Convective transport in the scrape-off layer of tokamaks

Abstract

A detailed study of blob formation, dynamics, and the associated convective transport in the scrape-off layer (SOL) of tokamak plasmas is presented. Dissipation level in the system, in addition to the blob size, is shown to play an important role in determining whether a blob propagates as a coherent object. Nonlinear SOL interchange/ballooning modes are shown to be capable of creating blobs near the separatrix without relying on the core or edge-plasma dynamics. Finally, the SOL density profiles under diffusive and convective transport assumptions are compared. In the convective regime, here assumed to be driven by the SOL interchange modes, two different scaling with the machine size R is found for the characteristic density “e-folding” length λn. When the dominant loss mechanism for the blobs is diffusive, the scale length becomes independent of machine size as the connection length increases. In the less typical case where the parallel losses along the open field lines dominate, λn∼R1∕2.