Nitrogen-seeded divertor detachment in TCV L-mode plasmas

Abstract

Most of the detachment experiments done to date on the Tokamak a Configuration Variable (TCV), both in standard and alternative divertor geometries, focused on L-mode integrated core density ramps. In view of extending these studies to high-power, high-confinement regimes, where impurity seeding will be necessary for detachment, the properties of nitrogen seeded L-mode detachment in TCV are assessed here with the extensive set of edge and divertor diagnostics and similarities and differences with integrated core density (ne) ramp detachment experiments are elucidated. It is found that in high current, reversed field plasmas, detachment at the outer target is achieved with N 2-seeding and density ramps, with target heat flux reductions of up to 90%, while the inner target only detaches with seeding. The Scrape-Off Layer radiation fraction reaches values of 60-80% and in all situations, a stable radiator can form around the Xpoint. The most striking difference between seeding and density ramp is the behavior of the upstream quantities. During the ne-ramp, a broadening of the upstream density profile (density "shoulder") occurs, concurrent with the outer target ion flux roll-over, while no such behavior occurs during nitrogen seeded detachment. Separatrix density, electron temperature and pressure also evolve strongly with increasing density, and are largely unaffected by the injection of nitrogen. Comparison of upstream and target pressures reveals that, in all cases, the outer target ion flux reduction coincides with the development of a parallel gradient of the total pressure. Common to all cases is also a reduction of energy confinement time with detachment, although this effect is weak for seeding at relatively high density. Studying the impact of the ∇B-drift direction in both nitrogen seeding and core density ramps reveals that drifts mainly affect the behavior at the inner strike point, highlighting the need to include drift in edge transport simulations.

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

“…Experimentally, the target power decreases both with increasing density and nitrogen seeding [11]. This is also found in the simulations, that can differentiate between the components of the target power and processes leading to its reduction.…”

“…These losses are dominated by impurity radiation (fig. 10), that has been found to strongly reduce I t , both on TCV with nitrogen seeding and other machines [11,43]. An underestimation of the impurity radiation in the simulations at high n e could explain why I t does not roll over (eq.…”

“…In TCV, core radiation and Z ef f increase strongly with nitrogen seeding at low, but not at high density [11].…”

“…Experimental core C 6+ density measurements show an increasing trend when I t rolls over, while the simulations indicate that the core C content decreases with density. A possible explanation for this may be an increase in main chamber sputtering due to the observed broadening of the SOL at high density [11]. The energy of this sputtered carbon is higher than for target sputtering due to the higher upstream plasma temperature, increasing the ionisation mean free path and, hence, core penetration.…”

“…In particular, heating power has been increased and divertor baffles have been installed to better separate the divertor from the main plasma and to reach regimes with higher divertor neutral pressures. In parallel, experiments employed nitrogen impurity seeding, as an alternative to density ramps, to achieve detachment [11] and to extend the studies to H-mode conditions [14]. Such active impurity seeding provides, in a carbon device, a more direct control on the impurity content than density ramps.…”