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Overview of the Helios Design: A Practical Planar Coil Stellarator Fusion Power Plant

C. P. S. Swanson, S. T. A. Kumar, D. W. Dudt, E. R. Flom, W. B. Kalb, T. G. Kruger, M. F. Martin, J. R. Olatunji, S. Pasmann, L. Z. Tang, J. von der Linden, J. Wasserman, M. Avida, A. S. Basurto, M. Dickerson, N. de Boer, M. J. Donovan, A. H. Doudna Cate, D. Fort, W. Harris, U. Khera, A. Koen, J. A. Labbate, N. Maitra, A. Ottaviano, R. K. Parmar, E. J. Paul, B. Reydel, A. van Riel, P. K. Romano, M. Savastianov, S. Saxena, S. Seethalla, S. Srinivasan, R. H. Wu, D. Nash, J. Priebe, M. Slepchenkov, S. Walsh, B. Berzin, D. A. Gates, the Thea Energy team

TL;DR

Helios presents a practical, steady-state fusion power plant concept based on planar coil quasi-axisymmetric stellarator design, pairing HTS coils with a tokamak-like X-point divertor and a sector-based maintenance scheme. The study integrates physics, engineering, and economics through high-fidelity simulations of equilibrium, MHD, turbulence, and neutronic transport, along with detailed engineering designs for coils, blanket, divertor, cryogenics, and power systems. Key findings include viable 1.1 GW thermal output and 390 MW net electric power, a 40-year coil lifetime with a 1.2 m plasma-coil gap, and a maintenance cadence of 84 days every two years achieving ≈88% capacity factor. The work argues for the practicality of a QA planar-coil stellarator family within current materials and HTS capabilities, with companion papers providing in-depth analyses of each subsystem to de-risk and validate the overall concept.

Abstract

Thea Energy, Inc. has developed the preconceptual design for "Helios," a fusion power plant based on the planar coil stellarator architecture. In this overview paper, the design is summarized and the reader is referred to the other papers for more detail. The Helios design is based around a two-field-period quasi-axisymmetric ("QA") stellarator equilibrium with aspect ratio 4.5 and a novel tokamak-like X-point divertor. The natural stability, low recirculating power, and steady-state capability of the stellarator are leveraged. Stability and transport are calculated using state-of-the-art, high-fidelity codes and grounded in measured performance of existing experiments. The electromagnetic coil set is high-temperature superconducting ("HTS") and consists of 12 large, plasma-encircling coils like the toroidal field coils of a tokamak, and 324 smaller, field-shaping coils. All coils are planar and convex. A maximum of 20 T on-coil is enforced, a value which has been achieved in existing large-bore HTS coils. There is a minimum of 1.2 m between plasma and coils, leaving space for tritium breeding blanket and neutron shielding. Because of this thick shielding, all coils have a minimum 40-year operational lifetime, the same minimum lifetime of the power plant system. 1.1 GW of thermal power and 390 MW of net electric power are produced. The shaping coils are individually controllable, enabling a uniquely configurable magnetic field for relaxed manufacturing and assembly tolerances and plasma control. A practical maintenance architecture is a primary driver of the design; maintenance is performed on entire toroidal sectors that are removed from between the encircling coils. A biennial maintenance cycle is estimated to take approximately 84 days, resulting in an 88% capacity factor. Rigorous engineering constraints such as temperature and stress limits are enforced.

Overview of the Helios Design: A Practical Planar Coil Stellarator Fusion Power Plant

TL;DR

Helios presents a practical, steady-state fusion power plant concept based on planar coil quasi-axisymmetric stellarator design, pairing HTS coils with a tokamak-like X-point divertor and a sector-based maintenance scheme. The study integrates physics, engineering, and economics through high-fidelity simulations of equilibrium, MHD, turbulence, and neutronic transport, along with detailed engineering designs for coils, blanket, divertor, cryogenics, and power systems. Key findings include viable 1.1 GW thermal output and 390 MW net electric power, a 40-year coil lifetime with a 1.2 m plasma-coil gap, and a maintenance cadence of 84 days every two years achieving ≈88% capacity factor. The work argues for the practicality of a QA planar-coil stellarator family within current materials and HTS capabilities, with companion papers providing in-depth analyses of each subsystem to de-risk and validate the overall concept.

Abstract

Thea Energy, Inc. has developed the preconceptual design for "Helios," a fusion power plant based on the planar coil stellarator architecture. In this overview paper, the design is summarized and the reader is referred to the other papers for more detail. The Helios design is based around a two-field-period quasi-axisymmetric ("QA") stellarator equilibrium with aspect ratio 4.5 and a novel tokamak-like X-point divertor. The natural stability, low recirculating power, and steady-state capability of the stellarator are leveraged. Stability and transport are calculated using state-of-the-art, high-fidelity codes and grounded in measured performance of existing experiments. The electromagnetic coil set is high-temperature superconducting ("HTS") and consists of 12 large, plasma-encircling coils like the toroidal field coils of a tokamak, and 324 smaller, field-shaping coils. All coils are planar and convex. A maximum of 20 T on-coil is enforced, a value which has been achieved in existing large-bore HTS coils. There is a minimum of 1.2 m between plasma and coils, leaving space for tritium breeding blanket and neutron shielding. Because of this thick shielding, all coils have a minimum 40-year operational lifetime, the same minimum lifetime of the power plant system. 1.1 GW of thermal power and 390 MW of net electric power are produced. The shaping coils are individually controllable, enabling a uniquely configurable magnetic field for relaxed manufacturing and assembly tolerances and plasma control. A practical maintenance architecture is a primary driver of the design; maintenance is performed on entire toroidal sectors that are removed from between the encircling coils. A biennial maintenance cycle is estimated to take approximately 84 days, resulting in an 88% capacity factor. Rigorous engineering constraints such as temperature and stress limits are enforced.

Paper Structure

This paper contains 21 sections, 12 figures, 1 table.

Table of Contents

  1. Introduction
  2. Summary of the design
  3. Plasma design and simulation
  4. Scoping studies, heating and fueling, and dynamic accessibility
  5. The stellarator equilibrium
  6. Energetic particle confinement
  7. Magnetohydrodynamic stability and evolution
  8. A note on the effects of an abrupt plasma termination in Helios
  9. Turbulence, transport, and profile prediction
  10. Electromagnetic coil physics design
  11. Divertor physics
  12. Engineering design of systems
  13. Electromagnetic coil engineering
  14. Divertor engineering and the first wall
  15. Neutronics, blanket, shield, and bioshield
  16. ...and 6 more sections

Figures (12)

  • Figure 1: The Helios equilibrium and coil set. The coils (silver) are planar and convex. The quasi-axisymmetric plasma equilibrium (red to blue) is shaded by the magnetic field amplitude on the surface produced by that coil set. Very good quasi-symmetry is visible.
  • Figure 2: Helios architecture, cut-away view. From interior to exterior, the plasma (green), the blanket (black and blue), the shaping coils (orange), the encircling coils (gray). The cryostat surrounds all with removable panels. The divertor pump ducts, cryogen delivery manifold, and microwave waveguides are also shown.
  • Figure 3: POPCON plot of the plasma operational trajectory (red) from zero plasma pressure through an ignited state. 10 MW of ECH power is required to start up. No operational constraints are violated.
  • Figure 4: Four toroidal cross-sections of the boundary of the Helios plasma equilibrium. The equilibrium was optimized for several techno-economic figures of merit including proxies for coil cost and complexity.
  • Figure 5: M3D-C1 simulation results. Poincaré sections of the magnetic field at $\phi=0$ (a-e), $\pi/4$ (f-j), and $\pi /2$ (k-o) shown as a function of time for $0 \leq t \leq 1600\tau_A$ (left to right). Perfect stability would be indicated by nested flux surface of different colors, and little change between times. Small island chains do appear. Stochastic regions occur at the edge but do not result in pressure profile flattening. No large-scale unstable modes are in evidence.
  • ...and 7 more figures