ELM ion energies in the ASDEX Upgrade far scrape-off layer

1. Introduction. Type I ELMs carry a significant fraction of their energy W ELM across the scrape-off layer (SOL) and are the dominant source of the first wall thermal load and impurity sputtering. In ITER, W ELM will be considerably larger compared to present tokamaks and the ELM-wall interaction may be an issue for the first wall lifetime and impurity production. New measurements of the ELM ion energies by a retarding field analyzer (RFA) in the ASDEX Upgrade (AUG) far SOL are presented. Type I and, for the first time, mitigated ELMs are captured by a RFA at different W ELM and the outer midplane separatrix distance r sep . A fluid model of the ELM filament parallel transport [1] is employed to obtain the characteristic range of the ELM-averaged filament radial propagation speeds v r

Physics conclusions in support of ITER W divertor monoblock shaping

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

Ion orbit modelling of ELM heat loads on ITER divertor vertical targets

The high heat flux areas on the vertical divertor targets in the ITER tokamak will consist of cuboid tungsten monoblocks bonded to copper cooling tubes. Three-dimensional ion orbit modelling is used to calculate the heating of tungsten monoblocks during ELMs at the inner vertical target, where the highest surface energy densities are expected. The presence of thin gaps between monoblocks results in exposed edges onto which the heat flux can be focused. ELM ions are focused by their gyromotion onto the magnetically shadowed, long toroidal edges of the monoblocks. The risk of monoblock edge melting is greater than the risk of full surface melting on the plasma-wetted zone. Alternative shaping solutions such as edge chamfering, filleting, and poloidal beveling do not show promise; the melt zone simply migrates to other locations on the monoblocks. Without ELM mitigation, there is a marginal risk of edge melting due to uncontrolled ELMs in the pre-nuclear phase of ITER operation, and an absolute certainty of it in the burning nuclear phase. To avoid edge melting altogether, the surface energy density would have to limited to less than 0.15MJ/m2. Keywords: ITER, Divertor, ELM heat load