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.
One important recent trend in deposition technology is the continuous expansion of available processes towards higher ion assistance with the subsequent beneficial effects to film properties. Nowadays, a multitude of processes, including laser ablation and deposition, vacuum arc deposition, ion assisted deposition, high power impulse magnetron sputtering and plasma immersion ion implantation, are available. However, there are obstacles to overcome in all technologies, including line-of-sight processes, particle contaminations and low growth rates, which lead to ongoing process refinements and development of new methods. Concerning the deposited thin films, control of energetic ion bombardment leads to improved adhesion, reduced substrate temperatures, control of intrinsic stress within the films as well as adjustment of surface texture, phase formation and nanotopography. This review illustrates recent trends for both areas; plasma process and solid state surface processes.
Expanded austenite, formed after nitrogen plasma immersion ion implantation or low energy nitriding of austenitic stainless, is characterized by a high nitrogen content CN of up to 20 at. % and an unusual fast diffusion, which in general cannot be described using a single diffusion coefficient. Here, the concentration dependent diffusivity is calculated for several experimental parameters and steel alloys. Two mathematical simplifications of the general diffusion theory, well justified for physical reasons, helped in solving the equations. First, a constant surface concentration was assumed, despite a constant nitrogen flux into the surface, and, second, only mobile nitrogen atoms in a stationary steel matrix were considered. Thus, it was possible to solve the Boltzmann–Matano equation and obtain the concentration dependent diffusion coefficient D(CN). In all cases, a step-like behavior, with a high value for high nitrogen contents and a low value for low ones, is found, with the transition point between a nitrogen concentration of 5 and 17 at. %, depending on the sample.
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