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

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.

• The paper introduces a novel fusion power plant design with planar coils that simplify manufacturing and maintenance while achieving robust plasma confinement.
• It employs high-temperature superconductors and quasi-axisymmetric optimization to enhance plasma-coil separation and ensure MHD stability.
• The integrated design yields 1.1 GW thermal output, an 88% capacity factor, and a tritium self-sufficient cycle, demonstrating practical fusion viability.

Overview of the Helios Planar Coil Stellarator Fusion Power Plant

Introduction and Design Rationale

The Helios design represents a significant departure from the traditional modular coil stellarator paradigm, leveraging a planar coil architecture compatible with established manufacturing techniques, high-temperature superconductors (HTS), and quasi-axisymmetric (QA) plasma optimization. The design process is informed by advanced systems analysis, high-fidelity simulation, and benchmarking to existing experimental results, prioritizing robust engineering constraints over aggressive plasma physics extrapolations.

Whereas previous stellarators such as ARIES-CS [najmabadi_aries-cs_2008, ku_physics_2008] relied on complex 3D modular coil forms with tight tolerances, Helios utilizes 12 large, planar, plasma-encircling coils and 324 individually addressable planar field-shaping coils, all convex and manufacturable by winding in tension. This configuration, enabled by advances in both magnetic equilibrium optimization [landreman_optimization_2022, jorge_single-stage_2023] and HTS coil engineering [hartwig_sparc_2023], allows increased plasma-coil separation (minimum 1.2 m) and significantly simplifies blanket, shielding, and maintenance architecture.

Plasma Physics and Equilibrium

Helios operates as a QA stellarator with two field periods, an 8 m major radius, aspect ratio 4.5, and a 6 T on-axis magnetic field. The equilibrium was optimized using DESC [dudt_desc_2020] for quasisymmetry (to facilitate energetic particle confinement), MHD stability (Mercier and ballooning criteria), and engineering proxies such as coil-field stress and placement.

Energetic alpha particle losses are reported at 6.6% of fusion product energy, sufficient for sustained ignition but with peak local wall loads up to 4 MW/m², necessitating targeted divertor and wall engineering. MHD stability assessed via TERPSICHORE [anderson_terpsichore_1990] and M3D-C1 [jardin_multiple_2012] yields growth rates below critical limits ($\gamma/\omega_A < 2\%$), and nonlinear simulations do not exhibit deleterious global instabilities or pressure flattening even with edge stochasticity.

Key operational metrics include a peak ion temperature of 20 keV, central density $2.1 \times 10^{20}$ m$^{-3}$, energy confinement time 1.8 s, and $H_{ISS04} = 1.4$ verified by first-principles gyrokinetic–transport simulation (GENE and Trinity3D), with configuration-tailored transport suppression via reversed magnetic shear.

Engineering and Maintainability

The Helios coil system is based entirely on planar coils: 12 toroidal field-like encircling coils (20 T max on-conductor, 20 K HTS, stresses under 800 MPa) and 324 circular shaping coils, designed for modularity and replacement. The shaping coils’ independent control affords both correction for assembly errors and dynamic adjustment for bootstrap or induced current effects.

Blanket and shielding employ a 50 cm thick, lead-lithium eutectic breeder (65% $^6$Li), EUROFER97 structure, and multi-layer neutron attenuation (WC, B$_4$C, borated water, HDPE), yielding a tritium breeding ratio (TBR) of 1.3 and projected coil lifetimes > 40 years based on fast neutron fluence limits [prokopec_suitability_2014, fischer_effect_2018]. The vanadium-alloy (V-4Cr-4Ti) first wall allows for 15 FPY operation at high neutron flux.

The divertor is a continuous, non-resonant, X-point configuration with tungsten, helium-cooled targets operating up to 10 MW/m², integrating baffle and dome systems inspired by tokamak solutions [kukushkin_effect_2007, galassi_numerical_2020].

Maintenance is sector-based: entire toroidal sectors of the radial build (first wall through shaping coils) can be removed between the large encircling coils without disturbing those coils, enabling 84-day biennial outages and an estimated 88% capacity factor—significantly improving on the port-based, high-part-count schemes required for compact modular coil designs.

Facility and Fuel Cycle

The power balance yields 1.1 GW thermal and 390 MW net electric output at nominal efficiency (40%). The plant is tritium self-sufficient with <2 kg startup inventory. The supporting electrical and control infrastructure is highly modular, with redundancy and fast-response architecture integrated for both main field supplies and startup ECRH systems (12 × 170 GHz gyrotrons).

The facility design encompasses full Rankine cycle thermodynamics, closed-cycle helium/lead-lithium cooling for critical in-vessel components, advanced cryogenics (10 MW required at 20 K), and a suite of modern diagnostic and control systems employing multi-rate FPGA/GPU architectures.

Implications and Outlook

Helios addresses longstanding obstacles for stellarator power plant deployment: coil complexity, tight plasma–coil clearance, blanket/maintenance impracticality, and divertor inefficiency. It accomplishes this via three synergistic developments:

1. Planar coil arrays: Allow conventionally manufacturable and replaceable magnet systems, with in situ error correction.
2. High-temperature superconductors: Enable both high magnetic field and increased coil–plasma separation for practical blanket and shield integration.
3. Quasi-axisymmetric optimization: Achieve compactness, robust stability, and improved energetic particle confinement with relaxed geometric constraints.

Helios sets conservative, experimentally grounded performance targets and demonstrates, via integrated physics and engineering modeling, that a practical QA stellarator power plant is viable with current or near-term high-field HTS technology. It positions the planar coil QA stellarator as a leading configuration for first-of-a-kind fusion electricity generation, pending experimental validation of specific physics and engineering risks (e.g., divertor heat load management, long-life HTS coil resilience, and fast ion loss mitigation).

Future theoretical and experimental work should focus on refining turbulent transport surrogates in QA geometry [landreman_how_2025], continued improvement in fast ion optimization [bonofiglo_fast_2025], engineering demonstration of large planar HTS coils [nash_prototyping_2025], and sector-based maintenance and remote handling validation.

Conclusion

The Helios concept articulates a technically and operationally credible stellarator fusion power plant based on a planar coil, QA architecture with conservatively engineered subsystems. By integrating advances in magnetic field optimization, superconducting technology, and maintainable plant design, Helios demonstrates plausible path to fusion power with high availability, inherent steady-state operation, and manageable in-vessel component lifetimes, establishing a major new pathway in magnetic fusion research (2512.08027).