Temporal evolution of blobs in the scrape-off layer

Experimental observations have revealed that the transport in the edge and scrape-off-layer (SOL) of toroidally magnetized plasmas is strongly intermittent and involves large outbreaks of hot plasma. These structures, often referred to as “blobs”, are formed near the last closed flux surface (LCFS) and propagate far into the SOL. The convective transport mediated by the blob-like structures prevails in virtually all confinement states, including edge-localized modes. They have a profound influence on the pressure profiles in the SOL, the ensuing parallel flows, and the power deposition on plasma facing components. The basic blob dynamics is well described in terms of nonlinear interchange motions [1-2] in the limit of cold ions, the so-called ESEL model. Experimental observations, however, show that the ion temperature in the blobs may be even higher that the electron temperature due to the rapid loss of hot electrons. Treatment of hot ions in the classical two fluid models is not tractable. However, gyro-fluid models are able to incorporate these effects in a much simpler way, but cannot account for collisional effects in a consistent way. Gyro-fluid and drift approximation two-fluid models are equivalent in certain limits. A comparison of the results from numerical simulations is therefore relevant. In this paper we present and compare the results from the 2D numerical codes; the electrostatic edge-SOL turbulence code, ESEL [1-2] and the gyro-fluid code G-ESEL, the later simulates the local and non-local [3] gyro-fluid equations. The focus of the investigations is the propagation of Gaussian “blob” like density structures. We examine the speed and the associated radial density transport as a function of blob amplitude and width. We observe an increase radial transport if finite ion temperature is included (G-ESEL).