Speaker
Description
While the conventional lower single-null (LSN) divertor will be tested in ITER, its extrapolation to power-plant conditions remains uncertain, motivating the exploration of further optimised solutions. Three geometrical parameters can be changed independently in the LSN configuration: poloidal length of the divertor legs, poloidal flux expansion at the target and radial position of the strike points. Additional magnetic nulls can be included in the path towards improved heat exhaust solutions. Each of these variations has an associated alternative divertor configuration (ADC): long-legged divertor (LLD), X-divertor, super-X divertor and X-point target. Different experimental campaigns on the TCV tokamak have investigated the impact of these ADCs on divertor power exhaust, showing varying degrees of improvement depending on the configuration. Modelling efforts, however, remain scarce and focused so far on interpretative modelling with mean-field codes lacking turbulent transport.
In order to improve the physics understanding of the ADCs, we performed turbulence simulations of TCV plasmas with different magnetic geometries, precisely the LSN and the four aforementioned ADCs mentioned. The simulations were carried out by using GBS, a first-principles, 3D, global turbulence code that self-consistently evolves the plasma dynamics with the neutral atoms modelled kinetically. A detailed comparative analysis of the considered configurations highlights the role of turbulence in particle and heat exhaust and its interplay with magnetic geometry, pointing out their relative strengths and weaknesses. All ADCs show the decrease in divertor heat load with respect to the LSN configuration. The LLD achieves this heat flux reduction through its increased divertor volume, while the turbulent dynamics are not affected. In contrast, the other configurations benefit from modifications of the turbulent dynamics. These findings constitute important elements for more reliable predictions of ADC performance in future devices.