Commonwealth Fusion Systems

P. Ball

An emerging industry of nuclear-fusion firms promises to have commercial reactors ready in the next decade. An emerging industry of nuclear-fusion firms promises to have commercial reactors ready in the next decade. Credit: Gretchen Ertl, CFS/MIT-PSFC, 2021 Commonwealth Fusion Systems - Collaborative team working on the magnet inside the test stand housed at MIT. Commonwealth Fusion Systems - Collaborative team working on the magnet inside the test stand housed at MIT.

A. Watterson, Amanda Hubbard, Rui Diaz-Pacheco, Ashleigh Francis, R. Granetz, James Logan, T. Golfinopoulos, Alexey Kaplan, P. Michael, Richard Murray, C. Sanabria, S. Schweiger, T. Toland, R. Vieira

PIT VIPER cables are rare-Earth barium copper oxide (REBCO)-based high-temperature superconducting (HTS) cables developed at the Massachusetts Institute of Technology and Commonwealth Fusion Systems. The cable design consists of a twisted copper former with grooves filled with stacks of REBCO tape, enclosed in a structural jacket. The assembly is then filled with solder in a vacuum pressure impregnation (VPI) process. One side effect of the VPI process, however, is potential damage to the superconductor that reduces its critical current and critical exponent. Damage may result from two mechanisms: prolonged exposure to elevated temperatures, and physical erosion of the copper stabilizer layer of the HTS tape, which leaves parts of the REBCO layer unprotected. In this experiment, the effect of extended time scales ($>2$ h) of flowing tin-lead-based solder exposure on HTS in PIT VIPER cables was tested, exploring for the first time exposure to flowing solder at a time scale that is particularly relevant to large-scale magnet manufacturing. During this study, two experimental samples were manufactured for electrical testing: one 2.5-m-long straight cable and one 20-m-long coiled cable. Each cable was exposed to molten tin-lead solder for 2.5 h during the VPI process, and then, electrically tested in a liquid nitrogen bath. Critical current, $n$-value, and resistance were measured. Critical current is compared to modeled values determined from characterization of the tape used, and degradation is assessed from this comparison. The measured critical current, when tested in a liquid nitrogen bath and under self-field, was uniform within reasonable experimental error. This result derisks solder degradation for the manufacturing of SPARC cable magnets.

J. Hillesheim, A. Creely, T. Eich, N. T. Howard, N. Leuthold, R. Sweeney, A. LeViness, A. O. Nelson, L. Nichols, R. Tinguely, M. Usoltseva, D. Battaglia, T. Body, C. Hansen, N. Logan, R. Mumgaard, P. Rodriguez-Fernandez, P. B. Snyder, B. Sorbom, John C. Wright

Commonwealth Fusion Systems plans to build ARC as the first fusion power plant at a site in Chesterfield County, Virginia, USA by the early 2030s. We present an overview of analysis comprising the physics basis of the ARC V3A design, a high-magnetic-field tokamak with $B_0=11.4 \ \text{T}$ , $I_p=12.0 \ \text{MA}$ , $R_0=4.62 \, \text{m}$ , $a=1.18 \, \text{m}$ . ARC V3A is designed to produce $P_{fus} \approx 1.13$ GW DT fusion power and deliver $\geqslant$ 400 MW net electric power to the grid. This overview includes quantitative analysis of fundamental issues for design of and operational plasma scenarios for a tokamak power plant, and lays out the design targets and strategic choices for ARC, including empirical fusion performance projections, assessment of H-mode access, ion cyclotron resonance heating simulations, alpha particle physics and time-dependent full-pulse simulations. This is complemented by topical papers on fusion performance and transport, disruption physics, boundary physics and magnetohydrodynamic stability. Critically, these studies identify key model uncertainties and physics risks to be retired through SPARC operation. Due to the modular nature of ARC, early results from SPARC can be incorporated into the design of the first ARC as well as subsequent replacements of the ARC vacuum vessel.

M. Reinke, B. Sorbom, M. Greenwald

The path to demonstrate how fusion energy can be used to generate net electricity is undergoing an important transition, shifting from relying almost exclusively on public funding to also being supported by a diverse set of private companies. This article discusses the motivations and processes by which peer review, a mainstay of publicly funded fusion science, translates to the research and development activities of private fusion companies. The perspective is from a team which has experienced this evolution first-hand, having transitioned from publicly funded fusion projects to working on the high-field tokamak path, supported or employed by Commonwealth Fusion Systems. We believe the continuation of peer review to be critical to the advancement of industry-led fusion science, but also acknowledge where it needs to have restrictions due to pursuit of some fusion technology that may need to remain proprietary. The discussion is expected to be generally applicable to any privately funded fusion endeavor, but necessarily draws upon the experience gained from developing the science, engineering, and technology basis for the SPARC tokamak and the planning for its future operation.

S. Oh, H. Kim, S. Nam, Y. Chu, D. Oh, H. Choi, J. Lee, W. Kim, Y. Jeong, H. Chang, J. Lee, S. Hahn

Full transposition of superconducting wires within a conductor may not be a requirement for high temperature superconductor (HTS) magnet for fusion applications. Already, conduction-cooled 20 T model coil built with non-transposed HTS stacks has been demonstrated by Commonwealth Fusion Systems (CFS). Here, we discuss 3 possible stacked HTS cryogen-cooled conductor concepts for toroidal field (TF) magnet applications. The first one is somewhat like LTS conductors, the second, HTS stacks are capped by copper stabilizer, the last one, only stacks without copper. We simulate a simplified TF magnet model, 12 T, size of KSTAR, using the 3 conceptual HTS conductor designs. A comparative thermo-hydraulic analysis for a fast charging case has been carried out and its implications on HTS conductor design are further discussed.