The peer-reviewed articles in this special issue of the Journal of Plasma Physics describe a comprehensive, self-consistent and robust baseline physics solution with conservative design margins for a practical fusion pilot power plant, Type One Energy’s ‘Infinity Two’. They present the Infinity Two fusion pilot power plant physics design basis that realistically considers, for the first time, actual power plant operating experience, the complex relationship between competing requirements for plasma performance, plant startup, construction logistics, power plant capacity factor and economics. The Infinity Two baseline physics solution makes use of the inherently favorable operating characteristics of highly optimized stellarator fusion technology using modular high-field magnets.

High-fidelity computational plasma physics analyses presented in these articles have enabled Type One Energy to substantially reduce the risk of meeting Infinity Two power plant functional and performance requirements. This unique and transformational achievement is the result of a global development program led by the Type One Energy plasma physics and stellarator engineering organization, with significant contributions from a broad coalition of scientists from national laboratories and universities around the world. Each of the interrelated analyses described in these articles utilized state-of-art computational modeling and simulation tools, together with high-performance computing and optimization techniques which have been benchmarked using the latest generation of magnetic confinement fusion research devices.

The ability of Type One Energy to establish a commercially viable, reasonably conservative, physics basis for the Infinity Two fusion pilot power plant rests upon access to a series of enabling technical developments over the past decade. These include the development of high-temperature superconducting (HTS) magnets demonstrated to produce the magnetic field strength necessary for high density plasma operation. Type One Energy has also secured exclusive rights, via Commonwealth Fusion Systems (CFS), to the proven Massachusetts Institute of Technology (MIT) HTS cable technology for use in stellarators. In addition, the emergence of exascale supercomputers now supports detailed full-physics, three-dimensional plasma physics simulations. Type One Energy has made use of a spectrum of high-performance computing facilities, including access to the US Department of Energy supercomputers such as the exascale Frontier machine at Oak Ridge National Laboratory, to perform its physics simulations. Collectively, these enabling technology access rights allow Type One Energy to efficiently develop a commercially viable fusion power plant design by refining, updating and upgrading the very successful W7-X large-scale, modular magnet, optimized superconducting stellarator science platform.

The consistent and robust physics solution for Infinity Two results in a deuterium-tritium (D-T) fueled, burning plasma stellarator with 800 MW of fusion power (although plasma physics solutions exist up to ∼3 GW fusion power). It is characterized by plasma with a resilient and stable magnetohydrodynamic (MHD) equilibrium exhibiting low neoclassical and turbulent transport losses, as well as tolerable direct energy losses to the first wall. The Infinity Two stellarator has sufficient room for both adequately sized island divertors and a blanket which provides appropriate shielding and tritium breeding. High magnetic field and optimized plasma geometry design choices for Infinity Two allow operation at high plasma density (and pressure) with a moderate plasma beta, resulting in a low bootstrap current (<5 kA) and suppression of plasma instabilities driven by energetic particles.

The practical engineering design of large-bore HTS magnets for fusion was clearly demonstrated in the landmark MIT/CFS papers on the SPARC Toroidal Field Model Coil project (Viera et al. Reference Viera2024). In that work, a planar HTS magnet operated successfully with a peak magnetic field of 20 T on the magnet coil at a temperature of 20 K. Subsequent work by Type One Energy, in collaboration with MIT, has resulted in the development of modified HTS cable technology for use in the non-planar magnets of modular coil stellarators, as shown in Riva et al. (Reference Riva2023). Similar to that in tokamak technology, thermal energy confinement in the stellarator improves with decreasing plasma particle orbit size and therefore benefits from higher magnetic field strength. The benefits of high-field magnets go beyond transport, including the myriad stellarator performance advantages of being able to operate at higher plasma density. As such, the development of practical HTS fusion magnets is a technical milestone fundamental to successful commercialization of magnetic confinement fusion energy. When combined with the inherent operational advantages of stellarator technology, HTS fusion magnets enable the design of a practical Infinity Two fusion pilot power plant based on a realistic physics solution.

Stellarator technology possesses natural engineering solutions to many of the challenges in developing a practical fusion power plant. This advantage is due to the stellarator’s inherent steady-state operating mode, which is achieved without the need for plasma current drive. The stellarator is proven to exhibit excellent plasma stability and associated reliable operation over a wide range of plasma parameters when utilizing the modular magnet architecture of HSX and W7-X. It can also achieve ignited plasma operation when properly designed. Many of the engineering challenges associated with both the geometrically complex shape of the stellarator and the use of superconducting modular coil magnets were demonstratively overcome at power plant-relevant shape and scale by W7-X. Stellarators, like all magnetic confinement fusion technologies, have historically faced plasma physics challenges in the areas of establishing good thermal confinement, identifying equilibrium solutions and providing sufficient energetic particle confinement and thermalization. Tremendous progress has been made in recent decades, building on the massive global investment in toroidal magnetic confinement fusion science, on finding solutions to these challenges. The use of advanced theoretical and computational tools to predict plasma performance and ultimately to optimize stellarator plasma behavior have led to these solutions. In fact, given the large design space and geometric complexities of stellarators, developing a viable high-field stellarator fusion power plant fundamentally requires a highly optimized plasma configuration. All these advances in stellarator plasma theory and computational methods over the past several decades, together with the emergence of high-field HTS magnets, have catalyzed development of the baseline Infinity Two fusion pilot power plant physics basis reported in this journal issue.

The Infinity Two stellarator design derived from the physics basis presented in these articles is a four-field period, aspect ratio 10 and quasi-isodynamic configuration utilizing the maximum-J (J is the 2nd adiabatic invariant of the particle motion) approach for good plasma confinement. A summary of the baseline plasma design solution is provided in the paper by Hegna et al.: ‘The Infinity Two Fusion Pilot Plant Baseline Plasma Physics Design’, and in table 1. High-fidelity transport modeling confirms the robustness of this solution, with a broad range of operational space available for high gain. Indeed, simulations show that ignited D-T operation is attainable at sufficiently large plasma density.

Table 1. Key parameters of the Type One Energy Infinity Two plasma physics design.

Not only does the Infinity Two physics basis provide the foundation for a modular magnet high-field stellarator with excellent fusion plasma performance, but the stellarator configuration offers compelling power plant economics. The stellarator configuration has enabled Type One Energy to architect a maintenance solution which supports good power plant capacity factors and associated levelized cost of electricity. It also supports favorable regulatory requirements for component manufacturing and power plant construction methods essential to achieving a reasonable over-night cost for Infinity Two.

Type One Energy has high confidence that the essential physics solution presented in this journal issue provides a good baseline stellarator configuration for the Infinity Two fusion pilot power plant. However, opportunities do exist to further improve plasma performance and decrease design margins that Type One Energy has initially incorporated into Infinity Two to manage first-of-a-kind risks. These Infinity Two design margins are established to accommodate uncertainties inherent in the results of computational models used to develop the Infinity Two physics basis. Computational modeling uncertainties include, for example, modeling the compatibility of island divertor performance with excellent core plasma confinement. Reducing these types of stellarator physics uncertainties and the associated need for increased Infinity Two design margins will be achieved through a design verification test program to be performed in 2029 on a subscale stellarator, ‘Infinity One’, which is derived from the essential features of Infinity Two. Infinity One is currently being designed and will be constructed and operated by Type One Energy in collaboration with its partner utility, the Tennessee Valley Authority (TVA). The TVA has made its recently retired Bull Run fossil plant in the state of Tennessee available for deployment of Infinity One and will provide the requisite support services for operation of Infinity One.

On 20 January 2025, Type One Energy entered into a Cooperative Agreement with TVA to jointly develop plans for a potential TVA Infinity Two fusion pilot power plant project in the Tennessee Valley region using Type One Energy stellarator fusion power technology. The TVA believes that this nominal 350 MWe Infinity Two fusion power plant could provide a complementary source of base load electrical generation for the region as early as the mid-2030s. The Infinity Two stellarator configuration described in the following journal articles is derived from the current baseline physics basis for this TVA fusion pilot power plant.

Type One Energy expects that its stellarator fusion power plant physics basis and associated design configuration will evolve as plasma theory, physics codes and optimization methods improve. Further details and refinement of the Infinity Two stellarator design presented in this study will emerge as the engineering for magnets, blanket, and shielding progresses, and the final design becomes more established. However, completion of this comprehensive, unified baseline physics basis for the Infinity Two fusion pilot power plant, as described in these journal articles, is a major programmatic achievement supporting Type One Energy ’s mission to commercialize stellarator fusion energy on a decadal time scale and an important milestone in building the future of energy.