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New analytical expressions for parallel transport coefficients in multicomponent collisional plasmas are presented in this paper. They are improved versions of the expressions written in Zhdanov [ Transport Processes in Multicomponent Plasma, English ed. (Taylor and Francis, London, New York, 2002)], based on Grad's 21N-moment method. Both explicit and approximate approaches for the calculation of transport coefficients are considered. Accurate application of this closure for the Braginskii transport equations is discussed. Viscosity dependence on the heat flux is taken into account. Improved expressions are implemented into the SOLPS-ITER code and tested for deuterium and neon ITER cases. Some typos found in Zhdanov [ Transport Processes in Multicomponent Plasma, English ed. (Taylor and Francis, London, New York, 2002)] are corrected.

SOLPS-ITER modelling databases of three tokamaks—ASDEX-Upgrade, JET and ITER with fluid drifts activated are compared to understand the dependence of edge plasma performance on machine size and other global parameters. Two medium Z extrinsic radiating impurity species (Ne and N) are considered. It is demonstrated that N is better kept in the divertor region than Ne in semi-detached and detached divertor conditions due to smaller first ionization potential (FIP effect). Together with the fact that Ne radiates more efficiently at higher plasma temperatures, this leads to an increase in the efficiency of Ne for divertor heat load control with increasing machine size. In larger machines such as JET and ITER Ne can be as efficient a radiator as N while for ASDEX-Upgrade Ne easily leads to radiation from the pedestal and loss of H-mode stability. The relative roles of various physical effects are compared for the three tokamaks based on both whole databases and in more details for chosen semi-detached regimes with comparable fraction of radiated power. It is shown that for smaller machines drift effects are more significant and divertor asymmetries more pronounced.

A problem of a flush-mounted probe in a fully ionized magnetized plasma (such as SOL plasma) is treated analytically and numerically for arbitrary probe size. The model describes the structure of electric field and particle fluxes, as well as the transport processes, in a plasma zone disturbed by the probe. The I-V characteristics and the structure of return current to the electrode are derived on this basis.

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