Nanostructure formation and D retention in redeposited-like W exposed to linear plasmas

Different kinds of W layers resembling the ones found after campaigns in tokamak were produced by Pulsed Laser Deposition: namely nanocrystalline, amorphous and porous W layers. Films were exposed to divertor relevant D plasma, in PSI-2; D retention as well as nanostructure formation were investigated. For nanocrystalline W films we found lamellar structures that coalesce with increasing D fluence. Instead, on amorphous W no lamellas were formed but a new random string-like shape. A higher oxygen content in the W layer results in an evolution of the shape of the nanostructures in straight parallel lines with acute angles. The presence of a porous structure morphology hinders the formation of surface nanostructures. Reviewing results from different linear devices including also bulk W, we observe that, when formed, nanostructures appear in general with a fluence threshold of 3-5 × 1025 D/m2 regardless the impinging D flux. D retention shows an unusual trend increasing fluence and is enhanced by the presence of amorphous structure and open morphology. Amorphous W films exhibit higher D retention (3 orders of magnitude) compared to crystalline W. When the amorphous W is annealed, retention returns to the standard values of bulk W regardless the presence of a layered structure parallel to the substrate. Porous W, thanks to the high surface to volume ratio and the presence of void distribution along the growth direction, favors higher recycling and thus limits the D uptake during exposure. The presence of O, in the investigated coatings, seems to have little effect on D retention.