qualitative agreement with the experiment. This is mainly determined by a deeper edge transport barrier and a weaker parallel SOL transport of He ions, with respect to D ions, and by the higher first ionization energy of He atoms, which results in a deeper penetration of recycled atoms into the plasma. The simulated He compression is, however, much smaller than the experimentally measured one, despite the introduction of additional, non-default physics components into the code. Helium gas transport in the subdivertor region towards the pumps is conductance-limited, but moderately enhanced by the entrainment of He atoms into the stronger, viscous deuterium gas flow via friction. The observed poor helium recycling poses challenges in view of the requirements of helium exhaust in future reactors. Our results emphasize the need to investigate further strategies to optimize helium pumping, to guarantee an efficient removal of helium ash in future burning plasmas. Additionally, the observed difficulty of SOLPS-ITER in reproducing the experimental observations suggests a careful evaluation of the currently available extrapolations of impurity transport towards future devices obtained via edge transport modelling.