Beyond power gain: Toward a comprehensive milestone framework for all fusion energy concepts

In this Perspective paper, we propose an inclusive set of milestone categories to track the progress of fusion concepts from scientific research through grid-connected electricity. We emphasize the importance of not only (1) plasma performance but also (2) the development of engineering, technology, and materials to demonstration-level maturity, some of which are fuel- and concept-specific, and (3) the application of systems engineering from concept to power plant. This framework reflects the diversity of paths toward commercial fusion energy and addresses system elements beyond plasma performance. Commercial viability must be a guiding consideration from the outset. Fusion development does not follow a simple sequential path of physics-based milestones; it requires concurrent progress in integrated physics, technology, engineering, and economic analysis to de-risk entire system designs. Different concepts present varying development needs, risks, and trade-offs—simplifying some subsystems while increasing challenges in others. Successful realization depends on coordinated development of all subsystems, ideally in parallel. Therefore, assessing a concept's maturity requires well-defined quantitative metrics and risk management spanning all critical elements. Our focus is on concept-agnostic metrics that apply across fusion approaches, including magnetic confinement fusion with long plasma durations and pulsed concepts such as inertial confinement, magneto-inertial, and magnetized target fusion. While most efforts center on the deuterium–tritium fuel cycle, alternative fuels such as deuterium–helium-3 (D–3He) and proton–boron (p–11B) merit consideration.