The nature of plasma fluxes to surfaces nearly tangential to the magnetic field

Plasma fluxes to edge structures, such as limiters, probes and divertor plates, are generally assumed to be due to parallel transport only-and all limiters and divertors have been designed on the basis of this standard model. A number of experimental results have indicated substantial fluxes to surfaces which are parallel, or nearly parallel to B-a result which is inconsistent with the standard model. A new model is presented which includes direct cross-field deposition on edge structures. In the case of limiters and probes this direct deposition process is amplified by a 2D or 3D concentration, or funnelling, effect which is induced in the confined plasma, inboard of the edge structure. As a result, the plasma wetted area of large edge structures such as inner wall limiters is substantially greater than indicated by the standard model-a generally beneficial effect. For sufficiently oblique angles between B and divertor tiles, it may be that heat and particle transport to the tiles is dominated by direct, cross-field transport. This would have major implications for the exploitation of divertor configurations with very large flux expansion and large wetted area