The implications of high-Z first-wall materials on noble gas wall recycling

Recent experiments in ASDEX Upgrade (AUG) have experienced surprisingly high He plasma impurity concentrations. Such high He concentrations have not been observed with. C walls, they were only observed since the increase in the W first-wall coverage of ASDEX Upgrade to 85%. The high He plasma concentration appears to be linked to the fraction of W surfaces open to plasma contact that are not covered by boronization layers and to the number of He glow discharges performed for wall conditioning prior to normal plasma operation. This points to the different retention and release properties of W and C for He. To elucidate these differences dedicated laboratory experiments have been performed. To study the retention, different types of W and C targets, including those used in ASDEX Upgrade, were implanted with He-3 at 200 and 600 eV and the amount of retained He was determined through thermal effusion spectroscopy (TES) and ion beam analysis methods. These experiments showed that W can retain up to 10 times more He than C depending on the energy of the implanted He-3. The TES measurement showed that both W and C start to out gas He at 400 K which is not reached at the first-wall in AUG. Therefore 3 the differences in the release of He from W and C surfaces due to particle bombardment was investigated. He implanted W and C substrates were exposed to He-4 or H-2 plasmas and the loss of He-3 was measured. For an H-2 plasma and 100 eV bias, three times more He is released from W than from C. For He-4 and 100 eV bias between 2 and 10 times more He-3 was released from W than from C depending on the type of W and C. The release rate of He from W and C through particle induced release was found to be equal. Therefore the stronger release of He from W can be explained by the higher retention of He in W.