The role of plasma-molecule interactions on power and particle balance during detachment on the TCV tokamak

Abstract

This paper shows experimental results from the TCV tokamak that indicate plasma-molecule interactions involving D + 2 and possibly D − play an important role as sinks of energy (through hydrogenic radiation as well as dissociation) and particles during divertor detachment if low target temperatures (< 3 eV) are achieved. Both molecular activated recombination (MAR) and ion source reduction due to a power limitation effect are shown to be important in reducing the ion target flux during a density ramp. In contrast, the electron-ion recombination (EIR) ion sink is too small to play an important role in reducing the ion target flux. MAR or EIR do not occur during N 2 seeding induced detachment as the target temperatures are not sufficiently low.The impact of D + 2 is shown to be underestimated in present (vibrationally unresolved) SOLPS-ITER simulations, which could result from an underestimated D 2 + D + → D + 2 + D rate. The converged SOLPS-ITER simulations are post-processed with alternative reaction rates, resulting in considerable contributions of D + 2 to particle and power losses as well as dissociation below the D 2 dissociation area. Those findings are in quantitative agreement with the experimental results.

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

“…2 , and thus their predicted impact on the formation of detachment, is low. However, this disagrees with experimental data that shows large numbers of molecular reactions involving D + 2 occurring during the onset of detachment [19]. It is hypothesised that the low D + 2 /D 2 ratio is caused by an incorrect mass re-scaling for the hydrogen molecular charge exchange rate to deuterium.…”

“…We can compare these simulation results to the post processing predictions created from the routines made in [19]. This can be seen in figure 4a, where we observe a significantly lower MAR rate in the corrected rate compared to the post processing pre- diction.…”

“…Recent studies have observed the difference between experimental and simulation data can be explained by the influence of MAR reactions as an ion sink [33]. This is surprising as current SOLPS simulations predict a very low D + 2 density in the plasma and as a result a low rate of MAR [18], contradicting the experimental data.…”

“…Therefore, a post-processing routine was created, which can be applied to simulation results, to approximate the adjusted reaction rates at higher D + 2 . However, this post-processing does not account for the the way these increases in reaction rates may affect the rest of the plasma and so a self consistent adjustment to the SOLPS simulation is needed [19]. This difference in the measured and simulated D + 2 density is theorized to be due to an inaccuracy in the calculation of the deuterium molecular charge exchange rate in EIRENE simulations.…”

Comparison between MAST-U conventional and Super-X configurations through SOLPS-ITER modelling

“…with drifts) of ongoing MAST-U experiments will bring even more understanding of the role of β and divertor closure on the divertor detachment threshold and the expansion of the detached region from the target towards the X-point. A more accurate treatment of molecular charge exchange in SOLPS-ITER would also likely lead to particle losses at higher temperatures due to Molecular-Activated-Recombination (MAR), as pointed out in [31], and may change the differences in particle balance between configurations. By validating such simulations against experiments, we would then be in a position to make predictions for the detachment access windows of future fusion reactors prototypes, such as STEP.…”