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Recent experiments on ST40 have begun to explore the X-point radiator (XPR) regime in H-mode discharges, with modelling support via SOLPS-ITER. Previous studies on TCV, AUG, WEST, and JET suggest that the XPR regime offers an operating space which reduces divertor heat loads while maintaining good core confinement through the introduction of a stable impurity radiation front above the X-point [1,2,3,4].
Early observations suggest as the radiative region forms, the radiation front crosses the X-point and resides on the core side, cooling the plasma edge significantly. This radiative region appears to form a banded emission structure which consists of impurity radiation then hydrogenic radiation, like what has been observed on AUG [1,2]. This appears to coincide with the detachment of the outer divertor leg. Compared to non-XPR discharges, these plasmas exhibit lower divertor heat fluxes (confirmed by IR thermography, and Langmuir probe measurements).
Initial spectroscopic analysis and SOLPS-ITER modelling suggest that the impurity radiation front mainly consists of sputtered carbon, which has been compressed towards the X-point via high field side (HFS) and low-field side (LFS) D₂ fuelling, and appears to lead to the formation of this radiative region.
Extrinsic seeding with lithium (via impurity powder dropping) and neon (via LFS puffing) in this regime are also under investigation. IR thermography, and Langmuir probe measurements suggest that this seeding further reduces divertor heat loads. SOLPS-ITER modelling with a lithium neutral particle source positioned on the upper outer divertor walls, suggests that lithium can be localised within the divertor region while avoiding upstream dilution and maintaining core performance. Overall, this regime appears to be promising for long-pulse operation by reducing erosion and power loading on plasma-facing components, without degrading core conditions.
References:
[1] – O. Pan, et al. “SOLPS-ITER simulations of an X-point radiator in the ASDEX Upgrade tokamak”, Nucl. Fusion, 63 016001 (2023).
[2] – M. Bernet, et al. “The X-Point radiating regime at ASDEX Upgrade and TCV”, Nuclear Materials and Energy, Volume 34, 101376 (2023)
[3] – N. Rivals, et al. “Experiments and SOLEDGE3X modeling of dissipative divertor and X-point Radiator regimes in WEST”, Nuclear Materials and Energy, Volume 40, 101723 (2024)
[4] – M. Bernet, et al. “Power exhaust by SOL and pedestal radiation at ASDEX Upgrade and JET”, Nuclear Materials and Energy, Volume 12, pp 111-118, (2017)