Description
Advancing a HELIAS-type stellarator toward DEMO requires a systematic reduction of critical physics uncertainties that directly impact reactor performance. This work discusses and prioritizes these uncertainties based on their influence on machine design, the urgency for near-term decisions, and the feasibility of closing knowledge gaps through current experimental access and tool maturity. The analysis focuses on five key areas: core transport, MHD stability, fast-ion confinement, plasma exhaust, and plasma-material interactions.
While W7-X has successfully validated neoclassical transport optimization, significant gaps remain. Turbulent transport must be characterized across diverse density-gradient regimes to fully understand configuration-dependent confinement. Furthermore, advanced core turbulence modelling is needed to incorporate electromagnetic effects at high pressure and Alfvénic mode coupling, both of which can significantly alter heat transport. As W7-X is currently a predominantly carbon device, establishing impurity-tolerant operation compatible with a metallic wall remains a critical challenge; multi-species impurity transport must therefore be addressed to ensure core-edge compatibility.
Fast-ion confinement at high-β, particularly regarding losses driven by Alfvén eigenmode coupling, represents another critical uncertainty affecting first-wall loading and requires high-fidelity quantitative predictions. Finally, island divertor physics presents several open questions: drift effects on heat and particle transport must be quantified, and the necessity of divertor closure for HELIAS needs validation. Ensuring consistent detachment, assessing helium ash removal against dilution limits, and verifying the robustness of divertor topology under finite-β variations are essential for defining a stable exhaust geometry. Addressing these priorities — specifically by executing integrated scenarios at 5–7 T with coupled core–edge modelling — will establish the physics foundation for next-step stellarator reactors.