Comparison of high density and nitrogen seeded detachment using SOLPS-ITER simulations of the tokamak á configuration variable

Abstract

First of a kind SOLPS-ITER simulations on TCV that include nitrogen have been performed to model recent nitrogen seeded detachment experiments.Based on spectroscopic measurements, a nitrogen recycling coefficient R N p ≈ 0.3 − 0.5 on the graphite walls of TCV is estimated. The experimentally observed decrease of core nitrogen density with increasing plasma density is reproduced and linked to a reduction of the ionisation mean free path in the SOL. Although the influence of sputtered carbon impurities from TCV's graphite wall cannot be fully eliminated, seeding nitrogen increases control over the total impurity density. This facilitates disentangling the effect of impurities from that of high upstream density on the main characteristics of detachment, namely target power and ion current reductions and the development of a parallel pressure drop. Increasing the density and the seeding rate reduce the power on the divertor targets in a different way: with density, the ion current increases and the target temperature strongly decreases, whereas seeding impurities decreases the ion current and affects less strongly the temperature. The reduction in ion current when seeding nitrogen is due to a lower ionisation source, which is not related to power limitation nor an increased momentum loss, but to a decrease of the ionisation reaction rate. Impurity seeding leads to less volumetric momentum losses (and hence pressure drop) than density ramps, for the same level of energy flux reduction. Additionally, main chamber sputtering of carbon is identified as a possible explanation for the missing target ion current roll-over during density ramps in the simulations.

Real-time feedback control of the impurity emission front in tokamak divertor plasmas

“…In this work, we control the position of the C-III emission front (465 nm) along the divertor leg. This emission front has been shown to be a good marker for a relatively low temperature region 26 , 27 . Note that the divertor plasma is mostly tied to the magnetic field lines, such that the front movement is predominantly in the direction along the divertor legs and approximately toroidally symmetric.…”

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

“…Post-processing N 2 seeded SOLPS-ITER simulations for TCV [55] in the same way results in a negligible impact of D + 2 in contrast to the impact during a core density ramp because the target temperature reach a minimum near 3.5 eV during these simulations. These higher target temperatures lead to a lower D 2 density and thus a lack of D + 2 , even when post-processing is applied.…”

B2.5-Eunomia simulations of Magnum-PSI detachment experiments: II. Collisional processes and their relevance

“…The resulting atomic product however, can undergo elastic collision or CX, and so these collisions are centered in a higher T e than elastic H + -H 2 collision as seen in figure 18. Elastic H + -H 2 collisions as a power loss channel at low electron temperatures are also found in several tokamak studies [5,10,36].…”

“…In this paper we have shown the importance of different collisional processes in a plasma undergoing detachment in Magnum-PSI and associating them with different plasma regimes. Several processes that are highlighted in this paper, namely MAR and elastic H + -H 2 collisions, have been found to occur in tokamak divertors undergoing detachment [5,10,35,36]. It is important to note that in the simulations of this paper and experiments, these processes are exclusively influenced by the increase of neutral background pressure via gas puffing of H 2 .…”