Wendelstein 7-X, the stellarator experiment in Greifswald, Germany, has built on knowledge gained from its predecessors to demonstrate the strong potential of the stellarator concept as a reactor path. This has sparked significantly greater interest across the fusion community in stellarator-based fusion reactors – leading to a wave of start-ups in Europe, the US, and Asia aiming to harness helical plasma confinement for practical fusion energy.
Stellarators offer several intrinsic advantages for a future fusion power plant — continuous operation, freedom from major disruptions, and no need for a large plasma current. Yet, some physics challenges remain unsolved. To make fast, decisive progress towards a First-of-a-Kind stellarator reactor, a team of experts from leading European laboratories, led by Marcin Jakubowski – a project leader of EUROfusion Work Package Stellarator, joined forces to identify the most critical physics questions ahead and to set sharp, actionable research priorities that will help to drive the stellarator program forward.
Following lively discussions within the stellarator community during the workshop organised in Greifswald, a report was compiled that systematically maps the critical physics uncertainties that must be resolved before a stellarator-based demonstration reactor, HELIAS First-Of-A-Kind (FOAK), can be designed and built with confidence. Titled ‘Key Physics Uncertainties and Related Investigation Needs towards Stellarator Reactors’, the document is a community-wide assessment, drawing on expertise from EUROfusion’s entire stellarator research programme. The report can be found here.
The report identifies and prioritizes these uncertainties, referred to as gaps, across five interconnected areas: scenario integration, fast particles and Alfvén waves, core transport, MHD equilibrium and rotational transform control, heat and particle exhaust and plasma–wall interaction. Each gap is assessed according to its potential impact on machine design and the current state of knowledge, using a newly developed Subjective Science Readiness Level (SSRL) scale ranging from 1 (emerging hypothesis) to 9 (gap fully resolved and integrated into the baseline design).
In the area of scenario integration, the primary challenge is developing reliable fueling schemes — in particular, predicting how cryogenic pellets ablate and how the resulting plasmoids drift in a stellarator geometry. For fast particles, reactor-relevant validation of improved fast-ion confinement at high plasma beta and of Alfvén eigenmodes-driven losses remains outstanding. Core transport modelling must advance to the point where temperature and density profiles can be predicted for reactor conditions, including the complex interplay of neoclassical, turbulent, and multi-species effects. In the domain of MHD equilibrium and stability, nonlinear behavior at high beta and the robustness of the island divertor topology under realistic operating conditions are priority concerns. Finally, for plasma–wall interaction and exhaust, the necessity of a closed island divertor concept for HELIAS and the predictive modelling of tungsten erosion and migration represent the most urgent open questions.
For each identified gap, the report defines concrete investigation needs and specifies whether they can be addressed through theory and simulation, experiments on existing facilities such as Wendelstein 7-X, or by means of advances in diagnostics, modelling tools, or new experimental devices. This structured approach is designed to ensure that research activities are aligned with the milestones of the stellarator reactor design process – so that no critical uncertainty is encountered too late to influence the engineering choices.
The report forms part of EUROfusion’s broader effort to build a strong scientific foundation for HELIAS First-of-a-Kind, the European stellarator path towards a fusion power plant, alongside the ongoing tokamak route. By identifying the most important physics steps early and setting clear research priorities, the stellarator community is creating a practical roadmap for the years ahead – one that will help turn current uncertainties into a more robust basis for future design decisions and reactor development.