Systematic investigation of the formation behavior of helium bubbles in tungsten

The formation of He bubbles in tungsten under exposure to a He plasma was systematically investigated using low energy (∼50 eV) He+ ions with a wide fluence range (∼1023 to 1026 m−2) in the linear divertor plasma simulator PISCES-A at several temperatures (523-973 K). TEM observations after thinning exposed W samples with FIB revealed that the layer thickness (>30 nm) of He bubbles largely exceeds the ion implantation range of a few nm. The size of He bubbles was found to increase with an increase in the sample temperature: it was around 10 nm at 973 K, while only small He bubbles of 1-2 nm were observed at <773 K. In addition, to obtain information on the initial formation behavior, in-situ TEM observations during He ion irradiation were also performed.