Carbon transport, deposition and fuel accumulation in castellated structures exposed in TEXTOR

In order to maintain the thermo-mechanical durability of ITER it is proposed to castellate the interior surface of the first wall and divertor by splitting them into small-size cells [W. Daener et al., Fusion Eng. Des. 61&62 (2002) 61]. A concern is the accumulation of fuel in the gaps of the castellation. In TEXTOR, molybdenum limiters were exposed in the scrape-off layer (SOL) plasma to assess fuel accumulation. The first limiter was exposed under deposition-dominated conditions. Carbon deposits were formed both on top surfaces and in the gaps. About 0.12% of the impinging D-fluence was found in the gaps. Another castellated limiter was exposed under erosion-dominated conditions. Deposited layers were found only on the plasma shadowed areas of the gaps. A significant amount of molybdenum from the limiter was found intermixed in the deposit. The gaps contained ∼0.03% of the impinging D-fluence. Modeling was performed to simulate carbon transport into the gaps.