Defect evolution in tungsten exposed to helium plasma and deuterium plasmas studied by slow positron beam

Comparison of defect formation in tungsten samples exposed only to a helium plasma and those exposed to both helium plasma and subsequent deuterium plasmas is performed via Doppler broadening spectroscopy in the positron annihilation technique (DBS-PA). The S parameters of tungsten samples after helium plasma exposure increased with the increase of the incident fluence, which indicates that low energy helium plasma could induce the formation of defects such as nano-sized helium bubbles and dislocations in tungsten. The S parameters of samples exposed to helium incident fluence of 7.9 x 10(21) He/m(2) enhanced after deuterium plasmas exposure, indicating that low energy deuterium plasmas could form defects in tungsten too. Unexpectedly, deuterium plasmas exposure did not significantly influence the S parameters of the tungsten sample with helium incident fluence of 3.9 x 10(22) He/m(2), and reduced the S parameters of the tungsten sample with helium incident fluence of 7.9 x 10(22) He/m(2). These variations of S parameters could be associated with the occupation of nano-sized helium bubbles by deuterium atoms. This indicates possible competition between the increased density of vacancy-type defects by deuterium plasmas exposure and the filling of nano-sized helium bubbles by deuterium atoms, reducing detectability of these defects.