Effects of impurities on deuterium retention in single crystal tungsten

The role of impurities in the retention of deuterium in single crystal tungsten implanted at room temperature has been investigated. To avoid the effects of air exposure, the D+ implantation and the subsequent D retention measurement using thermal desorption spectroscopy were performed in the same vacuum system without breaking vacuum. For fluences of 1 x 1023 D/m2 prominent desorption peaks were observed during TDS, one at ∼400 K and one at ∼600 K. The use of liquid nitrogen cooling of the ion beam aperture led to a reduction of the background gas impurities (O and C containing molecules) in the vacuum chamber, which consequently almost completely eliminated the post-irradiation TDS peak at ∼400 K. Reduction of these gas impurities by ∼50% has led to a reduction of ∼20% in the total deuterium retention. Furthermore, weakly bound deuterium in the W specimens was released when the vacuum system was mildly baked to ∼360 K before TDS, resulting in an approximately 50% reduction in the total amount of retained D. It was also found that post-implantation time delay before performing TDS leads to a significant loss of the implanted deuterium; ∼25% of the implanted deuterium escapes from the W specimens during the first 24 hours and ∼55% after about eight weeks.