Rapid diffusion of lithium into bulk graphite in lithium conditioning

One of the known lithium conditioning effects is the reduction of carbon impurities released from the graphite walls of a tokamak. However, little is known about the role of lithium in graphite–plasma interactions. Graphite intercalates lithium atoms between the hexagonal layer planes. The Li diffusion constant in the direction perpendicular to the basal planes of highly oriented pyrolytic graphite (HOPG) is measured for the first time using Rutherford backscattering spectrometry and is found to be D⊥=2.1×10−10 cm2/s with an activation energy of 0.26 eV, in the case of low temperatures (300 K) and low Li concentration (Li/C atomic ratio <3%). This value increases by a factor of two when HOPG is exposed to hydrogen plasma before lithium deposition. The implications resulting from the Li–graphite intercalation in lithium wall conditioning are discussed, focusing on Li-induced suppression of graphite sputtering.