Outer divertor of ASDEX Upgrade in low-density L-mode discharges in forward and reversed magnetic field: I. Comparison between measured plasma conditions and SOLPS5.0 code calculations

Abstract

Abstract. The scrape-off layer and divertor plasma conditions have been carefully analysed in dedicated ASDEX Upgrade experiments consisting of lowdensity L-mode discharges, with both forward and reversed toroidal magnetic field and plasma current. In forward field, the outer divertor plasma is in a lowrecycling regime with peak target temperature above 25 eV. In reversed field with similar main plasma parameters, the target temperature is below 15 eV and the density is 5 times as high as in forward field, indicating a higher recycling regime in the outer divertor. The SOLPS5.0 code package is used to model these divertor plasmas. Specifically, it is tested whether a combination of input assumptions exists that enables matching the code solution to all outer divertor diagnostic measurements, and whether these assumptions are compatible with constraints imposed by measurements outside the outer divertor. In forward field, a good level of agreement with multiple outer target measurements is found with assumptions that simultaneously match the measured density and temperature profiles at the outer midplane, where the uncertainty in radial position of the separatrix is ±0.5 cm. Similar approaches made previously for higher recycling regimes have not led to such a good consistency between all modelled and measured outer divertor parameters. In reversed field with higher recycling in the outer divertor, a solution consistent with the outer target Langmuir probe measurements cannot be obtained, at least not without significantly compromising the match to the upstream profile measurements. Significant mismatches are observed also between the modelled and measured upstream Mach number in forward field. These discrepancies question the global validity of the plasma solutions, and their origin is not yet clear. In Part II, the analysis of outer divertor conditions is complemented by local impurity migration studies, using the divertor plasma solutions presented in this paper.

A new experimental classification of divertor detachment in ASDEX Upgrade

“…Thus, the combination of both, the Γ dr Θ flow in the PF and the Γ dr r flow brings more plasma to the inner divertor in forward field and to the outer divertor in reversed field. It was further shown at ASDEX Upgrade that the E × B drifts have to be taken into account in order to simulate low density attached divertor plasmas correctly [39].…”

Physics of ultimate detachment of a tokamak divertor plasma

“…In particular, it was shown that introduction of the drifts in the plasma transport equations allows us to reproduce quantitatively the divertor asymmetry observed on JET and ASDEX Upgrade (e.g. Aho-Mantila et al 2017;Reimold et al 2017).…”

L-mode radiative plasma edge studies for model validation in ASDEX Upgrade and JET

“…Anomalous transport was described by radially varying transport coefficients which, together with the upstream separatrix density, were adjusted to achieve the best possible agreement with the measured low-recycling conditions in the outer divertor, in the absence of seeding (similar to [6,7]). Simultaneously, agreement with the measured temperature and density profiles at the outer midplane was requested (within measurement uncertainties), yielding high-recycling inner divertor conditions in the simulations, with density a factor of 2 higher than in the ASDEX Upgrade experiment.…”

“…The studies focus on two all-metal divertor tokamaks, ASDEX Upgrade and JET, and on low-density L-mode plasmas, in which at least one of the divertor legs is in a low-recycling regime. In the full-W ASDEX Upgrade, a good consistency between various divertor diagnostics has been observed for such conditions in the absence of seeding [6,7], suggesting that these divertor conditions are simple enough to be reproduced by the fluid code packages. The experimental results from N 2 seeded discharges are discussed in Section 2, followed by first qualitative modelling studies in Section 3.…”