Implantation and erosion of nitrogen in tungsten

Nitrogen puffing is routinely applied in nuclear fusion plasma experiments with tungsten walls to control the amount of power emitted from the plasma by radiation. However, as nitrogen is retained in significant amounts in tungsten it adds some complexity to the plasma-wall interaction. Basic questions concerning the interaction of nitrogen with tungsten, namely the energy and temperature dependent retention of nitrogen implanted into tungsten and the erosion of the formed tungsten nitride by deuterium, are still open. To address these questions, laboratory experiments with a mass-filtered ion source and sample analysis with in situ x-ray photoelectron spectroscopy (XPS) and nuclear reaction analysis were performed. The results of the implantation and erosion measurements were interpreted by means of simulations with a Monte-Carlo code describing the interaction of energetic particles with matter in the binary collision approximation. This required the development of a forward calculation, converting the simulated depth profiles into XPS intensity ratios. With appropriate settings, the experimental implantation and erosion results at ambient temperature are well described by the simulations. However, for increased temperatures it has been observed that there is an unexpected difference between implanting nitrogen into tungsten before heating the sample and implantation into a heated sample. The application of the developed forward calculation is not limited to the problems presented in this work but can be applied especially to all kind of XPS sputter-depth profiling measurements. Finally, simulations with the previously validated Monte-Carlo code are used to extrapolate the presented results on nitrogen retention to energies and particle compositions relevant for fusion experiments. These simulations make quantitative predictions on nitrogen retention in tungsten and on relevant time scales. The simulations also show that recoil implantation of nitrogen by deuterium significantly increases the effective implantation depth of nitrogen.