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

Abstract

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.

Castellated structures for ITER: Differences of impurity deposition and fuel accumulation in the toroidal and poloidal gaps

“…Modeling attempts to simulate the observed deposition pattern using the same assumptions as reported in [15] resulted in partial success only: a rough qualitative agreement between modeling and experimental data was achieved with a carbon atom reflection coefficient of 50%.…”

“…Runs were made with a simple particle reflection model, as was used in [15] for three particle reflection coefficients R=0, R=0.5 and R=0.9. 30.000 carbon atoms were started from the three source locations shown in the Fig.4b for the two different geometries.…”

“…Dedicated studies addressing the performance of castellated structures under plasma impact are ongoing on several tokamaks [10][11][12]. Series of experimental investigations with ITER-like castellated samples were performed in TEXTOR along with dedicated modeling to address the issues of carbon transport and fuel accumulation in the gaps [13][14][15] and on molten layer formation and its impact on the performance of castellation [16]. In a recent study [17] an attempt was made to optimize the shape of the castellation cells in order to minimize the carbon transport and fuel deposition in the gaps and the first comparative results were presented.…”

Investigations of castellated structures for ITER: The effect of castellation shaping and alignment on fuel retention and impurity deposition in gaps

“…Earlier it was noted that simple processes of particle reflection from the gap walls might satisfactorily reproduce experimental deposition patterns [6]. The new findings such as deposition at the bottom of the gaps showed that the modeling algorithms must be improved.…”

Formation of deuterium–carbon inventories in gaps of plasma facing components

“…In DIII-D a heatable sample was exposed to a detached divertor plasma to study the dependence of the gap deposition as function of material temperature. The derived growth rates together with results from the similar experiments in TEXTOR [9,10] can be finally used to obtain an estimate for the growth rate of the ITER in-vessel T-inventory due to deposition inside the gaps of the first wall.…”