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Description
To ensure an operating regime compatible with its tungsten divertor, ITER must operate at least in a partially detached state. Considering the required level of power crossing the separatrix ($P_{SOL}$) to sustain H-mode, such a partially detached state will be achieved through strong radiative power losses obtained via extrinsic (seeded) impurities. However, impurities can also influence plasma behaviour through additional mechanisms, such as modifying transport properties or affecting neutral penetration.
To quantify these effects, edge modelling has been performed for three JET pulses with and without neon seeding of the JET ITER-baseline scenario, characterized by high plasma current ($Ip=2.5\ MA$), input power ($P_{in}=27-33\ MW$) and triangularity ($\delta=0.38$) and a closed vertical target divertor configuration (VV). Using the Soledge2D and EDGE2D codes, transport profiles have been estimated in a comparative way, and by carefully matching outer-midplane kinetic profiles (electron density and electron/ion temperatures), Langmuir probe target profiles and total radiated power. Particle and heat fluxes from TRANSP interpretative simulations have been used to provide core boundary conditions for the edge model at top-of-pedestal region.
The results show that whilst increasing the seeding level an increase in particle diffusivity is required throughout the entire region between the core boundary and the far SOL to match radial (density) profiles. Conversely, the electron heat diffusivity decreases across the same region, except for a localized increase near the top of the pedestal. In all cases, reproducing the measured ion temperature at the separatrix requires the ion heat diffusivity to remain very low, likely close to its neoclassical value, in the near- and far-SOL.
To assess the impact of seeding on neutral dynamics, the estimated radial transport coefficient profiles were applied for cases with varying impurity concentrations corresponding to different levels of radiative dissipation. The results indicate that, with all other parameters held constant, the electron density at the separatrix decreases as the impurity concentration and total radiation increase as previously observed in (partially) detached conditions. This behavior results from the combined effect of reduced free energy available for ionization (the so-called power-starvation effect) and the different distribution of electrons originating from deuterium and impurity atoms. At constant pressure, as the available power decreases, the temperature drops while the electron density in the divertor region increases, reducing deuterium ionization and neutral penetration. Electrons originating from impurities only partially compensate for the loss of deuterium-derived electrons at the separatrix.