Helium retention of plasma facing materials

The helium retention properties such as retained amount, desorption temperature and activation energy of desorption were examined for graphite, B4C, SiC and tungsten. After the helium ion irradiation with a helium energy of 5 keV in an ECR ion source, these data were obtained by using a technique of thermal desorption spectroscopy. The amount of retained helium saturated at the fluence higher than (0.5-3)×1022 He/m2 for every material. The maximum retained amount was (1-5)×1021 He/m2. The maximum helium concentration in the implanted depth was compared with that of hydrogen similarly obtained. The atomic ratio, He/H, was 0.3 for graphite, 0.15 for B4C, 0.3 for SiC and 0.3 for tungsten. It was seen that a considerable large amount of helium was trapped in the plasma facing materials. The desorption temperatures were 570 K for graphite, 570 and 1200 K for B4C, 570 and 1140 K for SiC and 770 K for tungsten. The helium retention can be reduced by baking with a temperature of 600 K or 800 K for graphite or tungsten, respectively. However, such reduction becomes difficult for SiC or B4C, since the desorption temperature is very high. Thus, the effective shielding of the scrape-off layer in a diverted plasma has to work sufficiently to avoid the back flow of helium into the core plasma, when the helium emits during discharges.