Impurity transport in edge plasmas with application to liquid walls

The transport of impurity ions in a magnetically confined plasma is studied in the region between their origin at a material surface and the core plasma as defined by closed magnetic flux surfaces. The focus is physics understanding of the results of two-dimensional (2-D) transport modeling of the plasma and neutrals. A simple one-dimensional model is introduced to identify key processes and illustrate how such processes affect the core-edge impurity level. The 2-D simulation gives detailed results of scaling of the impurity level with parameters such as anomalous radial diffusion, hydrogen-plasma recycling, core power flux, and core-edge density. The results are obtained for a slab model of a tokamak, but by changing the magnetic connection length, scaling to other types of devices can be inferred. Lithium and fluorine impurities are considered explicitly as examples with low and moderate charge-state number, Z, for liquid wall materials; trends found for these cases provide guidance to the behavior of other impurities. The tolerable amount of impurity influx can be closely associated with the partial thermal collapse of the edge plasma. The results are used to provide a physics picture of previous results on the acceptable evaporative impurity flux from different types of liquid wall materials, and to show how these results can be expected to scale with parameters. (C) 2002 American Institute of Physics.