A New Scaling for Divertor Detachment

Authors: R.J. Goldston, J. A. Schwartz

Abstract: The ITER design, and future reactor designs, depend on divertor "detachment,"whether partial, pronounced or complete, to limit heat flux to plasma-facing components and to limit surface erosion due to sputtering. It would be valuable to have a measure of the difficulty of achieving detachment as a function of machine parameters, such as input power, magnetic field, major radius, etc. Frequently the parallel heat flux, estimated typically as proportional to Psep/R0 or PsepB0/R0, is used as a proxy for this difficulty. Here we argue that impurity cooling is dependent on the upstream density, which itself must be limited by a Greenwald-like scaling. Taking this into account self-consistently, we find the impurity fraction - see in linked report.

The absence of any explicit scaling with machine size is concerning, as Psep surely must increase greatly for an economic fusion system, while increases in the other parameters are limited. This result should be challenged by comparison with measurements on existing experiments. Nonetheless, it suggests that higher magnetic field, stronger shaping, double-null operation,, "advanced" divertor configurations, as well as alternate means to handle heat flux such as metallic liquid and/or vapor targets merit greater attention

Download PPPL-5315 (pdf 2.0 MB 14 pp)
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