Possibility of underestimation on sputtering yield of graphite and tungsten PFCs

Full text: In general, sputtering yield models for graphite and tungsten plasma-facing components (PFCs) to estimate the PFC lifetime and impurity influx in fusion devices has been developed without the morphological changes of surface which are induced by the deuterium plasma interaction with the PFCs. Previous studies on the morphological change of PFCs have been reported such as conical tip formation on graphite and blister formation on tungsten. Since the sputtering yield is increased with the incidence angle of ion, it is worth to study the effect of morphological changes of PFCs on the sputtering yield. In this study, it is focused on the mechanism of morphological changes of the graphite and tungsten PFCs by the deuterium plasma interactions and, in addition, it is observed that the increment of sputtering yields. Experiments were carried out with electron cyclotron resonance deuterium plasma and DC biased tungsten and graphite targets for the simulation of fusion edge plasmas. Sputtering yield is monitored by using the measurement of weight loss. For the graphite PFC, the formation of carbon-deuterium bonds by chemical reaction interacts with the sheath electric field results in formation of the conical tips on the graphite. Conical tip formation on the graphite increases the sputtering yield due to increase of local angle of ion incidence and additional energy transfer from backscattered ion. With the consideration of the morphological change of graphite, the sputtering yield is increased about 100% from the estimation of Roth model assuming the plane surface. For the tungsten PFC, deuterium ion irradiation generates the blister formation on the tungsten surface. If imagines the dome shaped blister, the sputtering is enhanced around the dome where the local angle of ion incidence is maximum. Notes that this side of dome is mechanically weaken due to plastic deformation during the blister formation of tungsten. The sputtering of blister edge may be enhanced due to morphological change and eventually top plate of blister is ejected and the small sized dusts can be generated. It results in the local increment of sputtering yield of tungsten PFC. Results suggest that the estimation of sputtering yield of PFCs interacted with the scrape-off layer (SOL) deuterium plasma requires the possibility of its enhancement due to the morphological changes of surface. (author)