Description
The emergence of private fusion ventures has renewed interest in compact tokamak design strategies, with proposals for smaller and more economical power plants based on various plasma performance and technological assumptions. Understanding the conditions under which these designs achieve their objectives requires systematic assessment of underlying modeling choices.
To address this question in the framework of the French PEPR SupraFusion program, a new system code, D0FUS, has been developed [1]. Mostly analytical and fully implemented in Python, it integrates plasma physics and technological constraints following the spirit of references [2,3]. Its high modularity enables systematic exploration of modeling choices and their consequences. D0FUS is open-source, has been benchmarked against published designs (ITER, EU-DEMO), and validated through comparisons with higher-fidelity codes (METIS for plasma physics, MADE and MADMACS for radial build).
Comprehensive parameter scans have been performed to assess the robustness of viable design points, corresponding to a so-called radial build that allows one to insert all necessary technological elements inside the magnetic axis. Two complementary aspects have been particularly scrutinised. First, high magnetic fields enabled by high temperature superconductors (HTS) are found to significantly expand the accessible design space from a plasma stability perspective [4]. However, optimal configurations depend critically on structural strategies. In conventional wedged designs, where the inner legs of the toroidal field (TF) coils are not in contact with the central solenoid (CS), large aspect ratio machines are favoured due to enhanced inboard space for structural reinforcements and more efficient field utilization (B0/Bmax). Accessing compact geometries requires alternative approaches such as bucking (contact between TF and CS coils) or advanced high-strength steels (e.g. N50H). Second, exploration of plasma physics model uncertainties demonstrates that choices of confinement scaling laws, bootstrap current model, prediction of the maximum achievable elongation, density limit and kink/Troyon stability boundaries substantially affect power plant size, performance, and economic predictions. Optimal model selection is therefore shown to be critical to minimize design uncertainties.
References
[1] Auclair, T., et al. (2025). The tokamak system code D0FUS and its first applications. Fusion Engineering and Design, 219, 115270.
[2] Freidberg, J. P. et al. (2015). Designing a tokamak fusion reactor — How does plasma physics fit in? Physics of Plasmas, 22(7).
[3] Johner, J. (2011). HELIOS: A zero-dimensional tool for next step and reactor studies. Fusion Science and Technology, 59(2), 308–349.
[4] Auclair, T., et al. (2026). Impact of mechanical constraints on tokamak design and implications for high field power plants. Submitted to Nuclear Fusion.