Recent progress towards a quantitative description of filamentary SOL transport

Abstract

A summary of recent results on filamentary transport, mostly obtained in the\nASDEX-Upgrade tokamak (AUG), is presented and discussed in an attempt to\nproduce a coherent picture of SOL filamentary transport: A clear correlation is\nfound between L-mode density shoulder formation in the outer midplane and a\ntransition between the sheath limited and the inertial filamentary regimes.\nDivertor collisionality is found to be the parameter triggering the transition.\nA clear reduction of the ion temperature takes place in the far SOL after the\ntransition, both for the background and the filaments. This coincides with a\nstrong variation of the ion temperature distribution, which deviates from\nGaussianity and becomes dominated by a strong peak below $5$ eV. The filament\ntransition mechanism triggered by a critical value of collisionality seems to\nbe generally applicable to inter-ELM H-mode plasmas, although a secondary\nthreshold related to deuterium fueling is observed. EMC3-EIRENE simulations of\nneutral dynamics show that an ionization front near the main chamber wall is\nformed after the shoulder formation. Finally, a clear increase of SOL opacity\nto neutrals is observed associated to the shoulder formation. A common SOL\ntransport framework is proposed account for all these results, and their\npotential implications for future generation devices are discussed.\n