Comparison of divertor seed impurity gases retention efficiency by SOLPS-ITER modeling

Abstract In the paper a SOLPS-ITER modeling of H-Mode scenarios with different seed impurity gases (N, Ne, Ar, Kr) is performed for ASDEX Upgrade geometry. These gases are compared from the point of view of their radiation efficiency and compression in the divertor, which is generally understood as the ratio of the divertor to upstream density. Three seeding levels of each impurity gas are considered: trace impurity level, level sufficient to radiate half of the incoming power and a so high seeding rate that the X-point radiator (XPR) starts to form. It is demonstrated that Ne has the lowest compression, the compression of Ar is bigger than that of N, and Kr has the best compression among gases considered. Such a ranking corresponds to the ranking of the impurity neutral atom ionization length, which for Kr is the shortest not only due to the smallest first ionization potential, but also due to the biggest ionization cross-section and the biggest mass (or the smallest thermal velocity). Additionally, for the given level of the radiated power fraction <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mrow> <mml:mi>rad</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> the seeding of Kr results in the smallest (among other radiants) effective charge <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>Z</mml:mi> <mml:mrow> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> in the confined region, which even for the proximity to XPR does not exceeds 1.5. It appears that the impurity compression strongly depends on whether coronal model or collisional-radiative (CR) one is used for the ionization/recombination rates calculation, or, in other words, whether such processes as step ionization (from excited levels) and 3-body recombination are taken into account. Since for Ar and Kr no results of CR model calculations are available in ADAS database used in SOLPS, the values of compression computed here for these radiants with coronal rates are underestimated. The compression appears to be more sensitive to the neutrals ionization rate than to the details of neutral flow from divertor to the pump, including interaction between neutral species.