Helium defect interactions in vanadium and beryllium

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Abstract
Thermal helium desorption experiments have been used to study the interaction of helium with trapping sites in the candidate fusion reactor materials vanadium and beryllium. The results concerning vanadium indicate that impurities play an important role in the behaviour of helium vacancy complexes. The impurities (probably oxygen) decorate the vacancies and cause helium to desorb at relatively low temperatures. Helium capture by vacancy type defects and nucleation of helium vacancy clusters is suppressed at irradiation temperatures higher than 550 K. The impurities seem to invoke vanadium to an anomalous behaviour compared to other BCC-materials e.g. Mo and W. Helium escaping from vacancy type defects in vanadium at 0.25 Tm (Tm =melting temperature) is one of these features, whereas in Mo and W release takes place at 0.4 T m . Some of the vacancies were observed to migrate at temperatures around 200 K, but most of the vacancy migration was delayed until 450 K, apparently by interactions with impurities. Impurity decorated vacancies produced by 1 keV helium injection could be removed by self-interstitials introduced into the vanadium by 50 eV argon bombardment. Results of neon, argon and krypton desorption experiments show also for these gases anomalous behaviour. Helium and argon desorption experiments in beryllium show helium and argon release by thermal vacancy assisted diffusion.