The prospects of an X-point target radiator as a reactor exhaust solution

Magnetic shaping of the tokamak divertor into an X-point target (XPT) configuration [1] have recently been shown in TCV Ohmic plasmas to trigger a secondary form of X-point radiation, called the X-point target radiator (XPTR), accompanied by a detached divertor [2]. New high-power experiments on TCV established stationary XPTRs through nitrogen seeding, using full available ECH power (2.5 MW for a ~1 m³ plasma) at low density (f_GW=0.1) and high plasma current (q₉₅=2.5). The detachment accessibility metric, (PB/R)/n_sep², is pushed to levels comparable to those anticipated in SPARC, conditions found to prohibit detachment on TCV in conventional divertor configurations. Impurity front position near the secondary X-point shows enhanced resilience against fueling, seeding, and heating perturbations at dynamic time scales. Exploiting this resilience leads to the passive mitigation of large transient heat loads caused by type-I edge localized modes (ELMs), with an XPTR appearing during each ELM crash. SOLPS-ITER simulations incorporating XPT geometry reproduce the detachment access benefits observed and highlight key roles played by drifts and parallel flows, representing a first step toward extrapolation to reactor conditions. Experiments are also being extended to a double-null X-point target (DN-XPT) configuration, which sustains four X-points simultaneously with real-time control of all separatrix spacings within one heat flux width. Overall, the XPTR offers a robust detached divertor regime with many benefits for a reactor: enhanced power handling capability with reduced reliance on impurity seeding; a stabilized detachment front acting as a “shock absorber” that can mitigate both transient reattachment and radiative collapse risks; and compatibility with good core performance. These results support the implementation of the XPT, currently planned for the SPARC reactor experiment and the ARC pilot plant, as a promising advanced divertor solution to address the power exhaust challenge in next-step tokamaks.



[1] B. LaBombard et al., Bull. Am. Phys. Soc. 58, 63 (2013)

[2] K. Lee et al., Phys. Rev. Lett. 134, 185102 (2025)