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.
Highlights
A magnetically confined fusion plasma is surrounded by a region of open field lines intersecting on material surfaces
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
The scrape-off layer (SOL) region can be further divided into two regions, a near and a far SOL.[1]
Summary
A magnetically confined fusion plasma is surrounded by a region of open field lines intersecting on material surfaces. Bohm and gyro-Bohm transport provide diffusion coefficients not larger than some m2/s in the SOL In the following, it is shown how the diffusion coefficients would vary in the case of the high density transition assuming no modifications in the sources and sinks for the purpose of illustration. Once the blob packing fraction and the filament velocity are known, the SOL length kn can be predicted.[22] the blob packing fraction provides a means to obtain reasonable diffusion coefficients or effective convective velocities, there are at least two caveats using blob packing fractions. The telegraph equation describes nonlocal, ballistic transport on the time scale of the plasma filaments and, as desired for transport codes, it provides a diffusive transport on equilibrium time scales It is consistent with the correlation characteristics in the SOL.
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