Fuel inventory and impurity deposition in castellated tungsten tiles in KSTAR: experiment and modelling

Abstract

Plasma-facing components have castellated structure for thermo-mechanical durability and integrity under high heat flux loads. However, fuel co-deposition in the grooves of the castellation may enhance fuel retention. In KSTAR, castellated tungsten tiles were tested to investigate the impact of tile shaping and misalignment on the retention. The tiles with poloidal and toroidal gaps of 0.5 mm were exposed at the divertor during a whole campaign encompassing 4364 s of plasma operation. Surfaces inside the gaps were analysed by means of 3He-based micro-NRA, ERDA and PIXE. Modelling of carbon deposition was performed with the impurity transport code 3D-GAPS assuming impurity penetration along the magnetic field lines with plasma-wetted areas defined by simple geometrical shadowing. The main deposited element is carbon with different concentration at the entrance of the groove, dependent on the tile shaping: 6 × 1017 cm−2 for a chamfered and misaligned gap and up to 283 × 1017 cm−2 for a flat and aligned gap. The deposition patterns are exponentially decreased to 4–10 × 1016 cm−2 inside the gap. Deuterium concentration in the gaps described above ranges, respectively, from 2 × 1017 cm−2 to 50 × 1017 cm−2 at the top of the groove and decreases to 1–4 × 1016 cm−2 following the carbon deposition trends. The highest carbon and deuterium densities are measured at the plasma-exposed side of the flat tile and aligned gap. Modelled deposition profiles reproduce qualitatively the experimentally observed trends.