Turbulent transport and the plasma edge

In magnetically confined fusion experiments, turbulent transport dominates over collisional transport in the edge gradient region and in the scrape-off layer (SOL). Traditionally concepts as Fick’s law are used to describe turbulent transport by effective diffusion coefficients and convective velocities.

These concepts are only well founded if the transport exhibits Gaussian statistics. In the last decade it has become increasingly obvious this is not the case [E.T. Lu, Phys. Rev. Lett. 74 (1995) 2511; D. Newman, B.A. Carreras, P. Diamond, T.S. Hahm, Phys. Plasmas 3 (1996) 1858]. Intermittency, long range correlations, and ballistic transport events are widely documented in the plasma edge.

The latter are characterized by the radial propagation of coherent structures – usually referred to as blobs – carrying energy, current and particle density across magnetic field lines. This ultimately renders the description of transport by mere use of effective mean transport coefficients useless, as this does not account for the effects of frequent extreme events, which have strong, lasting, and possibly destructive influence on plasma facing components.