CRAFT

Overview

The Compact Reinforced-Conductor Advanced Free-form Tokamak (CRAFT) is a conceptual design for a fusion pilot plant developed by the Princeton Plasma Physics Laboratory (PPPL). It represents a pathway toward commercially viable fusion energy by integrating several key innovations: the use of high-temperature superconductor (HTS) magnets, operation in a steady-state advanced tokamak regime, and a compact physical footprint. The primary goal of the CRAFT design is to demonstrate the production of net electricity (Q_engineering > 1) at a scale significantly smaller and potentially more affordable than large-scale international projects like ITER. By combining a high magnetic field with a high bootstrap current fraction, CRAFT aims to achieve a high fusion power density, a critical metric for the economic attractiveness of a fusion power plant.

Physics / Mechanism

The CRAFT design is founded on the favorable scaling of fusion power with the magnetic field. Fusion power density scales approximately as the fourth power of the toroidal magnetic field (B⁴). CRAFT leverages this by employing magnets made from Rare-Earth Barium Copper Oxide (REBCO), a type of HTS tape. Unlike traditional low-temperature superconductors, REBCO can operate at higher temperatures (20–30 K) and generate much stronger magnetic fields. The CRAFT design specifies a toroidal field of 7 T on the plasma axis, with peak fields on the conductor exceeding 14 T. This high field allows for stable confinement of a high-pressure plasma in a relatively small volume.

Operationally, CRAFT is designed as an "advanced tokamak." This implies a plasma scenario characterized by:

• High Beta (β): A high ratio of plasma pressure to magnetic pressure (β_N ≈ 4.5), indicating efficient use of the magnetic field for confinement.
• High Bootstrap Current Fraction (f_BS): A large fraction (f_BS > 80%) of the plasma current is self-generated by pressure gradients within the plasma itself. This drastically reduces the need for external, power-intensive current drive systems, which is essential for achieving net electricity and steady-state operation.
• High Confinement: The design assumes an energy confinement time consistent with established scaling laws (H98(y,2) ≈ 1.5), necessary to maintain the plasma temperature against energy losses.

A key engineering aspect is the management of immense electromagnetic (Lorentz) forces generated by the high-field magnets. The "Reinforced-Conductor" in the name refers to the structural design of the toroidal field (TF) coils. The REBCO tapes are embedded within grooved steel plates that provide structural support against these forces, a critical innovation for realizing compact, high-field devices. The design also incorporates a double-null divertor to handle the high heat and particle fluxes exhausted from the plasma core, a significant challenge in any high-power fusion device.

Historical development

The CRAFT concept emerged from decades of research at PPPL and within the broader fusion community. It builds upon the physics basis established by experiments like DIII-D, JET, and PPPL's own National Spherical Torus Experiment (NSTX-U), which explored the high-beta, high-bootstrap regimes characteristic of advanced tokamaks. The theoretical and computational tools used to model CRAFT's plasma stability and performance, such as the JSOLVER and TRANSP codes, are the result of long-term development efforts.

The enabling technology for CRAFT and similar concepts was the commercial availability of high-performance REBCO HTS tapes in the 2010s. This material breakthrough allowed designers to seriously consider magnetic fields and compact sizes that were previously unattainable. The CRAFT design study, led by physicist Jon Menard at PPPL, was presented in detail in a 2023 Nuclear Fusion paper. It represents a synthesis of advanced physics understanding and novel engineering solutions, directly addressing the U.S. Department of Energy's call for pilot plant designs that could accelerate the timeline to commercial fusion energy.

Current status

As of 2026, CRAFT remains a conceptual design study. It is not a funded construction project but serves as a reference point and a detailed, self-consistent proposal for a U.S.-based fusion pilot plant. The design is mature in its physics basis, with extensive modeling and simulation supporting its projected performance. Key parameters, such as the required confinement, stability limits, and current drive needs, have been analyzed and found to be consistent with the experimental database.

The engineering aspects, particularly the design and fabrication of the large, structurally reinforced HTS magnets, represent the leading edge of magnet technology. While smaller-scale HTS magnet demonstrations have been successful, building the full-scale, 11-meter-tall TF coils required for CRAFT is a major engineering undertaking that has not yet been attempted. The design continues to be refined through ongoing research at PPPL and in collaboration with other institutions, focusing on optimizing the magnet design, divertor solution, and the tritium breeding blanket concept.

Notable implementations

CRAFT itself is a singular design from PPPL. However, its core principles—compact size, high-field HTS magnets, and advanced tokamak operation—are shared by a new generation of fusion concepts, particularly in the private sector. Companies like Commonwealth Fusion Systems (with its SPARC and ARC designs) and Tokamak Energy are pursuing similar strategies. While the specific engineering solutions and plasma parameters differ, they all operate on the same premise that HTS technology enables a faster and potentially more economical path to fusion energy than the low-field, large-scale approach.

The CRAFT design is a prominent part of the U.S. national strategy for fusion energy development. It provides a public-sector, scientifically vetted blueprint that can inform policy decisions, guide research priorities, and serve as a benchmark against which other pilot plant proposals can be evaluated.

Open challenges

Despite its detailed design, CRAFT faces significant scientific and engineering challenges that must be addressed before construction could begin.

1. Magnet Technology and Manufacturing: Fabricating the 16 large, complex, reinforced HTS TF coils to the required tolerances is a primary challenge. Ensuring their reliability and quench protection over a long operational lifetime is critical and requires further R&D.
2. Exhaust Heat Management: The compact size of CRAFT leads to a very high power density on the divertor targets. The design relies on advanced divertor concepts, such as a Super-X or X-point target divertor, to spread the heat load. Validating these solutions at the relevant power levels is a major research area.
3. Steady-State Operation: Achieving and sustaining the targeted high-bootstrap-fraction plasma for long durations is a physics challenge. While supported by simulations, it requires precise control of the plasma profile and stability against various magnetohydrodynamic (MHD) instabilities.
4. Tritium Breeding and Fuel Cycle: The conceptual design includes a tritium breeding blanket, but engineering a robust and reliable system that can achieve a tritium breeding ratio (TBR) greater than 1.0 within the limited space of a compact machine is difficult. The full tritium fuel cycle, from breeding to extraction and reprocessing, must be demonstrated.
5. Materials Science: The structural materials and plasma-facing components must withstand extreme neutron fluences, heat loads, and mechanical stresses for years. Developing and qualifying these materials is a long-term challenge for the entire fusion field.

Outlook

The credible 5-15 year trajectory for the CRAFT concept involves progressing from a design study to technology maturation and component prototyping. In the next five years (2026-2031), the focus will likely be on de-risking the most critical technologies. This includes building and testing prototype segments of the reinforced HTS TF coils and conducting experiments on existing machines to further validate the physics of the advanced tokamak operating scenario. Research into advanced divertor solutions and neutron-resilient materials will also be a priority.

Looking out 10-15 years (2031-2041), if a decision were made to proceed with construction, this period would be dominated by detailed engineering design, site selection, and the industrial-scale fabrication of major components like the magnets and vacuum vessel. The timeline for a potential first plasma would extend beyond this 15-year window, contingent on sustained funding and the successful resolution of the key engineering challenges. The CRAFT study provides a valuable, integrated vision for a fusion pilot plant, and its evolution will be closely tied to the progress of HTS magnet technology and the broader U.S. national fusion program.

References

1. The Compact Reinforced-conductor Advanced Free-form Tokamak (CRAFT) for a Fusion Pilot Plant — Nuclear Fusion (2023)
2. Bringing the Sun to Earth: The US National Academies report on a strategic plan for US fusion research — Journal of Plasma Physics (2019)
3. Overview of the SPARC device — Journal of Plasma Physics (2020)
4. Fusion Energy Sciences Program — U.S. Department of Energy
5. Physics design of the ARIES-AT advanced tokamak burning plasma experiment — Fusion Engineering and Design (2003)
6. High-temperature superconducting magnets for fusion energy — Superconductor Science and Technology (2023)