On the melting threshold of toroidal gap edges of ITER divertor tungsten monoblocks under repetitive ELM transients

Abstract

Damage to plasma-facing components from transient heat loads, such as Edge Localized Modes (ELMs), poses a critical challenge for ITER’s tungsten (W) divertor monoblocks (MBs). Under the modified ITER Research Plan, the full actively cooled W divertor will be installed from the outset, while the provisional inertially cooled first wall used during ITER’s Start of Research Operations (SRO) will be replaced by the final, actively cooled variant before DT operation. Thus, damage to the divertor during SRO must be kept to the absolute minimum. This study investigates toroidal gap (TG) edge melting during repetitive Type I ELM loading in the SRO phase, first predicted in Gunn et al. (2017) to occur at much lower energy densities than for the top surface. The focus will be on the planned deuterium H-mode plasmas at half nominal toroidal field (2.65 T) and with plasma currents in the range 5–7.5 MA. Self-consistent 3D heat transfer simulations are performed to determine the ELM fluence thresholds for TG edge melting and assess the impact of ELM mitigation via frequency variation to identify the required mitigation level to minimize the melt damage at the ITER divertor during the SRO discharges. It is found that ELM frequencies above 20 Hz are necessary to completely avoid TG edge melting, even under worst-case assumptions. Moreover, for roughly equivalent conditions to those studied with a more simplified approach in Gunn et al. (2017), we find an approximate 25% lower heat load, which can be considered a reasonable agreement in view of the uncertainties in the ELM energy fluence specification and in the treatment of ion orbit dynamics in the gap vicinity.