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The X-point radiator (XPR) plasma regime displays favorable properties with regard to power exhaust in tokamaks: An H-mode-like confinement quality, a detached divertor, and the suppression of type-I ELMs are achieved simultaneously [1]. XPR scenarios may also pave the way for more compact and cheaper divertor solutions,as demonstrated on ASDEX Upgrade [2]. The parameter to control XPR stability and the fraction of power dissipated by radiation is the XPR height can be actively manipulated through impurity seeding and neutral gas fuelling.
This paper focuses on the perspectives for XPR scenarios in tokamak fusion
reactors. The XPR height depends on flux-surface geometry and seeding impurity. Among the impurities studied, argon shows the highest efficiency in radiating a large fraction of the heating power. In EU-DEMO, argon seeding resulted in an XPR height of 50 cm, corresponding to a dissipation of 90 % of the 150 MW heating power entering the pedestal region. With neon, only lower dissipation rates are achieved, and the XPR height depends only weakly on the neon concentration.
Furthermore, it is shown that even without impurity seeding, charge exchange power losses can be considerable at high neutral densities, increasing the heating power required to access the H-mode. The observed effect reproduces experimental trends in the power threshold, such as its dependence on density and on divertor closeness.
While a large flux expansion and the associated long connection length favor the occurrence of XPRs, less power is conducted to the XPR as the connection length increases, reducing the amount of dissipated power. These two opposing effects are investigated within a family of configurations related to the compact radiative divertor, where both parameters can be driven to extreme values.
These results were obtained from a reduced power and particle balance model [3] and an extension of it [4] which estimates the XPR height, the dissipated power, and the coupling to the upstream profiles. The calculated reduction in the pedestal gradient is consistent with the experiments and could explain the process of ELM suppression.
[1] M. Bernert et al., Nucl. Fusion 61, 24001 (2020).
[2] T. Lunt et al., Phys. Rev. Lett. 130, 145102 (2023).
[3] U. Stroth et al., Nucl. Fusion 62, 076008 (2022).
[4] U. Stroth et al., Plasma Phys. Contr. Fusion 67, 025001 (2025).