Castellated structures for ITER: the influence of the shape of castellation on the impurity deposition and fuel accumulation in gaps

Castellated structures are proposed for the first wall and divertor area in ITER. However, there is a concern that impurities and fuel will be deposited in the gaps of castellation leading to the undesirable tritium inventory. Optimization of the shape of cells in the castellated structures appears to be a possibility to reduce the impurity transport and fuel accumulation.A new tungsten castellated limiter has been exposed in TEXTOR. The castellation cells of the rectangular and of the roof-like geometries were exposed under the same plasma conditions, allowing a direct comparison of the carbon transport and fuel inventory in the gaps. The gaps of shaped cells contained a factor of 2–3 less carbon and deuterium, in comparison to those of rectangular cells.