Transmission electron microscopy of redeposition layers on graphite tiles used for open divertor armor of JT-60

Transmission electron microscopy, selected area electron diffraction and SEM studies were made on redeposition layers on graphite tiles removed from the lower-X-point divertor of JT-60. The observations were made, at poloidal and/or toroidal sections, in the inboard side zone of the inner-separatrix strike point. Top-surface layers of 0–6 μm-depth were correlated to the last 30-shots of the 2000-shots in the 1988 experimental campaign. In those shots, both limiter- and divertor-discharges were made with Ip below 2 MA and with neutral-beam-injection power below 25 MW. Graphene-Mo/Ti, or -Ni/Fe/Cr/Ti particulates co-deposition layers were attributed to disruptions. Columnar structures found at 20–30 mm from the strike point corresponded to divertor-discharges below 11 MW, while lamellar structures were attributed to the higher power divertor-discharges or to limiter-discharges. Columnar structures were ascribed to both low adatom-migration due to the lower redeposition temperature and the inclined incidence of carbon impurities.