Characterization of highly radiating nitrogen-seeded H-mode plasmas in unfavorable B at ASDEX Upgrade

The highly radiating nitrogen-seeded H-mode plasmas in unfavorable B has been characterized in the ASDEX Upgrade tokamak (AUG). Three levels of nitrogen puffing rates have been injected into a fully detached H-mode plasma, which is run in lower single null configuration with the ion B × ∇ B drift away from the X-point. A cold ( ∼ 1 − 2 eV) highly radiating ( ∼ 13.0 MW m − 3 ) region forms close to the X-point immediately after nitrogen seeding, as evidenced by measurements of the divertor Thomson scattering (DTS) and the two-dimensional bolometry reconstructions. In addition, the radiator moves further upwards above the X-point along the separatrix at the high-field side (HFS) with increasing nitrogen puffing rates, as evidenced by the Absolute eXtended UltraViolet (AXUV) measurements. The formation of the highly radiating regime is closely correlated to the modifications of the divertor plasma conditions. Along the line of sight of the DTS measurement, the electron temperature reduces down to a few eV, which initials near the X-point and further extends to the HFS scrape-off layer (SOL) simultaneously with the upward movement of the radiator, however, the electron temperature sustains ( ∼ 30 − 50 eV) at the low-field side (LFS) SOL with slightly decreased electron density. The highly radiating regime shows LFS/HFS divertor asymmetry, in contrast to that for H-mode plasmas in favorable B, suggesting that the drifts play an important role for the formation of the highly radiating X-point regime at AUG. The neutral particle flux increases significantly (factor of ∼ 2 ) in the private flux region, while it increases slightly ( < 20 % ) in the main chamber, thus suggesting an enhanced sub-divertor neutral compression with the formation of the highly radiating regime. Furthermore, a degradation of the pedestal electron density was observed with an enhancement of the electron temperature further inside the pedestal, and complete divertor detachment was achieved by nitrogen seeding with sustained plasma confinement. Finally, particle sources and flow patterns of deuterium and nitrogen ions have been analyzed by SOLPS-ITER modelling, confirming that the E × B drift plays an important role for the formation of the highly radiating regime in unfavorable B at AUG.