Lupus stellarator (Stellarex)

Overview

The Lupus stellarator, formally the Stellarex project, is a next-generation magnetic confinement fusion experiment designed to investigate and validate the performance of a quasi-axisymmetric (QA) stellarator at reactor-relevant plasma parameters. Located in Grenoble, France, and managed by a public-private consortium, its primary scientific goal is to achieve a plasma energy gain factor (Q_plasma) greater than unity in a steady-state regime. This objective distinguishes it from pulsed devices like most tokamaks and positions it as a key step toward a commercially viable stellarator power plant.

Stellarex's design philosophy centers on combining the intrinsic advantages of the stellarator line—namely, the absence of a large, inductively driven plasma current, which eliminates the risk of disruptive instabilities and enables continuous operation—with the favorable particle confinement properties of an axisymmetric magnetic field. By employing advanced stellarator optimization codes and high-temperature superconducting (HTS) magnets, the project aims to overcome historical challenges in stellarator performance, particularly neoclassical transport and energetic particle confinement, which have limited previous experiments. If successful, Stellarex will provide a critical proof-of-concept for the QA configuration as a leading candidate for a fusion power core.

Physics / Mechanism

The core of the Stellarex design is its quasi-axisymmetric magnetic configuration. Unlike classical stellarators, which possess a complex three-dimensional field, a QA stellarator is optimized to have a magnetic field strength |B| that is approximately symmetric in the toroidal direction when expressed in Boozer coordinates. This engineered symmetry is crucial for reducing neoclassical transport, the primary channel for energy loss in non-optimized stellarators. The reduction in transport arises because particles moving along the magnetic field lines experience a nearly constant |B|, similar to the environment in a tokamak. This significantly improves the confinement of both thermal plasma and high-energy alpha particles produced by deuterium-tritium (D-T) fusion reactions.

The specific magnetic geometry of Stellarex was developed using the STELLOPT and ROSE optimization codes, which simultaneously solve for magnetohydrodynamic (MHD) equilibrium, stability, and neoclassical transport properties. The optimization targeted a low-aspect-ratio (A ≈ 4.2) configuration with four field periods to maximize plasma volume while maintaining good confinement and stability against ballooning and kink modes. The resulting plasma shape is complex, requiring a set of 40 non-planar modular coils to generate the precise field.

To achieve the required on-axis field of 5.8 T in a steady state, Stellarex will use magnets wound from rare-earth barium copper oxide (REBCO), a type of high-temperature superconductor. HTS magnets offer significant advantages over traditional low-temperature superconductors, including a higher operating temperature (20–30 K), a larger temperature margin against quenches, and the potential for simpler cryogenic systems. The use of HTS is a defining engineering feature, enabling the high field strength and compact design necessary to test the Lawson criterion for ignition. Plasma heating will be accomplished through a combination of Electron Cyclotron Resonance Heating (ECRH) for plasma startup and core electron heating, and Neutral Beam Injection (NBI) for core ion heating and current drive-free operation.

Historical Development

The conceptual basis for Stellarex originates from the theoretical work on quasi-symmetry pioneered at the Princeton Plasma Physics Laboratory (PPPL) and the Max Planck Institute for Plasma Physics (IPP) in the late 1990s and early 2000s. Early experiments like the Helically Symmetric eXperiment (HSX) at the University of Wisconsin provided the first experimental validation of the quasi-symmetry concept, demonstrating significantly reduced transport compared to classical stellarators. The successful construction and operation of the Wendelstein 7-X (W7-X) in Germany, while based on a different optimization principle (quasi-isodynamicity), proved the feasibility of building and operating a large-scale, computationally optimized stellarator with complex superconducting coils.

The Stellarex project was formally proposed in 2019 by a consortium led by the French Alternative Energies and Atomic Energy Commission (CEA), building on decades of European expertise in plasma physics and superconducting magnet technology. The proposal gained momentum following advancements in HTS manufacturing that made large-scale, high-field magnets a credible engineering option. A key milestone was the 2021 publication of the finalized magnetic configuration and engineering design, which demonstrated a viable path to achieving Q_plasma > 1 within a manageable device size and cost envelope. The project received final funding approval from the EUROfusion consortium and private investment partners in late 2023, with site preparation in Grenoble beginning the following year. The design heavily incorporates lessons learned from the construction of ITER, particularly in the areas of magnet manufacturing, cryogenics, and remote handling.

Current Status

As of early 2026, the Stellarex project is in the advanced construction phase. The main cryostat hall and supporting infrastructure at the Grenoble site are nearing completion. The first major hardware procurements are underway, with contracts awarded for the HTS conductor and the vacuum vessel forgings. The HTS conductor contract, awarded to /companies/commonwealth-fusion-systems, represents one of the largest orders of REBCO tape to date, signaling a significant maturation of the HTS supply chain.

The winding of the first prototype non-planar modular field coil is scheduled to begin in Q4 2026 at a dedicated facility. This is considered the most critical-path item for the project, as the manufacturing tolerances for these complex coils are exceptionally tight. A full-scale winding and testing program is in place to validate the manufacturing process before commencing production of the 40 series coils. The vacuum vessel, a double-walled stainless steel structure, is being fabricated in sections by a European industrial partner. First plasma is tentatively scheduled for 2034, contingent on the successful delivery and assembly of the main magnetic and vacuum systems.

Notable Implementations

The Stellarex project is a large-scale collaboration involving several key institutions and industrial partners. The primary management and scientific leadership is provided by the CEA, with significant contributions from the EUROfusion consortium, which coordinates research activities across European laboratories. IPP in Germany provides expertise in stellarator theory and modeling, drawing from its experience with W7-X.

Key subsystems and their lead developers include:

• HTS Modular Coils: The design is a collaborative effort between CEA and the Swiss Plasma Center (SPC), with industrial manufacturing of the REBCO tape by Commonwealth Fusion Systems. The coil winding and testing will be performed on-site in Grenoble.
• Heating and Current Drive: The 55 MW heating system is a mix of technologies. The ECRH system (25 MW) is being developed by a consortium including IPP and KIT, while the NBI system (30 MW) is being designed by the Consorzio RFX in Italy, leveraging their experience from the ITER NBTF.
• Tritium Plant: The conceptual design for the tritium fuel cycle systems, including breeding blanket test modules, is being led by the UK Atomic Energy Authority (UKAEA). Stellarex plans to incorporate several Test Blanket Modules (TBMs) to study tritium breeding concepts in a stellarator environment, a critical step for future power plants.

Open Challenges

Despite a robust design, Stellarex faces significant scientific and engineering challenges. The foremost is the manufacturing of the complex, three-dimensional HTS modular coils to the required sub-millimeter precision. Any deviation from the specified geometry could introduce error fields that degrade plasma confinement and compromise the quasi-axisymmetric symmetry. Validating the structural integrity of the coils under immense electromagnetic forces during operation is another primary engineering hurdle.

From a physics perspective, while the QA configuration is predicted to have excellent energetic particle confinement, this must be experimentally verified at reactor-relevant energies and densities. The behavior of plasma turbulence in a high-beta, QA stellarator is an active area of research, and its impact on overall energy confinement remains a key uncertainty. Managing plasma-wall interactions and heat exhaust in the complex 3D geometry of the stellarator divertor is another critical challenge. The Stellarex divertor design is based on the island divertor concept, but demonstrating its ability to handle steady-state heat loads of up to 15 MW/m² will be essential for the success of the mission.

Outlook

The projected timeline for Stellarex aims for first plasma in 2034, followed by a multi-year commissioning and operational phase. The initial hydrogen and helium plasma campaigns will focus on machine commissioning and verifying the magnetic configuration. Deuterium plasma operations are expected to commence around 2037, with the goal of achieving high-performance, long-pulse scenarios.

The D-T campaign, aimed at demonstrating Q_plasma > 1, is planned for the early 2040s. Success in this phase would represent a landmark achievement for the stellarator concept and for fusion energy research as a whole. It would validate the quasi-axisymmetric stellarator as a viable path to a steady-state fusion power plant, potentially triggering the design of a demonstration power plant (DEMO) based on the Stellarex configuration. The project's emphasis on HTS magnets and steady-state operation positions it to provide crucial data for the design of future commercial fusion reactors, regardless of the ultimate performance achieved.

References

1. Overview of the Wendelstein 7-X project — Nuclear Fusion (2015)
2. Aspects of quasi-axisymmetric stellarator design — Physics of Plasmas (1999)
3. Experimental confirmation of the quasi-symmetric magnetic field in HSX — Nuclear Fusion (2007)
4. High temperature superconducting magnets for fusion energy — Fusion Engineering and Design (2021)
5. Recent advances on the stellarator concept — Plasma Physics and Controlled Fusion (2022)
6. The stellarator renaissance — Nature Physics (2016)
7. STELLOPT: A flexible stellarator optimization code — Plasma Physics and Controlled Fusion (2014)
8. Overview of the SPARC project — Journal of Plasma Physics (2020)