The diffusion limit of ballistic transport in the scrape-off layer
P. Mänz, C. Hufnagel, A. Zito, D. Carralero, M. Griener, T. Lunt, O. Pan, M. Passoni, B. Tál, M. Wischmeier, E. Wolfrum, ASDEX Upgrade Team
Abstract
At least the far scrape-off layer of magnetically confined fusion plasmas transport is intermittent and non-diffusive as observed by the appearance of plasma filaments. Transport codes using effective diffusion coefficients are still the main workhorse investigating the scrape-off layer and divertor regions. An effective perpendicular diffusion coefficient for intermittent filamentary dominated perpendicular transport in the scrape-off layer is motivated by the telegraph equation, describing an exponentially decaying correlated random walk. On short time scales, the telegraph equation describes the ballistic transport of filamentary structures with a typical velocity ub and correlation time τ. In stationary conditions, the corresponding diffusion coefficient is given by ub2τ. Since ub and τ can be determined experimentally, it is proposed to use ub2τ as an input for modeling or for interpretation of perpendicular transport in the far scrape-off layer.