An overview of fuel retention and morphology in a castellated tungsten limiter

A castellated tungsten test limiter composed of detachable segments was exposed to plasma discharges in the TEXTOR tokamak operated with graphite main limiters. Dismantling of the limiter enabled the analysis of surfaces located inside the castellation. The emphasis was on the determination of: (i) deposition and fuel retention; (ii) material mixing and new compound formation on plasma-facing surfaces and in the grooves of castellation. The investigation performed by means of accelerator-based ion beam analysis methods, microscopy and X-ray diffraction has brought several essential results: (i) deuterium retention on plasma-facing surfaces and in the castellation of metal PFC is strongly related to the co-deposition with carbon; (ii) both carbon and deuterium are detected only in narrow belts, a few millimetre broad, down the gap with the decay length of around 1.2–1.8 mm; (iii) the presence of copper droplets and tungsten oxide (WO2) has been identified in the gaps. Different pathways leading to the oxide formation are considered.