Strong correlation between D2 density and electron temperature at the target of divertors found in SOLPS analysis

A companion paper (Sang et al 2016 Nucl. Fusion (https://doi.org/10.1088/1741-4326/aa6548)) reports an assessment, using the SOLPS5.0 (B2-EIRENE) code, of the relative importance of two key aspects of divertor-baffle geometry: (i) divertor closure, and (ii) field-target angle. A wide range of the degree of divertor closure and field-target angle were modeled. An unexpectedly strong and simple correlation has been discovered in these data (and is reported here) between the electron temperature, T-et, and the D-2 density, n(D2t) at the target, for T-et < 10 eV and extending over two orders of magnitude for each correlate: T-et = 6.14 x 10(13) n(D2t)(-0.68) with R-2 = 0.98. The values of T-et, and n(D2t) are for each individual flux tube of the computational grid spanning two power decay widths outward from the separatrix. This may imply that achievement of low Tet reduces, essentially, to identifying the divertor-baffle geometry which achieves the highest gas density near the target. To try to identify the controlling physics involved, two-point model formatting (2PMF) has been applied to the code output; it finds an equally strong and simple correlation between the 2PMF volumetric power-loss factor, f(vol-pwr-loss), and n(D2t) for each flux tube: f(vol-pwr-loss) = 10.2 x 10(29) n(D2t)(-1.54) with R-2 = 0.93. While these trends are broadly as would be expected, the simplicity, tightness and span of the correlations are not understood at present. Additionally, since more of the volumetric power loss is due to impurities than to deuterium, and as the impurities do not radiate just at the target, it is not evident why f(vol-pwr-loss) is so strongly correlated with n(D2t). To address these questions, in future work 2PMF analysis will be extended to compute the individual contributions to f(vol-pwr-loss).