On defining scrape-off layer widths

A physically motivated, theoretical basis for defining scrape-off layer (SOL) widths is proposed. Applying it to theoretical models proposed for transport in the SOL shows how the width is affected by the form of the cross-field transport and the parallel transport to target plates. For a cross-field diffusivity chi(perpendicular to) and a parallel loss time tau(parallel to), the estimate Delta = C(chi(perpendicular to)tau(parallel to))(1/2), With C typically in the range 0.6-1.0 and chi(perpendicular to) and tau(parallel to) evaluated at the SOL-core interface. is found to provide a physically meaningful and accurate expression for the power SOL width Delta.