The investigation of structure, chemical composition, hydrogen isotope trapping and release processes in deposition layers on graphite sample surfaces exposed to DIII-D divertor plasma in extreme conditions

Graphite components, employed for the protection of the tokamak first wall, limiters and divertors, can be exposed to extremely high loads both during plasma disruptions and due to installation irregularities. Tile misalignments lead to leading edges that are heated under effect of extremely high plasma parallel thermal fluxes. Previously, a special sample design using DIMES mechanism has been described, in which the graphite sample was arranged to receive the parallel heat flux on a small surface area exposed to plasma from the outer strike point of a DIII-D discharge. In this work, the sample behavior was investigated during exposure to 12 s of plasma (3 s in each of four sequential discharges). The average heat flux onto the sample surface during one discharge was about 500 W/cm2 and the parallel heat flux ∼15 kW/cm2. The deposited coatings on the sample and a silicon collector are composed of pure carbon and by visual observation can be divided into several bands. The first band on the silicon collector is a region of intense carbon deposition, which is centered directly under the region of intensive plasma exposure and consists of great many irregularly formed grains. The average deposition thickness is ∼50–60 μm, but in some areas it is up to 140–150 μm. The second band is located toroidally upstream of the first along the graphite sample slot and has globular structure. The coating thickness in this band varies from 18 μm on the first band boundary to 10 μm on the silicon collector edge. The third band of background deposition is located downstream of the first band and is an amorphous carbon layer of thickness ∼4–5 μm.