Helium trapping in aluminium near the critical dose on blister formation

Blistering and flaking caused by energetic He ions emitted from the plasma in fusion reactors possibly contribute to first-wall erosion. In order to study their characteristics, the numbers of He atoms trapped in He-ion-irradiated Al samples have been measured by a He atom measurement system and every sample has been observed by a scanning electron microscope. The samples have been prepared from a polycrystalline plate and irradiated with 20 keV He ions at room temperature. The saw-tooth like variation of the trapped He atoms with the dose has three edges corresponding to the blistering, flaking and double flaking, respectively. The critical doses for the three events are found to be 4 x 1021, 7 x 1021, 12 x 1021 He atoms m-2, respectively. The average number of He atoms included in an event is 5.4 x 1010 He atoms in the case of the blistering and 2.1 x 1011 He atoms in the case of flaking. (orig.)