Effect of cyclic extremely high loads on behaviour of the ATJ graphite sample exposed to DIII-D divertor plasma

Graphite elements, destined for the protection of the tokamak first wall, divertors and limiters, can be exposed to extremal loads during plasma disruptions and owing to installation irregularities. Tileʹs leading edges are heated under effect of extremely high plasma parallel thermal fluxes. Behaviour of graphite sample, that was arranged to receive the parallel heat flux on a small surface area exposed to DIII-D divertor plasma, in extreme conditions of cyclic loading is analyzed. ATJ graphite sample was exposed to plasma during 3 s in each of four sequential discharges. The average heat flux on the sample surface was about 500 W/cm2 and the parallel heat flux ∼15 kW/cm2. The surface topography in various areas of the graphite sample after plasma influence has a rather various form and character. On the graphite sample surface it is possible conditionally to select nine characteristic bands and areas. As a result of intensive erosion under plasma thermal flux effect the leading edge surface represents a continuous solid destruction zone by depth from 0.8 mm and up to 2 mm in the central part, that exceeds 1 mm ledge size. The destruction zone extends at different angles to a horizontal plane and on front for various width, reaching in the centre of erosion zone of almost outside leading edge, i.e. about 5 mm. The total volume of ablated material from erosion band is ∼17–18 mm3.