Investigating the impact of the molecular charge-exchange rate on detached SOLPS-ITER simulations

Abstract

Plasma-molecular interactions generate molecular ions which react with the plasma and contribute to detachment through molecular activated recombination (MAR), reducing the ion target flux, and molecular activated dissociation (MAD), both of which create excited atoms. Hydrogenic emission from these atoms have been detected experimentally in detached TCV, JET and MAST-U deuterium plasmas. The TCV findings, however, were in disagreement with SOLPS-ITER simulations for deuterium indicating a molecular ion density ($D_2^+$) that was insufficient to lead to significant hydrogenic emission, which was attributed to underestimates of the molecular charge exchange rate ($D_2+D^+\rightarrow D_2^++D$) for deuterium (obtained by rescaling the hydrogen rates by their isotope mass).

In this work, we have performed new SOLPS-ITER simulations with the default rate setup and a modified rate setup where ion isotope mass rescaling was disabled. This increased the $D_2^+$ content by $>\times 100$. By disabling ion isotope mass rescaling: 1) the total ion sinks are more than doubled due to the inclusion of MAR; 2) the additional MAR causes the ion target flux to roll-over during detachment; 3) the total $D\alpha$ emission in the divertor increases during deep detachment by roughly a factor four; 4) the neutral atom density in the divertor is doubled due to MAD, leading to a 50\% increase in neutral pressure; 5) total hydrogenic power loss is increased by up to 60\% due to MAD. These differences result in an improved agreement between the experiment and the simulations in terms of spectroscopic measurements, ion source/sink inferences and the occurrence of an ion target flux roll-over. Extrapolating simplified scalings of divertor molecular densities (TCV \& MAST-U) to reactor-relevant densities suggests the underestimation of molecular charge exchange could strongly impact divertor physics (neutral atom density, ions sinks) and hydrogen emission (which has implications for detachment control) in deeply detached conditions, warranting further study.

SOLPS-ITER predictive simulations of the impact of ion-molecule elastic collisions on strongly detached MAST-U Super-X divertor conditions

“…We find that MAR does not play a significant role in our simulations. However, we note that MAR (as well as molecular activated dissociation-MAD), involving D + 2 , may be underestimated in SOLPS-ITER as the reaction rates are specified for hydrogen and in certain cases re-scaled to deuterium using a simplified mass re-scaling [27,28]. As one would expect, a higher divertor electron density and lower plasma temperature near the target is achieved in the fuelling scan.…”

“…Non selfconsistent post-processing of these simulations has been performed for diagnostic purposes in [25], which showed that this does not play a significant role in the seeded simulations, but plays a significant role in the density ramp cases after the detachment onset; resulting in significant levels of MAR, hydrogenic emission and MAD-leading to plasma energy losses and additional neutral atoms. This post-processing, which is not self-consistent, is expected to exaggerate the molecular effects by roughly ×2 [28]. It is possible that repeating this study with updated ion-conversion cross-sections may change the solutions discussed in this work such that ionconversion plays a more significant role.…”

“…Note that the EIRENE / AMJUEL reaction rates are specified for hydrogen and in certain cases are re-scaled to deuterium using a simplified mass re-scaling [27]. In the case of H.2 3.2.3, this can result in an underestimation of the cross-section, especially at detachment-relevant temperatures [25,28].…”

“…At this stage, it is important to emphasise the following. Given the documented discussion in literature about ion-conversion [25,27,28,38], the default ion-conversion cross-sections used by SOLPS-ITER may need to be updated. Ion-conversion cross sections are underestimated in detached conditions, particularly for deuterium.…”

“…By default, EIRENE performs ion isotope mass rescaling of this rate, < σv > D(T) =< σv > H(T/2) . As < σv > H(T) decays strongly at T < 1 eV, this leads to a strongly underestimated rate at detachment-relevant conditions for deuterium [28]. Non selfconsistent post-processing of these simulations has been performed for diagnostic purposes in [25], which showed that this does not play a significant role in the seeded simulations, but plays a significant role in the density ramp cases after the detachment onset; resulting in significant levels of MAR, hydrogenic emission and MAD-leading to plasma energy losses and additional neutral atoms.…”