Mitigation of Edge-Localised Modes with an X-Point Target Divertor in TCV

In high-confinement (H-mode) plasmas, large transient power loads created by type-I edge-localized modes (ELMs) can irreversibly damage the walls of future fusion reactors [1]. Recent experiments in the Tokamak à Configuration Variable (TCV) suggest that type-I ELMs can be strongly mitigated by the X-point target (XPT) divertor. This alternative configuration, considered for the forthcoming SPARC reactor [2], features a secondary X-point that can serve as a virtual target for the plasma [3]. Motivated by previous results [4], new TCV discharges optimized for ELM measurements reveal that type-I ELM peak heat fluxes can be reduced by more than 85% at the outer strike points (OSPs) of the XPT when compared to ELMs with similar energy losses in conventional single-null (SN) plasmas. A CIII-filtered fast camera shows that the CIII emission peaks at the secondary X-point during the ELMs, resembling a transient X-point target radiator [5]. The XPT strike points (SPs) 2 and 3 remain detached, and SP4 receives a small, spread amount of heat, whereas the SN OSP reattaches. The level of mitigation in the XPT allows TCV to operate in a type-I ELMy H-mode with target heat loads as low as those in grassy ELM regimes with the same injected power.

[1] T. Eich et al. Nucl. Mater. Energy 12 84–90 (2017)

[2] A. Q. Kuang et al. J. Plasma Phys. 86, 865860505 (2020)

[3] B. LaBombard et al. Nucl. Fusion 55 053020 (2015)

[4] H. Raj et al. Nucl. Fusion 62, 126035 (2022)

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