Physics conclusions in support of ITER W divertor monoblock shaping

TLDR

The ITER tungsten divertor’s design hinges on monoblock front surface shaping, constrained by a 0.3 mm maximum radial step between adjacent blocks in high‑heat‑flux target areas. The study aimed to strengthen the physics basis for monoblock shaping decisions by coordinating experiments and simulations on edge power loading and melt dynamics. The authors used coordinated experiments and simulations, assuming optical projection of heat loads and magnetic shadowing, to compare shaped versus unshaped monoblock surfaces under stationary and transient loads with and without melting, assessing the impact of shaping on melt dynamics. The study confirmed that the geometric approximation for leading‑edge power loading is valid and recommends monoblock top‑surface shaping to shadow poloidal gap edges, thereby mitigating high‑heat‑flux loads.

Abstract

The key remaining physics design issue for the ITER tungsten (W) divertor is the question of monoblock (MB) front surface shaping in the high heat flux target areas of the actively cooled targets. Engineering tolerance specifications impose a challenging maximum radial step between toroidally adjacent MBs of 0.3 mm. Assuming optical projection of the parallel heat loads, magnetic shadowing of these edges is required if quasi-steady state melting is to be avoided under certain conditions during burning plasma operation and transiently during edge localized mode (ELM) or disruption induced power loading. An experiment on JET in 2013 designed to investigate the consequences of transient W edge melting on ITER, found significant deficits in the edge power loads expected on the basis of simple geometric arguments, throwing doubt on the understanding of edge loading at glancing field line angles. As a result, a coordinated multi-experiment and simulation effort was initiated via the International Tokamak Physics Activity (ITPA) and through ITER contracts, aimed at improving the physics basis supporting a MB shaping decision from the point of view both of edge power loading and melt dynamics. This paper reports on the outcome of this activity, concluding first that the geometrical approximation for leading edge power loading on radially misaligned poloidal leading edges is indeed valid. On this basis, the behaviour of shaped and unshaped monoblock surfaces under stationary and transient loads, with and without melting, is compared in order to examine the consequences of melting, or power overload in context of the benefit, or not, of shaping. The paper concludes that MB top surface shaping is recommended to shadow poloidal gap edges in the high heat flux areas of the ITER divertor targets.