There have been major developments recently in the understanding of the very edge of tokamak plasmas. New experimental analyses, coupled with new heuristic theoretical understanding, has led to a simple and accurate model for the width of the narrow region of plasma that is scraped off the plasma edge and ultimately strikes material surfaces, the Scrape Off Layer or SOL. Furthermore, the model predicts instability at the edge, due to the strong pressure gradient in the SOL, at just the point in edge density where good plasma confinement is experimentally found to be lost. It is possible to deduce from these results the scaling for the density of impurities needed to radiate away the power from a tokamak fusion power system. New ideas are now being developed to provide a low-Z impurity, lithium, in the form of a dense vapor cloud localized at a distance from the main plasma, to allow for robust dissipation of the SOL heat flux, and a collaboration is under development between PPPL and Magnum-PSI to test this concept.
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