The diffusion limit of ballistic transport in the scrape-off layer

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 u b 2 τ. Since ub and τ can be determined experimentally, it is proposed to use u b 2 τ as an input for modeling or for interpretation of perpendicular transport in the far scrape-off layer.