Power sharing in WEST double-null plasmas

Abstract This study investigates L-mode power sharing between divertor target plates in double-null (DN) magnetic configuration on the tungsten (W) Environment Steady-state Tokamak (WEST). A series of discharges scanning the distance between the separatrices at the outer mid-plane ( δ R sep ) were analyzed using Langmuir probes, embedded fiber Bragg gratings (FBGs), and calorimetry to characterize heat flux profiles and total deposited power in attached regime. Results show that DN configurations could significantly reduce the peak heat flux measured by FBGs on the lower divertor by over 70% compared to Lower Single-Null as power is more evenly distributed between upper and lower divertors. However, the heat flux is shown to be highly sensitive (peak value multiplied by 2) to millimetric variations of the δ R sep . These findings reinforce the DN configuration’s potential for mitigating peak heat loads in next step fusion devices, while highlighting the need for precise magnetic control in attached regime. Power deposition remains asymmetric in connected DN, with up to 89% of power directed to outer targets and 68% to the lower divertor, consistent with the ballooned nature of radial transport at the low-field side and downward B B × ∇ ∇ B drift effects. To quantify these effects, we develop a revised power sharing model building on Brunner’s formulation, incorporating localized sources at the inner ( S i ) and outer ( S o ) mid-plane, a vertical drift bias b , and a cooling factor κ diss . The best-fit parameters suggest that approximately 58% of the exhaust power preferentially flows downward due to vertical drifts, and that a secondary inner mid-plane source contributes up to 13% of the total power in WEST. The decay lengths obtained from the fit are in agreement with those independently measured from heat flux profiles, further validating the model.