Numerical modelling of metallic melt damage in contemporary tokamaks and future fusion reactors
Abstract
The European Union recognizes fusion energy as the most promising long-term solution for clean virtually unlimited energy, with the construction of ITER and the engineering design of EU-DEMO being short-term and mid-term objectives of the EUROfusion Consortium roadmap. The provision of plasma-facing components (PFCs) with sufficient lifetime constitutes one of the major remaining technological challenges in the development of magnetic confinement fusion reactors. In ITER, currently under construction with first operation scheduled at 2025, the PFC integrity is primarily threatened by fast transient power loading owing to magnetohydrodynamic (MHD) instabilities such as edge-localized modes, vertical displacement events or major disruptions. In EU-DEMO, currently undergoing its conceptual design phase (2021-2027), the PFC heat load capability could be surpassed during regular plasma ramp-up/-down and undesired events such as H-L transitions, major disruptions or vertical displacement events. The generated metallic melt will be subject to strong plasma-induced volumetric forces that will displace the material, causing large-scale surface deformations which may severely compromise power handling. Hence, the reliable modelling of melt events as well as the realistic assessment of their possible consequences constitute a priority topic in contemporary fusion research.
The MEMOS –U code, developed and maintained by the Complex Plasma Group of the Space & Plasma Physics division of KTH, is the only European code that is capable of simulating the motion of induced metallic melts coupled with the thermoelectric response of large-area wetted PFCs, thus providing realistic surface deformation profiles. The general numerical problem falls under the category of free-surface MHD flows with phase transitions. MEMOS-U solves the incompressible resistive thermoelectric MHD set of equations within the shallow water approximation and the magnetostatic approximation coupled with the heat convection-diffusion equation. The MEMOS-U predictive capability has been successfully tested in multiple dedicated EUROfusion experiments that deliberately achieved PFC melting by edge localized modes, major disruptions and steady state heat loads. The series of experiments featured different material compositions (tungsten vs beryllium), exposure geometries (leading edge vs sloped), cooling (active vs passive) and electrical connections (grounded vs floating). These extensive validation activities have been partly funded by EUROfusion (TSVV, WP-DES, WP-TE, WP-PWIE), the ITER Organization (three contracts from 2016-date) and the Swedish Research Council (2018-05273_VR, 2021-05649_VR).
With this proposal, we request for the resources that are necessary for (i) further benchmarking of the MEMOS-U code with a series of new dedicated EUROfusion experiments (simultaneous niobium versus iridium exposures in ASDEX-Upgrade, simultaneous plasma pre-irradiated versus pristine tungsten exposures in WEST, splashing tungsten exposures in ASDEX-Upgrade, splashing tungsten exposures in WEST), (ii) predictive runs of MEMOS-U for different ITER scenarios, (iii) predictive runs of MEMOS-U for different EU-DEMO scenarios.
