A novel method for treating MAR in EMC3-Eirene, and first applications to W7-X

Abstract This paper presents a novel ‘prediction-correction method’ for treating molecule-assisted recombination (MAR) in the three-dimensional (3D) boundary plasma transport code—EMC3-Eirene. In this approach, certain MAR products are first isolated and removed from the particle trajectories in Eirene and recorded as predictions, which are then implicitly corrected/reprocessed in EMC3 and subsequently fed back into Eirene as an ‘external’ source to compensate for the removed particles. Compared to the conventional scheme used in boundary plasma modelling, this new method exhibits enhanced numerical stability. It should be broadly applicable to other 2D fluid-kinetic edge plasma transport calculations. Using a typical detached plasma from W7-X as an example, we apply the latest version of the EMC3-Eirene code to perform, for the first time, a self-consistent analysis of the role of volume recombination processes, including MAR and electron–ion recombination (EIR), in a 3D divertor, namely the island divertor. Intrinsic carbon is assumed to be the only impurity species, and the radiation 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:mi>rad</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> is used as a control parameter. The simulation demonstrates that both EIR and MAR increase with <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:mi>rad</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , with the total volume recombination rate reaching approximately 30% of the total neutral source at <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:mi>rad</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> = 0.9. The MAR contribution is typically around a factor of three of that of the EIR when the radiation is located in the edge magnetic islands outside the last closed flux surface. There are no noteworthy effects of volume recombination on detachment performance regarding the power load on the target, the neutral pressure in the divertor chamber, or the distribution of impurity radiation. Nevertheless, volume recombination significantly changes the relative population of atoms and molecules in front of the targets, which may be generally important for boundary plasma spectroscopy in fusion devices.