A study of the influence of plasma–molecule interactions on particle balance during detachment

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 core 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 underestimatedThe 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.

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

“…Around that point in the discharge the divertor ion source no longer increases linearly and ultimately rolls-over (∼ 0.47 Greenwald fraction), which accounts for a large fraction of the observed ion target flux loss (Fig. 2a) [33]. This experimental evidence suggests that the outer divertor ion source is being limited by the power entering the recycling region: 'power limitation'.…”

“…In this paper we build on those previous experimental results [33] and use the BaSPMI [32] technique to investigate plasma detachment in TCV further to provide a more complete view of previous findings.…”

“…Recent developments in our spectroscopic analysis technique (BaSPMI [32]) have enabled the inference of the impact of plasma-molecule interactions through their effect on the hydrogenic line series intensities. Initial application of this technique to the TCV tokamak (n e ∼ 10 20 m −3 ) divertor plasma indicated that plasma-molecule interactions A c c e p t e d M a n u s c r i p t can: 1) have a strong impact on the observed Balmer line series; and 2) result in a large ion sink through MAR, that is significantly larger than electron-ion recombination [33].…”

“…The spectroscopic analysis in this work is performed using the BaSPMI technique (described in detail elsewhere [32,33,8]), which uses electron density estimates derived from a Stark broadening analysis in combination with D α − D γ measurements. Following previous research [32,33], the contribution of D + 3 is ignored by BaSPMI. Further information can be found in Appendix D.…”

“…This may be attributed to the presence of highly vibrationally-excited molecules, where the isotope difference between creating D − and H − is reduced [43,44]; this question is further examined in sections 2.2, 4.1. [40,41] (green) with a linear fit from before detachment [33,8]. Spectroscopically inferred atomic and molecular-activated ion source (red) from [33], electron-ion recombination ion sink (EIR -blue) from [33], total molecular activated recombination ion sink (MAR -magenta) from [33], D − related MAR ion sink (cyan) (new ) -all integrated over the outer divertor volume.…”