Study of the temperature-dependent nitrogen retention in tungsten surfaces using X-ray photoelectron spectroscopy

Nitrogen is foreseen as seeding species in future magnetic confinement fusion reactors in order to reduce the power load from the plasma onto the divertor target tiles by radiative cooling. As a side effect it also gets implanted into the tungsten wall and forms tungsten nitrides (WxN). The temperature-dependent WxN formation was investigated in dedicated laboratory experiments. N ions of 300 eV kinetic energy were implanted into W samples under ultra-high vacuum conditions in the temperature range 300 K to 800 K. The N retention in W was monitored and quantitatively analysed by X-ray photoelectron spectroscopy (XPS). A method to calculate the statistical error of the measured data based on Bayesian statistics was developed. Argon sputter depth profiling was combined with XPS to measure N in W depth profiles which were compared with simulated N implantation profiles. Annealing of samples implanted with N at 300 K does not cause a loss of N up to 800 K. However, the retained N amount decreases linearly with increasing implantation temperature. It was found that this reduction is due to ion-irradiation-induced N release at elevated temperatures. Over the whole temperature range N diffusion into depth was not observed. N accumulation measurements showed no evidence for a phase transition in the WxN layer. However, high resolution XPS measurements revealed that below 600 K a second photoelectron peak occurs in the N 1s signal which can be attributed to different local atomic arrangements of WxN.