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Description
This work investigates stationary power exhaust in the X-Point Target (XPT) divertor, combining experiments in the TCV tokamak with SOLPS-ITER simulations. Power exhaust is a major challenge for magnetic confinement fusion: future reactors will face intense heat fluxes channelled through a narrow scrape-off layer onto divertor targets, exceeding material tolerances if unmitigated. Detached operation will therefore be essential, requiring divertor concepts that dissipate power and momentum via impurity radiation and plasma–neutral interactions while preserving core performance.
The XPT introduces a secondary X-point in front of the divertor target, splitting the divertor leg into two branches. It is predicted to reduce target loads and broaden the detachment window relative to a Lower Single-Null (LSN) configuration. Recent TCV results confirmed this improved XPT performance up to Lengyel metric levels, characterising the detachment challenge, relevant to future reactors. However, extrapolation and magnetic design optimization require understanding the mechanisms driving the XPT advantage and validating modelling tools.
Ohmic L-mode density ramps and nitrogen-seeded experiments with auxiliary heating in TCV are compared against SOLPS-ITER simulations. These simulations include realistic geometry, multiple impurities, kinetic neutrals, drifts, currents and improved sheath conditions, using an optimized setup validated across several TCV divertor configurations.
The simulations reproduce the reduced peak target power loads in the XPT relative to the LSN, while radiated power remains similar, indicating a dominant role of non-radiative processes, particularly radial transport. Radial transport in the XPT is dominated by macroscopic drifts due to two geometric features near the secondary X-point: nearly toroidal field lines that enhance sensitivity to cross-field transport and a low-potential secondary private flux region generating an ExB convective cell. This causes a strong particle redistribution, consistent with spectroscopy, and reduce target density in one branch depending on toroidal field direction. In simulations this yields a higher peak target temperature in that branch compared to the LSN, a trend not observed experimentally. Code experiments explore this discrepancy by indirectly testing kinetic effects, turbulent transport and performing geometry scans to account for reconstruction errors. The latter reveals strong sensitivity, within fractions of a power decay length, to the separation between the main and secondary separatrices. The simulated temperature rise is mitigated by impurity seeding, consistent with experiments, and correlates with a distinct ionization pattern that allows seeded species to better access high-power flux tubes in the XPT.
These results advance the understanding of the XPT physics and assess SOL modelling capabilities and remaining challenges.