Blob stability and transport in the scrape-off-layer

High-density plasma filaments extended along the magnetic field, which look like "blobs" in the plane perpendicular to B, have been observed in the scrape-off-layer (SOL) of many plasma devices. These objects become polarized and subsequently ExB drift to the wall carrying a significant flux of particles. This mechanism may account for the observed nondiffusive, intermittent transport in the far SOL of tokamaks. Previous work has examined simple models of blob propagation and shown that the radial convective velocity u(x) has the scaling u(x)proportional toy(b)(-2), where y(b) is the poloidal blob dimension. Thus, the radial particle flux is sensitive to the details of the blob size and shape distributions; these in turn are affected by various secondary instabilities which cause blob fragmentation. A simple model of blob instability (driven by curvature in the presence of sheath boundary conditions) is studied by 1D calculations and 2D simulations to provide insight on the stability and dynamics of blobs. It is shown that a competition between the time scales for instability and transport determines the range of allowed blob sizes and shapes and constrains the overall radial velocity. (C) 2003 American Institute of Physics.