Heliac

Overview

The Heliac, an acronym for Helical Axis Stellarator, is a type of stellarator confinement device for fusion energy research. Its defining characteristic is a magnetic axis that is not planar but instead traces a helical path in three-dimensional space. This is typically achieved by arranging a set of toroidal field (TF) coils helically around a central circular conductor, often called the 'l-coil' or 'hard core'. The interaction between the fields from the TF coils and the central conductor creates a magnetic field with a strong, built-in rotational transform. This high rotational transform provides robust magnetohydrodynamic (MHD) stability, theoretically allowing for stable plasma confinement at high beta (β), the ratio of plasma pressure to magnetic pressure.

Like all stellarators, Heliacs are intrinsically capable of steady-state operation because they do not require a large, inductively driven plasma current for confinement, unlike the tokamak. This eliminates the need for current drive systems and removes the risk of major disruptions, which are sudden, catastrophic losses of plasma confinement. The Heliac's design specifically targets the optimization of magnetic well and shear to control instabilities, aiming to achieve a high-beta equilibrium that could lead to a more compact and economically attractive fusion power plant.

Physics / Mechanism

The magnetic configuration of a Heliac is generated by a combination of external coil sets. The primary components are:

1. Toroidal Field (TF) Coils: A set of planar, circular coils whose centers are arranged along a helical path around a central ring.
2. Central Conductor (l-coil): A circular coil interlinking the TF coils. A large current in this coil generates a strong poloidal magnetic field.
3. Vertical Field (VF) Coils: A pair of large circular coils, typically located above and below the main assembly, used to control the radial position of the plasma.

In a 'flexible Heliac' design, an additional helical coil is wound around the central conductor, providing further control over the magnetic field properties. The superposition of the fields from the TF coils and the central conductor creates nested magnetic flux surfaces with a helical magnetic axis. The number of TF coils per field period (M) and the number of field periods (N) are key design parameters.

The rotational transform (ι) in a Heliac is very high, often exceeding unity (ι > 1) across the plasma radius. This is a direct consequence of the helical axis. A high transform is beneficial for avoiding low-order rational surfaces that can lead to MHD instabilities and island formation. The configuration can also be designed to have a magnetic well—a region where the magnetic field strength increases outwards from the magnetic axis—across the entire plasma volume. A magnetic well provides stability against interchange modes, a type of pressure-driven instability. The combination of high rotational transform, magnetic shear (the radial variation of ι), and a magnetic well allows Heliacs to access stable, high-beta plasma regimes that are difficult to achieve in other confinement concepts. The theoretical beta limit in some Heliac configurations can exceed 5%, a key target for the Lawson criterion in a power plant scenario.

Historical development

The Heliac concept emerged from theoretical work in the late 1970s and early 1980s aimed at creating stellarator configurations with improved stability properties. The initial idea was developed independently at Princeton Plasma Physics Laboratory (PPPL) and the Australian National University (ANU). The first experimental device to test the concept was the SHEILA Heliac at ANU, which began operation in 1985. It was a small device used to validate the formation of helical-axis magnetic surfaces and study basic plasma properties.

This was followed by the construction of larger, more flexible devices. The H-1 Heliac (later H-1NF) at ANU, commissioned in 1992, was designed as a flexible research facility, allowing for a wide range of magnetic configurations to be explored. It has been instrumental in studying MHD stability, plasma fluctuations, and wave heating in Heliac plasmas. Concurrently, the TJ-II Heliac was designed and built at CIEMAT in Madrid, Spain, achieving its first plasma in 1997. TJ-II is a four-field-period device known for its extreme flexibility in modifying its magnetic configuration, enabling detailed studies of the link between magnetic topology and plasma transport. The Compact Toroidal Hybrid (CTH) at Auburn University in the United States, which began operation in 2005, is a hybrid device that can superimpose an Ohmically-driven plasma current onto a Heliac configuration to study the physics of current-carrying stellarators and disruption mitigation.

These experiments have collectively demonstrated the key principles of the Heliac: the successful formation of stable, helical-axis plasmas and the ability to control the magnetic configuration to influence confinement and stability.

Current status

As of 2026, Heliac research continues at a few key institutions worldwide, focusing on validating theoretical models and addressing specific physics challenges. The primary active devices are H-1NF, TJ-II, and CTH. Research has shifted from basic concept validation to more detailed transport and stability studies relevant to a future fusion reactor.

Recent work on TJ-II has focused on understanding the role of energetic particles in stellarator plasmas, which is critical for alpha particle confinement in a burning plasma. Experiments have used Neutral Beam Injection (NBI) to create fast ion populations and study their confinement and interaction with MHD modes. TJ-II has also made significant contributions to understanding the physics of plasma turbulence and transport barriers in non-axisymmetric systems. For instance, studies have shown that modifying the magnetic configuration can trigger transitions to improved confinement regimes, analogous to the H-mode in tokamaks.

At H-1NF, research emphasizes the study of Alfven eigenmodes, MHD stability boundaries, and the development of advanced plasma diagnostics. The flexibility of H-1NF allows for systematic scans of magnetic parameters to benchmark and validate complex 3D physics codes like VMEC and STELLOPT. CTH continues to explore the physics of plasmas that bridge the gap between pure stellarators and tokamaks, providing unique data on how a 3D magnetic field can be used to control and mitigate current-driven instabilities.

Notable implementations

• H-1 National Plasma Fusion Research Facility (H-1NF): Located at the Australian National University, H-1 is the largest stellarator in Australia. It is a three-field-period flexible Heliac with a major radius of 1.0 m. Its research program is a cornerstone of the Australian fusion effort, focusing on fundamental plasma physics, wave-plasma interactions, and diagnostic development. It serves as a national facility for researchers across the country.

• TJ-II: Operated by the Laboratorio Nacional de Fusión at CIEMAT in Spain, TJ-II is a four-field-period flexible Heliac with a major radius of 1.5 m. It is a prominent device in the European fusion program and has been highly productive, contributing significantly to the understanding of stellarator transport, stability, and the physics of internal transport barriers. Its operational program is closely coordinated with the broader European effort, including the Wendelstein 7-X stellarator.

• Compact Toroidal Hybrid (CTH): Situated at Auburn University in the USA, CTH is a five-field-period device with a major radius of 0.75 m. Its unique capability is the ability to induce a significant toroidal current, allowing it to explore a wide range of hybrid stellarator-tokamak scenarios. This research is directly relevant to understanding and controlling disruptions in tokamaks and exploring alternative operating regimes.

Open challenges

Despite its theoretical advantages, the Heliac concept faces significant scientific and engineering challenges that have prevented it from becoming a mainstream path to a fusion reactor.

1. Engineering Complexity and Access: The defining feature of a Heliac—the central conductor interlinked with the TF coils—creates a topologically complex and constrained geometry. This makes assembly, maintenance, and diagnostic access extremely difficult. For a power plant, this design would pose severe challenges for installing components like the tritium breeding blanket and divertor.

2. Neoclassical Transport: While stellarators are optimized to reduce turbulent transport, they suffer from neoclassical transport, which arises from particle drifts in the complex 3D magnetic field. In the low-collisionality regime of a reactor, this can lead to significant energy losses. While Heliacs can be optimized to reduce these losses, they remain a concern, particularly for impurity and alpha particle confinement.

3. Coil Forces and Stresses: The complex, non-planar coil geometry and high magnetic fields result in enormous and complex electromagnetic forces. Designing support structures that can withstand these forces while providing sufficient access is a major engineering problem. The central conductor, in particular, is subject to immense stress.

4. Plasma-Wall Interaction: The 3D shape of the Heliac plasma complicates the design of a divertor for handling heat and particle exhaust. The magnetic field lines intersect the wall in a complex pattern, and managing the resulting heat loads in a steady-state environment is a critical, unsolved issue.

Outlook

The 5-15 year trajectory for Heliac research is likely to be one of continued fundamental physics investigation rather than a direct path to a power plant. The concept's engineering complexity has led to it being superseded in large-scale experiments by quasi-symmetric designs like Wendelstein 7-X and the Helically Symmetric eXperiment (HSX), which offer improved neoclassical confinement with more modular coil designs.

However, the existing Heliac devices will remain valuable for fusion science. They will continue to serve as testbeds for validating the advanced 3D physics codes that are essential for designing any future stellarator. Research at TJ-II, H-1NF, and CTH will provide crucial data on MHD stability at high beta, energetic particle behavior, and turbulence in complex 3D fields. These findings will be directly applicable to the design of next-generation stellarators, even if they do not follow the Heliac configuration.

In the longer term, the Heliac is unlikely to be the chosen configuration for a commercial fusion reactor due to its engineering challenges. The lessons learned from the Heliac program—particularly regarding the benefits of high rotational transform and the importance of integrated design considering physics and engineering constraints—have been instrumental in guiding the evolution of the broader stellarator concept towards more practical and optimized solutions.

References

1. The H-1 Heliac — Nuclear Fusion (1992)
2. First plasmas in the TJ-II flexible heliac — Nuclear Fusion (1999)
3. Design and construction of the Compact Toroidal Hybrid — Fusion Science and Technology (2005)
4. Overview of the TJ-II stellarator results — Nuclear Fusion (2013)
5. Stellarator and Heliac Devices — Reviews of Modern Physics (1988)
6. Energetic particle physics in the TJ-II stellarator — Nuclear Fusion (2017)
7. Recent results from the H-1 Heliac — Plasma Physics and Controlled Fusion (2004)