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

All plasma-facing components (PFC) in ITER will be castellated, i.e. composed of small blocks separated by narrow grooves (∝ 0.5 mm) in order to reduce thermally-induced stress. Narrow grooves of castellation are considered as a potential trap for species migrating to these gaps. Therefore, co-deposition of fuel together with material eroded from the wall may be decisive for the overall tritium inventory in a device with a huge number (over one million in ITER) of grooves. This calls for detailed studies of castellated structures exposed to the plasma for long- or short-term in present-day tokamaks. A castellated tungsten test limiter (macro-brush structure) was exposed to plasma discharges in the TEXTOR tokamak operated with graphite main limiters and an Inconel liner. The limiter was composed of twelve detachable segments, each brased to a copper base. Dismantling of the segments enabled the analysis of surfaces inside the castellation. The emphasis was on the determination of: (i) deposition and fuel retention on the plasma-facing surface and in the gaps and (ii) material mixing and new compound formation inside the castellated structure. The study performed by means of nuclear reaction analysis (NRA), Rutherford backscattering spectroscopy (RBS), electron microscopy, X-ray diffraction (XRD) and other methods has brought several essential results. (a) Deuterium retention on plasma-facing surfaces and in the castellation of metal PFC is strongly related to the co-deposition with carbon; (b) Both carbon and deuterium are detected only in narrow belt, a few mm broad, down the gap with the decay length of about 1 -1.5 mm; (c) The presence of copper droplets and tungsten oxide has been identified in the gaps of the macro-brush limiter. This is probably the first-ever identification of tungsten oxide on PFC. Different pathways leading to the oxide formation are considered. The implications of these results for a long pulse operation of a tokamak with castellated wall components are discussed. (orig.)