Plasma surface interactions in impurity seeded plasmas

With tokamak devices developing towards higher heating powers, and carbon plasma facing components being increasingly replaced by high-Z materials like tungsten, impurity seeding for radiative power dissipation gains more importance. This review summarizes the core and divertor radiative characteristics of potential seeding species, namely noble gases and nitrogen. Due to its radiative capability below 10 eV, nitrogen turns out to be a suitable replacement for carbon as a divertor radiator. For typical plasma parameters and high radiation levels, it becomes the most important eroding species for high-Z plasma facing components. Nitrogen exhibits pronounced storage in near-surface tungsten layers in an about 1:1 W/N atomic ratio, which may effect W sputtering. While the inter-ELM erosion of tungsten can be almost completely eliminated by electron temperature reduction, type-I ELMs remain an effective sputtering source. Since a large ELM cannot be significantly ameliorated by radiation, impurity seeding has to be integrated with a benign ELM scenario, like the type-III ELMy H-mode or active ELM control by pellets or resonant magnetic perturbations.