Speaker
Description
Future tokamak reactors are expected to generate divertor heat fluxes that greatly exceed the technological limits of plasma-facing components if unmitigated. While a partially-detached conventional single-null (SN) divertor is currently foreseen for ITER, alternative divertor concepts based on strategic magnetic shaping and neutral baffling are under investigation to improve power exhaust performance [1,2]. Among these concepts, the X-point target (XPT) divertor has shown significant promise in facilitating detachment onset and improving detachment front stability [3]. This work discusses experiments conducted on TCV (R=0.88 m, a=0.25 m, BT=1.4 T) to explore the parameter space of the XPT’s power handling capabilities. A high-power, high-plasma-current scenario was developed with up to 2.5 MW of injected electron cyclotron heating, while sustaining L-mode operation at low core line-averaged densities of 1.5–2 × 10¹⁹ m⁻³ (Greenwald fraction fGW=0.08–0.1). This scenario is enabled by a novel model-based density controller with real-time density profile estimation, together with an XPT divertor shape controller providing feedback control of the X-point separation in flux coordinates. The Lengyel metric, (PsepB/R)/nsep², which characterizes the divertor detachment challenge for a given scenario, approaches values relevant to future fusion reactors. Compared with the SN configuration, the XPT exhibits a substantial redistribution of impurity line emission and radiation during nitrogen seeding—an effect absent in the SN case—indicating an earlier onset of detachment. In addition, the peak parallel heat flux to the outer target is reduced by up to 50% in the XPT configuration without compromising the inner target. Guided by recent SOLPS-ITER simulations, which revealed the significant roles of macroscopic E×B drifts and X-point separation in the physical basis of XPT's heat flux mitigation mechanisms, a new set of field-reversal experiments at the same power was performed on TCV to validate these effects, and comparisons of these features will be presented.
[1] C. Theiler et al., Nucl. Fusion 57, 072008 (2017)
[2] K. Verhaegh et al., Commun. Phys. 8, 215 (2025)
[3] K. Lee et al., Phys. Rev. Lett. 134, 185102 (2025)