A New Scaling for Divertor Detachment

Rob Goldston, Matt Reinke, & Jacob Schwartz

The ITER design, and future reactor designs, depend on divertor detachment to limit heat flux to plasma-facing components. It would be valuable to have a measure of the difficulty of achieving detachment as a function of machine parameters. Frequently the parallel heat flux, estimated typically as proportional to Psep/R or PsepB/R, is used as a proxy for this difficulty. We argue that SOL impurity cooling is dependent on the upstream separatrix density, which itself must be limited to a fraction of the Greenwald density. Taking this into account, as well as modern understanding of the SOL width, we find the impurity fraction required for detachment scales dominantly as Psep/Bp, with no explicit size scaling to mitigate the absolute power scaling.