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The plasma background and impurity transport in the scrape-off layer (SOL) of the WEST tokamak are modeled for three discharges over a Psep power scan ranging between 1.5 and 2.35 MW. An extended modeling grid is used to investigate far-SOL sourcing from the WEST baffle and lower divertor. Divertor Langmuir probe data at the targets and upstream data from reciprocating Langmuir probes and interferometry/reflectometry is used to create a simulated background plasma with the OSM and EIRENE codes. W-I spectroscopic data from WEST at the lower divertor and baffle is used to estimate the sputtered W flux as an input to the impurity transport code DIVIMP. A potential mechanism for far-SOL impurities from the baffle to transport into the near-SOL is proposed and discussed in terms of the model’s force balances. Analysis of simulated impurity transport showed similar impurity density at the separatrix for the mid and high-power cases despite the high-power case having 10% more sputtered W. This is explained by the combination of differing trends in the sourcing and screening with scaling power.

Highlights • Heat exhaust issues are studied by SOLPS simulation for CFETR phase II with argon seeding. • Low core impurity density can be achieved for high electron density due to both the decrease in the required impurity concentration for fixed radiation power and the improvement of the impurity screening of divertor plasma. • The simulation results are fitted by the Matthew law, which can provide the relationship between the n e , Z eff , and P rad. Abstract For the future fusion reactor, where no intrinsic radiative impurity, like carbon, is allowed to exist, impurity seeding is an indispensable way to exhaust the power across the separatrix. Meanwhile, the impurity concentration has to be limited in order to avoid significant dilution of the fuel in core plasma. For CFETR phase II with a fusion power ∼1 GW, the radiative divertor with argon seeding is studied by SOLPS simulation. Under a high radiative power fraction ∼80% of the power across the separatrix, a density scan is performed for different deuterium ion density at the core-edge boundary. The divertor plasma is fully detached and impurity radiation is concentrated in the divertor region. The relationship between the electron density n e , effective charge Z eff and radiated power P rad are well fitted to the Matthew law, which is considered helpful to find a consistent operation scenario by further core-edge integrated modeling for CFETR phase II. [ABSTRACT FROM AUTHOR]

Since tungsten is considered as the optional divertor target material for the future fusion devices, e.g., China Fusion Engineering Test Reactor (CFETR), it is crucial to have a good screening of the tungsten impurity to keep a low core tungsten concentration. In this paper, based on the two-point model, possible divertor plasma parameters are predicted in high recycling regime for CFETR. Further analysis of tungsten ionization lengths shows that when target temperature is low enough (less than 15 eV), the increase of upstream density may cause an increase of ionization length, and thus a degradation of divertor tungsten screening effect may happen. The 2-D simulations of tungsten transport are then performed by using Osm-Eirene-Divimp edGE [onion skin model (OSM)-EIRENE-DIVvertor IMPurities (DIVIMP)] code package. The target plasma parameters, in the region of those predicted by the two-point model, are set as input in OSM-EIRNEN to generate the background plasma. DIVIMP code is used to further simulate the tungsten impurity transport process. The results reveal that although the decrease of target plasma temperature may reduce the tungsten impurity prompt redeposition rate, the benefit of a lower erosion rate can make compensation. Therefore, to control the core tungsten concentration, lower target plasma temperature is preferred in high recycling regime. [ABSTRACT FROM AUTHOR]

Abstract: A radiating divertor approach was successfully applied to high performance ‘hybrid’ plasmas [M.R. Wade et al., in: Proceedings of the 20th IAEA Fusion Energy Conference, Vilamoura, Portugal, 2004]. Our techniques included: (1) injecting argon near the outer divertor target, (2) enhancing the plasma flow into the inner and outer divertors by a combination of particle pumping and deuterium gas puffing upstream of the divertor targets, and (3) isolating the inner divertor from the outer by a structure in the private flux region. Good hybrid conditions were maintained, as the peak heat flux at the outer divertor target was reduced by a factor of 2.5; the peak heat flux at the inner target decreased by 20%. This difference was caused by a higher concentration of argon at the outer target than at the inner target. Argon accumulation in the main plasma was modest (n Ar/n e ⩽0.004 on axis), although the argon profile was more peaked than the electron profile. [Copyright &y& Elsevier]

Abstract: Changes on transport and radiation properties of intrinsic carbon under the influence of the Dynamic Ergodic Divertor depend on the modal composition of the induced error field. For 12/4 mode operation, significant carbon screening is not observed up to half of the nominal DED maximum current achieved so far. On the contrary, for 3/1 DED operation the very efficient mechanism of expulsion of highly ionized carbon ions, observed to occur locally at the foot prints of the 3/1 helical structure and explained in terms of frictional forces, leads to a global carbon decontamination of the central plasma. This result has been established using a new multi-chord diagnostic, which allows to distinguish global from local carbon behavior. [Copyright &y& Elsevier]

Abstract: We report on impurity transport modeling using the DIVIMP code which is able to qualitatively reproduce the poloidal variation of non-recycling impurity penetration factor (PFNR) found in C-Mod experiments: a lower PFNR is computed at the inboard (3.6%) and divertor target locations (0.7%) than at the outboard (11%). By artificially increasing the modeled inner SOL plasma flow to correspond to measured values, a better quantitative agreement between modeled and measured PFs is achieved. We have also roughly reproduced the observed penetration factor for recycling impurities both in time dependence and magnitude. The model has shown that under attached conditions, the majority of recycling impurity ions flow into the confined plasma through the outboard side separatrix. For detached conditions the impurity influx across the separatrix is more concentrated near the divertor. [Copyright &y& Elsevier]

Abstract: A local island divertor (LID) experiment has begun on LHD, with the aims of controlling edge recycling and improving the plasma confinement. The fundamental divertor functions of the LID have been demonstrated in the recent experiments. From the particle flux profile measurements on the LID head it was found that the particles diffusing out from the core region are well guided along the island separatrix to the LID head. Owing to the closed configuration around the LID head, evidence of the high efficient pumping was observed, together with a strong capacity to screen impurities. The first results of edge modeling using the EMC3-EIRENE code are also presented. [Copyright &y& Elsevier]

Abstract: JET plasmas with reverse magnetic field feature a different SOL flow than those with normal field. The observed carbon fuelling efficiency from injecting methane gas was similar in reverse and normal field. EDGE2D modeling used an externally applied force to create the SOL flows, without specifying the origin of the force. The resulting flow agreed reasonably with the experimental values between the separatrix and 4cm mid-plane depth in the SOL. The effect of the flow on the calculated carbon screening was 5–15% higher carbon fuelling efficiency for the low flow velocity with reverse field. [Copyright &y& Elsevier]