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Abstract

Liquid metal divertors, in which solid tungsten plasma-facing components are replaced by a liquid metal confined within a porous tungsten capillary structure (CPS), are a promising approach to power exhaust in future fusion devices. In ASDEX Upgrade, a liquid tin module (LTM) was installed and exposed using the divertor manipulator. The unexpectedly high tin (Sn) content observed in the main plasma is attributed to the ejection of tin droplets into the core plasma, likely caused by subsurface bubble formation and burst within the liquid metal layer. This motivates the search for alternative liquid metals with lower susceptibility to such instabilities. One promising candidate is gallium (Ga, Z = 31).

The goal of this work is to assess, through predictive integrated modelling, the feasibility of a gallium liquid metal divertor in COMPASS Upgrade—a tokamak currently under construction. COMPASS Upgrade will have a major radius R0 = 0.894 m with the possibility to reach high field (BT ∼ 5 T) and high current (Ip ∼ 2 MA). Currently, power handling experiments with a special liquid lithium and tin divertor module based on the capillary porous system technology are planned. Gallium is not part of the current experimental plan, making numerical modelling an essential first step to evaluate its compatibility with H-mode operation.

This work describes integrated numerical modelling applied to COMPASS Upgrade simulated discharge by Fiesta+METIS for scenario # 24300 with BT = 4.25 T, Ip = 1.3 MA with 3.7 MW auxiliary heating (2.7 MW NBI +1 MW ECRH). For the numerical analysis, the COREDIV code is used, which self-consistently simulates core and SOL plasma. The gallium source is determined by both evaporation and sputtering processes, caused not only by the main plasma ions but also by impurity species.

At present, atomic data for gallium is incomplete; in particular, ionization and recombination rates, as well as line radiation coefficients for all ionization states, are not yet available. For this reason, germanium (Ge, Z=32), which has a similar atomic number, is used as a proxy in the density equations.

The analysis is performed for different incidence angles (α = 1°–10°), backplate temperatures (T₀ = 500–600°C), tungsten layer thicknesses (dW = 0.003 m, 0.005 m, 0.010 m, 0.015 m), and gallium layer thicknesses (dGa = 0.001 m, 0.003 m, 0.005 m), considering two cases: with and without prompt redeposition. The simulations yield divertor plate temperatures below 5 eV, under which conditions it was found that for incident angle α ≤ 5° and within the analysed ranges of tungsten and gallium thicknesses gallium is compatible with the operation in H-mode.