DEUTERIUM RETENTION IN N-SEEDED AND UNSEEDED TUNGSTEN LAYERS OBTAINED WITH DCMS AND M-HIPIMS TECHNIQUE

Abstract

The retention of hydrogen isotopes in tungsten co-deposited layers is a subject severely understudied in the literature compared to hydrogen isotope implantation in bulk tungsten specimens. However, impurity seeding in future next-generation fusion devices can reduce the sputtering threshold of tungsten and potentially increase the contribution of the tungsten layers to the global tritium retention problem. This paper reports on the development of fusion-relevant tungsten-deuterium and nitrogen-seeded tungsten deuterium layers using direct current (DCMS) and multipulse high-power impulse magnetron sputtering techniques (HiPIMS). The four layers resulting from co-deposition in argon-deuterium and argon-deuterium-nitrogen atmosphere were investigated from the deuterium retention and release point of view. Supplementary the crystalline structure of all samples and particularly the composition of the nitrogen-seeded layers were also studied. The tungsten-deuterium layers produced using m-HiPIMS technology showed a higher crystalline coherence compared to layers deposited with DCMS in part owned to the enhanced ion bombardment of the forming layers during deposition. On the other hand, the nitrogen-seeded layers showed without exception the formation of the tungsten-nitride compound. Here on the contrary the crystalline coherence as revealed by X-ray diffraction measurements measurement showed that nitride formation is favored for DCMS compared with m-HiPIMS. The elemental areal densities of the layer constituents were analyzed using Rutherford Backscattering Spectrometry. In this case, only the two nitrogen-seeded layers were analyzed revealing that a higher nitrogen content by a factor of ~2 is retained in DCMS deposited layer compared to HiPIMS.The release of deuterium was higher in the case of m-HiPIMS layers and it was shown that despite the deposition method the release behavior occurs predominantly at temperatures above 800 K suggesting a high energy trapping of deuterium in tungsten layers. Nitrogen seeding influences both the retention mechanisms and increases the deuterium retention by two orders of magnitude in seeded layers compared to unseeded ones.