Investigations of the magnetic field dependence of blob velocity and size with Li-BES at ASDEX Upgrade

Abstract

At the edge of fusion plasmas, intermittently expelled density filaments, so called blobs [1, 2], are propagating through the scrape-off layer (SOL), which determines perpendicular transport, and can lead to a degradation of plasma facing components. Since this degradation is critical for the first wall in future fusion devices, an understanding of the generation and the propagation of blobs is needed. In toroidal plasmas, blobs are thought to be solitary density perturbations appearing together with an electric dipole from poloidal charge separation as a result of magnetic field inhomogeneities. Depending on the plasma regime, the dipole can radially accelerate the blob via the E ×B drift to a greater or lesser extent, and a relation between the size of a blob a and its radial velocity vr can be derived. A compact expression for a multi-regime scaling which takes into account three different effects (sheath-dissipation, inertia and divergence of ion polarization current) was derived by Theiler et al. [3]: