The quest to harness fusion energy, and to realise its promise of providing clean, firm power for humanity’s future, has spanned decades and continents. A global group of scientists and engineers has made enormous advances in science and technology in this time, bringing the field to the point where well-capitalised commercial entities are able to take the final step to build a fusion power plant. While challenges remain, our collective understanding of the physics of the type of fusion machine called a tokamak has advanced to the point that we can confidently design a fusion power plant that is on track for early commercial deployment. In parallel, we have advanced our technology to the point that such a power plant will be not only feasible, but will be economically attractive to energy markets.

At Commonwealth Fusion Systems, we are confident in our scientific and technological path to the ARCFootnote ¹ fusion power plant. Commonwealth Fusion Systems (CFS) has announced the location for the first ARC fusion power plant in Chesterfield County, Virginia, in a rapidly growing energy market and in a community excited about the future of fusion energy. The ARC fusion power plant must be a commercial product that meets the needs of customers and the market in order to ensure widespread deployment as quickly as possible. Along these lines, we have also secured our first customer, with Google signing up to buy 200 MW of the electrical output of ARC, roughly half of the 400 MW that the plant is designed to produce.

This special issue describes the firm foundation of physics understanding supporting the ARC tokamak design. In the early years of tokamak research, the complexity of the physics governing plasmas in tokamaks was not fully appreciated, but researchers have made considerable advances in the past six decades and we now have confidence in our ability to project the plasmas created in present and future machines within reasonable uncertainties. The work by early tokamak pioneers and the dedicated work by researchers around the world mean that tokamaks are the best understood fusion concept from a physics perspective, which is one of the reasons that CFS chose to pursue this technological approach.

This knowledge was used to design the SPARC tokamak (Creely et al. Reference Creely2020; Greenwald Reference Greenwald2020), which has the goals of achieving net fusion power production and closing key remaining physics questions on the path to the ARC fusion power plant. The SPARC tokamak is now under construction in Devens, Massachusetts, and is scheduled to begin operation in the next few years. In parallel, understanding of tokamak plasma physics and the distillation of that understanding into predictive tools has continued, based on experimental and computational research around the world. The articles in this special issue apply state-of-the-art physics understanding and tools to scope out the design and operational space of the ARC high-field tokamak. The physics described in these papers complements significant technology development efforts in high-field magnets, material science, molten salt blankets, tritium management and processing and remote handling, all of which combine to enable development of the ARC fusion power plant.

Given that there are remaining open questions that SPARC will answer and that the engineering design of ARC is not yet complete, these articles describe the present iteration of the ARC tokamak design, called ARC Version 3A (V3A), with the understanding that the ARC design will continue to iterate as we learn more. We do not anticipate large deviations of the final ARC design from the parameter space laid out here, such that the conclusions drawn here can be applied to the final ARC design. The articles in this collection highlight where there remain key physics questions that SPARC plans to answer, complementing previous discussions of the scientific value of SPARC (Creely et al. Reference Creely, Brunner, Mumgaard, Reinke, Segal, Sorbom and Greenwald2023). This collection also highlights areas in which the world scientific community outside of SPARC has the opportunity to contribute to cutting-edge research that will improve the first generation of fusion power plants. ARC is architected such that these results can be incorporated into relatively late design changes in the components closest to the plasma without significantly impacting the rest of the plant.

Commonwealth Fusion Systems and its partners are publishing this series of physics basis papers because we believe that science is best advanced through public discourse and that peer review is the best validation of our work in these areas on the forefront of human knowledge (Reinke, Sorbom & Greenwald Reference Reinke, Sorbom and Greenwald2023). It is the same reason that we have published other scientific articles in preparation for SPARC operation and why we plan to publish scientific results from SPARC. Commonwealth Fusion Systems is building off of the collective physics understanding of generations of scientists worldwide, and we plan to continue to publicly advance the field of plasma physics.

The rapid pace of advancements in fusion science and technology from both the academic community and fusion industry make it a time of great anticipation in fusion. Many new fusion machines are aiming to begin operation in the next few years, several of which are targeting net energy production. We expect to see the first generation of fusion power plants, such as ARC, soon after, and to finally realise fusion’s potential as the next transformative energy source for human civilisation. As the history of fusion research has taught us, fusion is hard, but transformative change is rarely easy and a fusion energy future is worth pursuing with relentless determination.