On defining scrape-off layer widths

A physically motivated, theoretical basis for defining scrape-off layer (SO L) widths is proposed. Applying it to theoretical models proposed for trans port in the SOL shows how the width is affected by the form of the cross-fi eld transport and the parallel transport to target plates. For a cross-fiel d diffusivity chi(perpendicular to) and a parallel loss time tau(parallel t o), the estimate Delta = C(chi(perpendicular to)tau(parallel to))(1/2), Wit h C typically in the range 0.6-1.0 and chi(perpendicular to) and tau(parall el to) evaluated at the SOL-core interface. is found to provide a physicall y meaningful and accurate expression for the power SOL width Delta.