Investigation of atomic and molecular rates in plasma-edge simulations through experiment-simulation comparisons

Abstract

Divertor plasma detachment is likely needed for the function of magnetically confined nuclear fusion. It greatly reduces the particle and heat flux incident on a target, and thus reduces the sputtering and heat loading on the target. It is therefore advantageous to have accurate simulations of plasma behaviour in the divertor to design future Tokamak divertors.A common simulation package used for this purpose is SOLPS-ITER, which has previously been observed to be underestimating the contribution of molecular effects to plasma detachment. To correct this, its reaction rate for molecular charge exchange was altered. This alteration resulted in a significant increase in the density of D + 2 ions within the divertor. The particle balance and Dα emission data was then computed from the simulation outputs. These were compared to experimental data and a set of post-processing routines for the original simulations that predicts the effect of a corrected charge exchange rate.In comparison to the post-processing predictions: the increase in Molecular activated Recombination and the depth of the target flux rollover was less than predicted. This is due to the post-processing not being self consistent and it does not account for how the alterations in reaction rate would affect the background plasma.Compared to the experimental data: the correction showed a significant improvement in the agreement of general trends, such as the increased MAR reaction rate and the rollover of target flux. However, there were still significant disagreements in the magnitudes of various reaction rates, suggesting further improvements to SOLPS-ITER are still needed.

Spectroscopic investigations of detachment on the MAST Upgrade Super-X divertor

“…As a post-processing approach is applied to convert the molecular density scalings to molecular ion densities in the simplified emission model, the strongly underestimated D + 2 /D 2 ratio in SOLPS-ITER does not impact our results directly. They can, however, affect our results indirectly as including a more correct molecular charge exchange rate self-consistently can result in different scalings for the molecular/neutral atom densities [49]. Preliminary analysis of these updated simulations shows that they would not alter the molecular density scaling as function of T e significantly, but they would increase the neutral density by up to a factor 2 at lower temperatures (T e < 2.5 eV) due to the increased level of molecular dissociation.…”

The role of plasma–atom and molecule interactions on power & particle balance during detachment on the MAST Upgrade Super-X divertor

“…This was attributed to underestimation of molecular charge exchange in Eirene, particularly for deuterium and tritium [3,13,28,29]. Increasing the molecular charge exchange (D 2 + D + → D + 2 + D) cross-section by disabling ion isotope mass re-scaling from H to D, both through post-processing the TCV SOLPS-ITER simulations [3], as well as self-consistent SOLPS-ITER simulations [28,30], led to an improved match between the experiment and simulation.…”