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Abstract.The use of nitrogen seeding to reduce the edge plasma temperature has recently been successfully applied in ASDEX Upgrade. While the plasma performance was significantly improved compared to other seeding species like Ar or Ne, questions remained as to the interaction of nitrogen with a tungsten first wall. In particular the formation of thick tungsten nitride layers with reduced melting temperature and increased physical sputtering was a concern. Therefore dedicated laboratory experiments have been performed to investigate the interaction of W surfaces with N plasmas. Tungsten coated Si samples were exposed to N ions from plasma and ion gun sources at energies from 20 eV to 10 keV and W surface temperatures from 300 to 750 K. After exposure to the N plasma with fluences of up to several 10 23 N m 2 the N content in the samples was measured by nuclear reaction analysis. The sputter erosion was determined by measuring the thickness change of the W layer by RBS. The formation of W nitride phases was investigated in separate XPS experiments where the samples were implanted in situ with keV N ions. It was found that only very small fractions of N are accumulated in the W surface and that N is bound in a nitride state. At temperatures above 600 K the nitrides are no longer stable which further reduces the N uptake into the W. Moreover the accumulation of N in the surface leads to a decrease in W physical sputtering due to the lower W concentration at the surface.

The evolution of carbon/boron deposition and the deuterium inventory were determined during the transition from a carbon dominated to a full tungsten ASDEX Upgrade. In the carbon dominated machine about 17 g of carbon were deposited at the inner divertor and in remote areas during one standard discharge campaign. Main carbon sources were the ICRH antennae protection limiters in the main chamber. After coating these limiters with tungsten the carbon deposition decreased to 35 g. The remaining carbon originated mainly from erosion at the outer divertor strike point. Transition to a full tungsten machine resulted in a further decrease of the carbon deposition to about 1 g. 1.31.7 g deuterium was trapped in codeposited carbon/boron layers in the divertor and in remote areas during the carbon dominated campaigns. The deuterium inventory decreased to 0.140.22 g in the full tungsten machine.

The formation of endothermic carbides on Fe and Ni is studied using X-ray photoelectron spectroscopy (XPS) by deposition of carbon films from the vapor phase and subsequent annealing steps. The reaction between carbon and metal substrates is measured by shifts in the C 1s photoelectron peaks. By comparison with two elementary carbon photoelectron energies determined from carbon films on unreactive Au substrates, a carbide peak in the C 1s spectra on reactive Fe and Ni substrates is identified. The carbides formed after deposition of carbon films at room temperature are located at the interface between carbon film and metal substrate. We report on the behavior of elementary carbon films with respect to film thickness and thermal treatment leading to carbide formation and carbon diffusion into the bulk.

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