Intermittent transport in edge plasmas

The properties of low-frequency convective fluctuations and transport are investigated for the boundary region of magnetized plasmas. We employ a two-dimensional fluid model for the evolution of the global plasma quantities in a geometry and with parameters relevant to the scrape-off layer of confined toroidal plasmas. Strongly intermittent plasma transport is regulated by self-consistently generated sheared poloidal flows and is mediated by bursty ejection of particles and heat from the bulk plasma in the form of blobs. Coarse grained probe signals reveal a highly skewed and flat distribution on short time scales, but tends towards a normal distribution at large time scales. Conditionally averaged signals are in perfect agreement with experimental measurements. It is well established that the cross field transport of particles and heat near the edge of magnetically confined plasmas is strongly intermittent. This is observed in a variety of devices including linear [1, 2] as well as toroidal configurations [3, 4]. Detailed investigations of the spatial fluctuation structure have revealed strong indications that the intermittent nature of particle and heat transport is caused by localized structures in the form of plasma “blobs” propagating radially far into the scrape-off-layer (SOL) of toroidal plasmas [2, 5, 6]. It was suggested that this is caused by a dipolar vorticity field formed by the charge separation in a density blob due to guiding-center drifts in a curved inhomogeneous magnetic field [7]. In this contribution we will provide a self-consistent description of the intermittent particle and heat flux and link it to the of the emergence and evolution of such blob-like structures. We base our investigations on a novel model for interchange turbulence in slab geometry for the outboard midplane of a toroidal device [8]. The model includes the self-consistent evolution of the full profiles in the edge/SOL. A local Boussinesq-like model, where the “background” profile is separated from the fluctuations, fails to provide a realistic description. The geometry comprises distinct production and loss regions, corresponding to the edge and SOL of magnetized plasmas. The separation of these two regions defines an effective last closed flux surface (LCFS), though we do not include magnetic shear in our model. In the edge region, strong pressure gradients maintain a state of turbulent convection. A self-regulation mechanism involving differential rotation leads to a repetitive expulsion of hot plasma into the SOL, resulting in a strongly intermittent transport of density and heat.