Obtaining reactor-relevant divertor conditions in tokamaks

It is argued that the paramount boundary plasma issue for DT reactors is likely to be the erosion wear of the plasma facing components, PFCs, and that a number of potential solutions all require the achievement of not only low temperature (10 eV) but also high density (1021 m−3) in the divertor. Estimates are made of the minimum heating power, Pheat, required to achieve a divertor target temperature of Tt = 5 eV and density nt > 1021 m−3, based on four recent hypotheses or scalings for the width of the power footprint on the target, λqt. Each of these result in predictions of how the required minimum Pheat depends on device size, namely as R, R3/2 or R2. The absolute magnitude for the required values of minimum Pheat is found not to vary significantly among the four power scalings; for the most part a factor of order ~2 for a significant range of R. The four hypotheses/scalings for λqt are empirically based; however, they draw on measurements made in tokamaks that did not have divertors operating primarily under these conditions. In order to establish if any of these power scalings are applicable, they are compared with measurements from a set of DIII-D discharges with high nt ~ 0.35 × 1021 at 5 eV. It is found that all four power scalings match the experimental measurements to within the uncertainties. The main objective is to determine what power is needed to achieve the required divertor conditions in future devices, for both reactor and simulator tokamaks, and therefore the approximate agreement of the four, strongly empirical, power scalings increases confidence that this may be possible.