Interaction of nitrogen plasmas with tungsten

Abstract

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.

Influence of nitrogen pre-implantation on deuterium retention in tungsten

“…Nitrogen plasma exposure was performed with a working pressure of 0.25 Pa applying a gas flow of 50 sccm N 2 gas. For these conditions it was shown that N 2 + is the dominant ion species [6,7]. It contributes about 85 % to the total ion flux with minor contributions from N 3 + and N + ions ( 5-10 % each) [7].…”

“…For these conditions it was shown that N 2 + is the dominant ion species [6,7]. It contributes about 85 % to the total ion flux with minor contributions from N 3 + and N + ions ( 5-10 % each) [7]. The ion flux of the main species was about 410 18 N 2 + /m 2 s [7], which resulted in a total N fluence of about 1.510 22 N/m 2 for 30 min plasma exposure.…”

Comparing deuterium retention in tungsten films measured by temperature programmed desorption and nuclear reaction analysis

“…Such copper interlayers have been used earlier in our laboratory to avoid peeling of magnetron-sputtered W films [38]. Fig.…”

Nitrogen retention in ASDEX Upgrade

“…Different from hydrogen or noble gases, N is retained in the walls by forming compounds such as tungsten nitrides [3]. On the other hand, N does not form layers on top of the original wall material and the diffusion of N in tungsten is negligible below 800 K [4].…”