COMMONWEALTH FUSION SYSTEMS’ HIGH-FIELD PATH TO FUSION ENERGY D.F. Brunner for the SPARC, ARC, TFMC, CSMC, and WHAM Teams Commonwealth Fusion Systems Devens, MA, USA Email: [email protected] Abstract Commonwealth Fusion Systems (CFS), in collaboration with the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center (PSFC), has demonstrated high-field superconducting magnets at the performance and scale needed for fusion energy. Following this success, CFS has raised in total over $2B in private capital and is now building the SPARC high-field tokamak to demonstrate net energy as well as doing early design and R&D for ARC, its fusion pilot plant. This paper will cover CFS’ achievements to date and its plans for getting to a fusion pilot plant. CFS spun out of the MIT PSFC in 2018 to commercialize the high-field tokamak path to fusion energy, building on decades of experience in high-field magnet engineering and tokamak plasma physics at the PSFC. Recognizing the potential that high-temperature superconductors (HTS) had for use in high-field magnets, CFS and MIT set out on a 3-year research program to develop the first fusion-scale HTS magnet, the SPARC Toroidal Field Model Coil (TFMC). In parallel they ramped up efforts to design the SPARC tokamak, which uses the new HTS magnet technology and will demonstrate net fusion energy. While the TFMC was the main target of CFS’ HTS magnet R&D, it has also been working on a high-field HTS mirror coil pair for the Wisconsin HTS Axisymmetric Mirror (WHAM), high-cyclic-loading and low-AC-loss HTS cables, and the SPARC Central Solenoid Model Coil (CSMC). The success of the TFMC program gave CFS’ investors the confidence to fully fund completing the design, construction, and initial operation of SPARC to demonstrate commercially relevant net fusion energy. At the time of this paper, the SPARC facility is well under way: all of the major buildings are weather-tight and ready for tokamak components, most major systems are being procured, and the first SPARC magnet is being produced in CFS’ magnet factory. To get fusion energy on the grid as fast as possible, CFS is not waiting on SPARC results to start on design and R&D for its fusion pilot plant, ARC, and is ramping up these activities. The TFMC was at the center of CFS’ and PSFC’s first three years of HTS magnet R&D [Hartwig2023]. The initial magnet R&D behind the TFMC was carried out in 12 months and the TFMC went from a “blank sheet” through design, manufacturing, assembly, and testing in only 27 months. HTS, in contrast to the low-temperature superconductors (LTS) being used in ITER and other major superconducting tokamaks, can go to much higher magnetic fields, allowing for smaller machines that have lower capital cost and shorter construction times. The mission of the TFMC, shown installed in its test facility in Fig. 1, was to retire the principal risks of utilizing large-scale HTS magnets in high-field fusion devices. These risks include computational modeling and design of no-insulation magnets, fabrication processes, electromechanical loads approaching ~1 GPa, novel cryogenic cooling scheme for high heat removal, and internal, demountable low resistance joints. The secondary mission of TFMC was to develop ancillary HTS magnet technology suitable for use in fusion devices, such as high-current HTS current leads and HTS cold bus systems, and foster a robust supply chain, primarily in the scale-up of HTS performance and volume from key manufacturers. While smaller HTS magnets have been operated at comparable fields, the TFMC was ~100 times larger in size and stored magnetic energy (110 MJ). The TFMC was brought up to full performance on its first test run in September 2021, demonstrating the great potential of the new technology. The TFMC Program achievement its main objectives: It achieved >20 T peak field-on-conductor in a large-bore magnet at SPARC-relevant conditions while confirming the no-insulation magnet physics and operation, particularly the current, voltage, power, and temperature distribitions. It confirmed the feasibility of massively parallel, single pass, machined cooling channels within the winding pack. The demountable pancake-to-pancake joints internal to the magnet winding pack proved to be simple and robust with resistances in the 1 nΩ range. High-fidelity electromagnetic and thermo-mechanical models of no insulation magnets were validated such that confidence in the design and operation of the full-scale SPARC TF was achieved. The Program developed relationships, processes, and qualification of materials, instrumentation, tooling, fabrication processes, and external vendors to enable confidence and speed in the construction of the SPARC TF magnet. The development of the REBCO supply chain achieved an unprecedented manufacturer scale-up needed for the TFMC and is continuing for SPARC. Finally exploration of the passive, self-protecting resilience to current sharing and quench in representative-scale, fusion-relevant no insulation REBCO magnets was explored, indicating key areas where CFS and MIT are improving the magnets for SPARC. A series of six papers on the TFMC program and facility are under review for publication in IEEE Transactions on Applied Superconductivity [1]. While the TFMC was developed specifically for SPARC and with a long-term pathway to ARC in mind, its underlying no-insulation technology is also useful for other high-field, DC magnet applications. CFS has a magnet business unit that is actively engaging with partners to utilize this technology. CFS is building the mirror coils for the University of Wisconsin (UW) Madison WHAM, an ARPA-E funded high-field magnetic mirror [2]. These are a pair of HTS magnets with 5 cm diameter and 17 T “warm” bores. The first of these magnets, shown in Fig. 2, has been built and operated to full field; the second is under construction. Another application category for non-insulating magnet technology is magnets for energy generation by wind. In collaboration with Hinetics, CFS is designing high-field, no-insulation HTS magnets suitable for use in wind generators. HTS magnets, in addition to offering higher fields, are also much lighter and use far less rare earths than the permanent magnets used in today’s wind generators. In parallel with the TFMC no-insulation HTS magnet R&D, CFS and MIT have executed a series of insulated cable-based HTS magnet R&D programs. Through this collaboration, VIPER cable technology was developed [3]. The VIPER cable is composed of a stack of twisted HTS tapes soldered into a channel in a copper former and wrapped in a structural steel jacket; it can include an embedded cooling channel in the copper former. Tests of the VIPER technology were primarily done through a series of cables tested at the SULTAN facility at École Polytechnique Fédérale de Lausanne. To simulate the forces a cable would experience incorporated into a magnet, the cables are tested with current flowing through them in a background 11 T magnetic field. VIPER cables tested there survived forces, cyclic loading, and stresses and strains expected in SPARC, demonstrating the feasibility of using insulated HTS magnets for SPARC’s major pulsed magnets (central solenoid and poloidal field coils). In addition to immense forces, SPARC’s pulsed coils will also generate significant internal heat through “AC-losses”. To address this, a variation on the VIPER cable technology, the PIT-VIPER, was developed. PIT-VIPER cables have the copper former divided into separate petals, which are insulated from each other to break up return current paths and reduce AC losses. These were initially tested at Brookhaven National Lab in a background magnetic field and demonstrated ~20x reduction in volumetric heating under rapid magnetic field change, confirming the viability of the cables for SPARC-level applications [4]. The technology developed for insulated cable-based superconducting magnets will be used in the SPARC poloidal field and central solenoid coils as well as the current leads. With these successes in VIPER and PIT-VIPER R&D, CFS is proceeding with the ARPA-E funded SPARC CSMC to demonstrate the viability of using HTS in pulsed, high-field superconducting magnets. The CSMC is using the same test facility at MIT as the TFMC. The CSMC is a 4-layer, 10-turn insulated HTS magnet, with the mission to demonstrate SPARC-level pulsed magnetic fields (>4 T/s). The first layer, shown in Fig. 3, has been successfully tested at MIT to 43.3 kA, confirming model predictions and giving greater confidence that the technology can be successfully used in SPARC. These first tests included over 70 quenches and demonstrated the superiority of optical fiber Bragg gratings for quench detection over voltage taps in the demanding HTS magnet environment. The full CSMC will be tested in the coming months. Finally, CFS, MIT, and Type One Energy are collaborating on adopting the VIPER cable technology for use in stellarator coils. While VIPER cables have primarily been fielded in planar magnets with relatively simple bends, stellarator magnets typically have very complex 3D geometry and stringent requirements on winding accuracy. The first tests of complex, stellarator-relevant 3D bends with VIPER cables have met their requirements [5]. In parallel with the TFMC program, CFS and its collaborators developed the conceptual design for SPARC, a compact (R0=1.85 m), high-field (B0=12.2 T) deuterium-tritium burning tokamak employing the TFMC and CSMC magnet technologies. The physics basis for SPARC was published [6-12], demonstrating that the machine could achieve a scientific gain of Q=11 using the same conservative assumptions as ITER (i.e., H98y,2=1.0 in ELMy H-mode), giving confidence that it can meet its mission of Q>1 with ample margin. Empirical performance predictions were backed up by first-principles modeling indicating similar levels of gain. SPARC will be heated with 25 MW of Ion Cyclotron Range of Frequencies (ICRF) at 120 MHz, ensuring there’s enough power to enter into H-mode. The ICRF system, unlike previous generations based on vacuum tubes, has been designed and successfully prototyped to use exclusively solid state electronics; this offers the advantage of significantly less cost, better component availability, less development time, and higher reliability. The small size results in short current relaxation times and short total pulse current flattop length (~10 s); this enables SPARC to operate with an inertially-cooled tungsten divertor. The SPARC team is aiming to demonstrate Q>1 as soon as possible after the first plasma while minimizing total fusion power. Subsequent experimental campaigns will focus on raising machine performance to higher power and higher gain to study and retire risks for high-field tokamak fusion, including burning plasmas and high-power exhaust in advanced divertors. The SPARC Facility is presently being constructed at the CFS campus in Devens, MA, Fig. 4 and 5. Included on the campus are the CFS Headquarters as well as the Magnet Factory, Fig. 6, in which the HTS magnets for SPARC are being assembled and tested. The five major buildings of the SPARC Facility: Tokamak Hall, Diagnostics Building, Power Building, RF Building, and Utility Building are all weather tight and ready for assembly of tokamak components; the RF coax being the first tokamak-specific equipment installed with the cryostat base following shortly after. The SPARC vacuum vessel, Fig. 7, has been in fabrication for over a year, with expected completion in 2024. To get to demonstration of net fusion energy as fast as possible, SPARC was done as a “fast track” project where design and build were done in parallel. As such, the final designs of many systems are closing as construction is completing on the buildings. At the time of this writing, the majority of equipment for SPARC is out for procurement and, of critical importance, the most of the HTS needed for the SPARC magnets is on site. With SPARC well underway, CFS and its collaborators are laying the groundwork for the ARC fusion pilot plant. CFS’ ARC concept has grown out of the ARC-class devices developed at MIT [13,14]. Like SPARC, ARC is a compact, high-field deuterium-tritium burning tokamak. However, ARC must be larger to be able to include sufficient blanket to shield the HTS magnets, enabling an economic lifetime, as well as to breed sufficient tritium. SPARC will be a critical part of the R&D for ARC, both demonstrating the HTS magnets integrated into a high-performance fusion machine as well as closing gaps in tokamak physics. CFS is ramping up R&D for ARC on systems not included in SPARC; including demountable HTS magnets, remote maintenance, fusion nuclear materials, high-temperature FLiBe molten salt, and high throughput tritium processing. ACKNOWLEDGEMENTS Work supported by Commonwealth Fusion Systems. Parts of this work was funded under the INFUSE program – a DOE SC FES public-private partnership – and ARPA-E. REFERENCES [1] HARTWIG, Z.S., et al., The SPARC Toroidal Field Model Coil Program, Submitted to IEEE Trans. Appl. Supercond., preprint available: https://arxiv.org/abs/2308.12301. [2] RADOVINSKY, A., et al., Design of high field HTS coils for magnetic mirror, IEEE Trans. Appl. Supercond. 33 5 (2023) 1-5. [3] HARTWIG, Z.S., et al., VIPER: an industrially scalable high-current high-temperature superconductor cable, Supercond. Sci. Technol. 33 11 (2020) 11LT01. 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