PFC / JA - 96 - 5 The Influence of Anomalous Diffusion on Parallel Ion Transport in Edge Plasmas

In edge plasmas, such as the tokamak scrape-off layer, transport parallel and perpendicular to the magnetic field are strongly coupled. It is shown that this leads to an unconventional form of the parallel transport laws. In particular, if the radial transport is governed by anomalous diffusion, the parallel ion transport cannot be entirely classical. This general phenomenon is demonstrated explicitly within a simple model where the anomalous diffusion is driven by weak electrostatic turbulence. Since the parallel and perpendicular flows balance each other in the plasma edge, anomalous terms proportional to the radial shear of the parallel velocity are shown to enter into, and modify, the usual Spitzer problem for parallel ion transport. As a result, parallel ion transport is driven not only by parallel gradients, but also by radial gradients, which is of importance for the flow of impurities in the tokamak edge. A new type of parallel thermal force is found resulting from the combined action of anomalous diffusion, parallel velocity shear, and a radial temperature gradient. PACS Numbers: 52.25.Dg, 52.25.Fi, 52.35.Ra, 52.25.Gj. 'Present address: UKAEA Government Division, Fusion, Culham, Abingdon, Oxon, OX14 3DB, U.K.