X-point radiator and power exhaust control in configurations with multiple X-points in TCV

The power exhaust of fusion plasmas remains a critical focal point of tokamak research, as the unmitigated heat flux reaching the wall in future reactors is predicted to greatly exceed material limits. The X-point radiator¹ is a promising scenario that can provide high radiative losses and can achieve strong divertor detachment, but the radiation region’s proximity to the confined plasma may lead to a higher core impurity concentration and degraded core energy confinement. The Snowflake minus (SF)² – an alternative divertor configuration that features a second divertor X-point and enhanced connection length, L_∥ – was shown on the TCV tokamak to exhibit a stable radiation region between the configuration’s two X-points, thus located at some distance from the core. However, this was not accompanied by an improvement in divertor-core compatibility compared to the Single Null (SN)³. The inter-X-point radiation appears to be limited to flux surfaces with enhanced L_∥, moving towards the core with decreasing X-point separation, again with little change in divertor-core compatibility.

To accentuate any effects of additional X-points on power exhaust, an extreme divertor shape was developed in TCV, named the Jellyfish (JF). The JF features three divertor X-points and up to 4x longer L_∥ than an equivalent SN.  We observe strong target heat flux reductions in regions of enhanced L_∥ compared to the SN and SF. As in the SF, the longer L_∥ in the JF only appears to increase radiation locally, rather than increasing the total divertor radiated power. Interpretative modelling with the plasma edge code EMC3-EIRENE is used to deduce the role of cross-field transport, along with impurity and neutral dynamics, in these regimes.

¹M. Bernert et al. 2021 Nucl. Fusion 61 024001

²D. Ryutov, V. Soukhanovskii 2015 Phys. Plasmas 22 110901

³S. Gorno et al. 2023 Plasma Phys. Control. Fusion 65 035004