Speaker
Description
Fuel and impurity density control is essential to ensure reliable tokamak operations. This requires a deep understanding of the physical mechanisms determining the transport of particles in the plasma edge and the pumping of these in form of neutrals.
Divertor compression is the main physics-based figure of merit characterizing particle exhaust efficiency. High-fidelity transport models succeed in reproducing the experimental compression of the main plasma species, but underestimate impurity compression, at least in H-modes (A.Zito,NF2025). Reduced models suggest that a possible reason for this mismatch is the absence, in high-fidelity models, of MHD-related phenomena, such as edge localized modes (ELMs). ELMs are indeed regarded as an efficient mechanism for flushing plasma particles from the core, especially impurities, thereby increasing their transport towards the divertor (T.Pütterich,JNM2011).
The impact of ELMs on particle exhaust has now been quantitatively assessed by performing accurate, time-dependent SOLPS-ITER simulations of a type-I ELM cycle on an ASDEX Upgrade discharge, contrasted with high-time-resolution measurements across the plasma edge/SOL and divertor (M.Cavedon,PPCF2017), including He, Ne and Ar as trace impurities. To this aim, new code capabilities have been implemented to allow more sophisticated time-varying transport assumptions, to better resolve the fast ELM-related transients.
The periodic collapse of the pedestal has been mimicked through a modulated increase of edge radial transport coefficients, to emulate the sudden loss of confinement and consequent explosive ejection of particles and energy from the core. Additionally, a periodically increased anomalous parallel flow velocity in the SOL is used to emulate the free-streaming motion of the hot core-ejected filaments (D.Moulton,PPCF2013).
As a result, it is found that the ELM-averaged divertor compression of the main species is weakly affected, but that of impurity species is significantly increased w.r.t. comparable simulations but without ELMs, with effect scaling with the impurity charge. The impact of ELM bursts on the subdivertor gas density depends on a complicated interplay between pedestal collapse, setting the relative particle loss per species from the core, and increase in temperature of the divertor, affecting the leakage of recycled particles. For impurities, whose H-mode inter-ELM edge transport follows the neoclassical predictions, a higher pedestal, also scaling with the impurity charge, results in a higher relative loss of particles from the core once this collapses, explaining the numerical observations. The achieved results highlight the importance of properly including transient events when performing predictive simulations in view of future devices, especially when concerning impurity transport.