Improved core-edge compatibility using impurity seeding in the small angle slot (SAS) divertor at DIII-D

Abstract

Impurity seeding studies in the small angle slot (SAS) divertor at DIII-D have revealed a strong relationship between the detachment onset and pedestal characteristics with both target geometry and impurity species. N2 seeding in the slot has led to the first simultaneous observation of detachment on the entire suite of boundary diagnostics viewing the SAS without degradation of core confinement. SOLPS-ITER simulations with D+C+N, full cross field drifts, and n–n collisions activated are performed for the first time in DIII-D to interpret the behavior. This highlights a strong effect of divertor configuration and plasma drifts on the recycling source distribution with significant consequences on plasma flows. Flow reversal is found for both main ions and impurities affecting strongly the impurity transport and providing an explanation for the observed dependence on the strike point location of the detachment onset and impurity leakage found in the experiments. Matched discharges with either nitrogen or neon injection show that while nitrogen does not significantly affect the pedestal, neon leads to increased pedestal pressure gradients and improved pedestal stability. Little nitrogen penetrates in the core, but a significant amount of neon is found in the pedestal consistent with the different ionization potentials of the two impurities. This work demonstrates that neutral and impurity distributions in the divertor can be controlled through variations in strike point locations in a fixed baffle structure. Divertor geometry combined with impurity seeding enables mitigated divertor heat flux balancing core contamination, thus leading to enhanced divertor dissipation and improved core-edge compatibility, which are essential for ITER and for future fusion reactors.

SOLPS‐ITER EU‐DEMO modelling with drifts and kinetic neutrals

“…The possible way to increase the radiation is the radiating impurity seeding. Experiments on JET, ASDEX-Upgrade, and DIII-D [2][3][4] demonstrate the possibility of decreasing the energy loads to the divertor targets by the seeding nitrogen, neon, and argon as radiating impurities. Analysis of the experiments on different tokamaks shows that the decrease of power flux and electron temperature in divertor depends on the tokamak size and geometry non-linearly.…”

Detached regime with highly radiating X‐point: Physics and modelling

“…[1,2] Experiments on impurity seeding using different gases (mainly N, Ne, and Ar, more rare Kr) are performed on many machines for several years, [3][4][5][6][7][8][9][10][11][12] and they are supported by analytical models and numerical modelling. [13][14][15][16][17] It is widely accepted that as the impurty seeding rate increases, target operation regime moves from the attachment (strong heat flux density peak near the strike point) through the partial detachment (notable reduction of the heat flux density in the strike point vicinity), the pronounced detachment (notable decrease of maximal heat flux density and maximal electron temperature in the far scrape-off layer [SOL]) to the full detachment (flattening of heat flux density and electron temperature along whole target with typical values below 1 MW∕m 2 and 2 eV correspondingly). [5] The disadvantage of intensive impurity seeding is the possible impurity leakage from the divertor, which leads to the impurity penetration into the confined region and to the fuel dilution.…”

DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy

“…With these advances, experimental data using impurity injection and SOLPS-ITER simulations with full drift effects show that for the small angle slot (SAS) baffling geometry in the upper divertor of DIII-D, divertor detachment and pedestal performance can be optimized through magnetic geometry and choice of impurity species [43,44]. Experiments with nitrogen injection show that a larger quantity of impurity is required to detach the SAS divertor plasma (figure 10(a)), and a higher density of nitrogen appears in the pedestal and core plasma (figure 10(b)) when the outer strike point (OSP) is in the outer corner of the slot (red) compared with the OSP positioned on the inner slanted surface of the SAS (blue) [43,44]. SOLPS-ITER modeling with full cross-field drifts and both carbon and nitrogen impurity charge states is required to reproduce these effects.…”

“…(c) pedestal electron density and (d) electron temperature profiles with the OSP on the inner slanted baffle comparing a reference case without impurity injection (red), a case with nitrogen injection (blue) and a case with neon injection (green). Reproduced with permission from [43].…”