ARC Fusion Power Plant Physics Basis

13 min read Original article ↗
Skip to main content Accessibility help

This collection of five peer-reviewed articles describes the foundation of physics understanding that supports the ARC™ power plant design from Commonwealth Fusion Systems® (CFS).

The articles and an accompanying editorial, from authors at CFS and several collaborating institutions, describe the tokamak’s core plasma physics behaviour; the ways it will handle challenges like disruptions and heat exhaust; and SPARC tokamak projects that will inform the ARC tokamak’s physics. 

For more details, check the CFS press release, CFS blog post and Cambridge University Press story.

ARC fusion power plant rendering

Image courtesy of Commonwealth Fusion Systems

"These papers show why CFS and our partners are confident in the physics underlying the ARC fusion power plant. Designing a fusion power plant that we truly intend to build soon meant that we had to ask the right questions, sharpen our tools, and focus on the top priority information that SPARC will teach us."

- Alex Creely, Commonwealth Fusion Systems, USA - Guest Editorial co-author

"This special issue highlights the important role that peer-reviewed science can play in accelerating fusion commercialization. The ARC papers provide the broader community an opportunity to critically examine the physics basis for a fusion power plant while demonstrating that fusion companies like CFS can meaningfully contribute to the open scientific literature without compromising intellectual property."

- Troy Carter, Oak Ridge National Laboratory, USA - Journal of Plasma Physics Associate Editor and Special Issue Editor

"I am delighted that JPP is able to publish this new collection of papers on the ARC Physics Basis. Our publication of the SPARC Physics Basis in 2020 established what we hope is a gold standard for peer-review scrutiny and publication of the intellectual case for commercially funded fusion projects. That the case for SPARC is now followed by this case for its younger and bigger sibling is testimony both to the lively pace of innovation in the field and to the value of scientific peer review in an environment where basic science meets concerted effort at commercial viability. It is also just exciting to all of us in plasma physics that a case for a prototype fusion power plant can be specifically discussed on such a serious level. As these papers enter scientific circulation, I look forward to seeing a wave of further theoretical and modelling work, as well as lively debates and competing ideas, motivated by them."

- Alex Schekochihin, University of Oxford, UK - Journal of Plasma Physics Editor

Contents

Editorial

Research Article

  • Power and particle exhaust for the ARC fusion power plant

  • Part of:
  • Thomas H. Eich, Thomas A.J. Body, Tom P. Looby, Sean B. Ballinger, Alexander J. Creely, Jon C. Hillesheim, Philip B. Snyder, Nathan T. Howard, Rebecca Masline, Michael R.K. Wigram, George R. Tynan
  • Journal:
    Journal of Plasma Physics / Volume 92 / Issue 3 / 2026
    Published online by Cambridge University Press:
    04 June 2026, E66

  • To successfully show that fusion is an attractive energy source, the ARC$^{\scriptstyle \mathrm{TM}}$ fusion power plant will need to operate with a robust, integrated power and particle exhaust solution. To maximise ARC’s fusion power output while avoiding excessive erosion of the plasma-facing components, we will need to radiatively dissipate most of the power crossing the last-closed flux surface, injecting radiating impurities such as argon or neon to access divertor detachment. Divertor detachment will need to be integrated with a high-performance core plasma, and with efficient impurity pumping to prevent the accumulation of helium ash in the core. To access and control detachment in high-performance plasmas, we have designed ARC with up–down-symmetric divertors, with secondary X-points in long, tightly baffled outer legs. Using a core-edge modelling workflow, we predict that with this divertor design, ARC will access detachment with modest argon seeding in the divertor (${c_{Ar,div}}\sim {0.9\,\%}$), which should have minimal impact on the core (${\Delta Z_{\textit{eff},\textit{core}}}\lt {0.5}$) for reasonable argon enrichment (${c_{Ar,div}/c_{Ar,\textit{core}}}={6}$). Due to the high predicted divertor neutral pressure (${p_{\textit{div}}}\sim {20\,\mathrm{Pa}}$), we predict that ARC will sufficiently pump helium to limit ash accumulation in the core (${c_{\textit{He},\textit{core}}}\lt {2\,\%}$) for a helium enrichment of ${c_{\textit{He},\textit{div}}/c_{\textit{He},\textit{core}}}={0.4}$. ARC’s divertor design is expected to increase the stability of a detachment front in the outer divertor leg, helping to prevent divertor reattachment during smaller heat-flux transients such as scrape-off-layer filaments associated with the quasi-continuous exhaust regime. However, this buffering will not be sufficient to prevent divertor reattachment during large type-I edge-localised modes (ELMs), and as such these will need to be avoided on ARC. Experiments on SPARC will be used to select an integrated scenario which avoids or mitigates type-I-ELMs while maintaining access to detachment, good core fusion performance and sufficient impurity exhaust. SPARC experiments will also be used to finalise ARC’s divertor design, by studying the impact of magnetic and first-wall geometry on detachment stability, impurity enrichment and neutral baffling under conditions similar to those expected for ARC. In conclusion, our analysis finds that ARC will have a viable power and particle exhaust solution which is compatible with high-power operations, and this solution will be validated in experiments on SPARC.

  • Performance and transport in the ARC tokamak

  • Part of:
  • N.T. Howard, P. Rodriguez-Fernandez, J. Hall, M. Muraca, A. Saltzman, A. Ho, J.C. Hillesheim, A.J. Creely, T.H. Eich, T. Body, P.B. Snyder, C. Holland
  • Journal:
    Journal of Plasma Physics / Volume 92 / Issue 3 / June 2026
    Published online by Cambridge University Press:
    04 June 2026, E67

  • The ARC$^{\textrm {TM}}$ tokamak, a high-field ($B_T$ = 11.4 T) fusion power plant, under development by Commonwealth Fusion Systems, is studied using a suite of integrated modelling tools to predict its fusion power generation ($P_{fus}$), transport and confinement properties. Analysis is based off an ARC operational point scoped first with zero-dimensional (0-D) plasma operational contour (POPCON) modelling to produce 1.13 GW of fusion power. A suite of integrated modelling tools (TRANSP, ASTRA and TORAX) were applied to predict the performance and kinetic profiles of the ARC design point, yielding a range of predicted performance spanning from ${\sim} 900$ to 1300 MW in rough quantitative agreement with POPCON predictions. The sensitivity of these results to uncertain modelling inputs was probed using scans of pedestal boundary conditions around EPED-predicted values (total pressure and temperature ratios), tungsten concentration and seperatrix density around their nominal assumptions. Pedestal pressure and pedestal top $(T_i/T_e)$ play a large role in 1.5-dimensional performance predictions, able to modify the predicted $P_{fus}$ by a factor of 2 within reasonable assumptions. High-fidelity core nonlinear gyrokinetic profile predictions, performed using CGYRO (Candy et al. 2016 J. Comput. Phys., vol. 324, pp. 73–93) coupled with the PORTALS (Rodriguez-Fernandez et al. 2024 Nucl. Fusion, vol. 64, 076034; Phys. Plasmas, vol. 31, 2024, 062501) framework, yield substantially lower performance ($P_{fus} = 677$ MW) compared with 0-D and medium-fidelity modelling for nominal assumptions, showing that there is non-negligible uncertainty between models and that future work on SPARC may help resolve discrepancies. Lower overall performance results from significantly reduced volume-averaged densities and temperatures, along with reduced levels of density and temperature peaking. Turbulence and transport are largely dominated by ion temperature gradient across the profile, confirmed by both linear stability and the response of the nonlinear fluxes to changes in gradients, with some impact of kinetic ballooning modes in the deep core. This work represents one of the most complete scoping of potential fusion power plant conditions performed to date. The extensive integrated modelling provides confidence in ARC performance approaching 1 GW, while nonlinear gyrokinetic modelling results in open questions into the physics of density and temperature peaking in fusion-power-plant-relevant operational space. A discussion of results and the role that the SPARC tokamak (Creely et al. 2020 J. Plasma Phys., vol. 86, 865860502) will play in informing ARC design, performance and operation is presented.

  • ARC physics basis–magnetohydrodynamics

  • Part of:
  • N. Leuthold, N.C. Logan, D.A. Burgess, A.O. Nelson, S. Benjamin, C. Hansen, A. Kumar, C.F.B. Zimmermann, F. Carpanese, A.J. Creely, J.C. Hillesheim, M. Muraca, C. Paz-Soldan
  • Journal:
    Journal of Plasma Physics / Volume 92 / Issue 2 / April 2026
    Published online by Cambridge University Press:
    14 April 2026, E49

  • ARC is designed to produce ${400}\,\textrm {MW}$ of net electricity and prove the commercial feasibility of a fusion power plant. In order to achieve this goal ARC has to operate with optimal core performance in a stationary scenario that minimises wear on the first wall and divertor. This requires avoiding or mitigating magnetohydrodynamic (MHD) instabilities which have the potential to not only degrade the plasma core but also lead to deleterious transient heat loads on plasma facing components. Therefore, this work aims at characterising the MHD stability of the high performance ARC scenario and inform the design of error field correction coils. Firstly, simulations of vertical displacement events show that an in-vessel coil is not needed and instead the poloidal shaping coils can be used to control vertical stability. These simulations also inform the demands on the corresponding coil power supplies. Stability analysis of the ideal kink mode with or without a conducting wall and kinetic effects suggests that the ARC baseline scenario operates deeply in the stable region. Using RDCON, tearing modes at the $m/n=2/1$ and $3/2$ surfaces (with poloidal mode number $m$, and toroidal mode number $n$) are shown to be linearly stable, and including thermal transport effects in the rational surfaces lead to further stabilisation. However, other transient plasma instabilities can seed neoclassical tearing modes (NTMs). The marginally stable width of NTMs in ARC strongly depends on the internal inductance and can fall below ${0.1}{\,\,\%}$ of the normalised poloidal flux. Furthermore, an empirical cross-machine model of the $n=1$ error field leading to a disruption predicts a critical error field larger than SPARC but smaller than ITER. Three-dimensional coils can be designed with the Generalised Purturbed Equilbium Code based on a simple model that calculates the maximum correctable error field that is limited by the neoclassical toroidal viscosity torque. Broad scans of different coil geometries identify a set of 2 rows of off-midplane coils to be a suitable solution. It is also determined that such a set of three-dimensional coils is capable of correcting $n=2$ error fields to some degree and creating strong enough $n=2$ or $n=3$ edge resonant perturbation fields for the suppression of edge-localised modes at reasonable coil currents. The final design of the first ARC will be further informed by results from SPARC.