The role of the target electron temperature as a key detachment parameter

Since its introduction 40 years ago [Mahdavi et al, 1981, PRL 47 1602], the simple analytic 2 Point Model, 2PM, has been widely used to aid understanding of divertor behavior including detachment. For each magnetic flux tube in the edge, the 2PM relates target values of T_et and n_et to the n_eu and q_||u, the ‘upstream’ values at the divertor entrance or outside midplane. The utility of 2PMs for analyzing detachment is limited without some means of estimating with practically usable analytic approximations, the magnitude of the volumetric loss terms needed in the analytic modeling, i.e. loss of power, momentum and particles (volume recombination). Recently a surprising discovery has emerged from analysis of numerical edge code modeling of detachment, e.g. using SOLPS, namely the existence of strong correlations between T_et, i.e. the value right at the target, and all 3 of the (normalized) volumetric loss terms, thereby providing the missing piece needed for 2PM analysis of detachment [Stangeby, 2018, PPCF 60 044022]. For some of these terms, the strong correlations seem to be at least partially explicable in terms of known physics but for others even a rough explanation remains to be identified. Nevertheless, subsequent independent code studies have confirmed these strong correlations and recently experimental studies on JET have provided the strongest evidence to date for their physical existence [Lomanowski, 2020 APS Invited]. Recent analysis of SOLPS-ITER solutions for ITER [Stangeby, Lore, Canik, Pitts and Bonnin, tbp] show additional strong correlations between T_et and: the power flux deposited on the target; the ion flux deposited on the target; the neutral pressure at the target; the plasma density at the target. Implications for development of reduced models for detachment are discussed.