Description
High heat loads on the plasma-facing components of tokamak divertors place strict constraints on the performance of future fusion reactors [1]. In 2025, ASDEX Upgrade (AUG) carried out the first campaign with its upgraded upper divertor [2], designed for high power operation (Psep/R > 10) in alternative divertor configurations (ADCs), e.g. the X-divertor, snowflake divertor, and compact radiative divertor (CRD), with a full tungsten wall. A broad range of plasma parameters and comprehensive diagnostic coverage have provided a valuable experimental dataset for comparisons between modelling and experiments in ADCs.
SOLPS-ITER modelling contributes to interpreting the power exhaust capability of ADCs by accounting for the combined effects of geometry, cross-field transport, impurity radiation, and neutral particle behaviour. In the X-divertor, the simulation reproduces the temperature drop, observed by divertor Thomson scattering at the upstream side of the large flux expansion region relative to Langmuir probe measurements at the target. Comparisons of simulations with and without drifts highlight the critical role of cross-field transport in interpreting geometrical effects; neglecting drifts may lead to an overestimation of divertor plasma cooling and the neutral plugging effect. A similar observation has also been reported from TCV [3]. In the lowfield-side snowflake-minus divertor, drift-driven convective fluxes are found to be essential for explaining the density increase near the secondary X-point and the power distribution between the near- and far-SOL in experiments. The SOL dissipation capability of ADCs is investigated in terms of the maximum achievable radiation fraction in the SOL, the influence of divertor configuration on the X-point radiator access threshold, and power and momentum removal channels through cross-field transport. Additional experimental observations, such as density increases in the far SOL and persistent 3D effects, remain subjects for future studies using advanced modelling tools, including turbulence/filament dynamics and fully 3D geometries.
[1] M. Wischmeier et al., J. Nucl. Mater. 463, 22 (2015). [2] T. Lunt et al., Nucl. Mat. Energy 12, 1037 (2017). [3] M. Carpita, et al., AAPPS-DPP2025, Fukuoka, Japan.