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VIPA HTS magnet technology

VIPA (Vertically-aligned, Internally-cooled, Pancake-stacked, and Armored) is a proprietary high-temperature superconductor (HTS) conductor and magnet technology developed by Tokamak Energy Ltd. It is designed to enable the construction of high-field, compact spherical tokamaks for fusion energy production.

Overview

VIPA (Vertically-aligned, Internally-cooled, Pancake-stacked, and Armored) is a high-temperature superconductor (HTS) magnet technology developed by Tokamak Energy Ltd. for use in compact spherical tokamaks. The technology addresses the significant engineering challenges of building powerful magnets capable of withstanding the extreme forces and thermal loads inherent in a fusion power plant. By using HTS tapes, specifically Rare Earth Barium Copper Oxide (REBCO), VIPA magnets can generate much stronger magnetic fields (over 20 T) at higher operating temperatures (20–30 K) compared to traditional low-temperature superconductor (LTS) magnets. This capability is a key enabler for the spherical tokamak concept, which aims to achieve fusion conditions in a smaller, more cost-effective device by operating at higher magnetic fields.

Physics / Mechanism

The VIPA architecture is a multi-faceted solution designed to manage the mechanical, thermal, and electrical properties of HTS tapes in a demanding fusion environment. The name itself describes the core engineering principles:

  • Vertically-aligned: REBCO tapes are thin, flat conductors with highly anisotropic properties. To maximize their strength against the immense outward hoop stress generated by Lorentz forces in a toroidal field coil, the tapes are aligned with their strongest axis (the wide face) oriented vertically, parallel to the main electromagnetic forces. This orientation optimizes the conductor's load-bearing capacity.

  • Internally-cooled: A key innovation of the VIPA conductor is the integration of a cooling channel directly within the conductor assembly. This channel allows a cryogen, such as liquid nitrogen or gaseous helium, to flow in close thermal contact with the HTS tapes. This design provides efficient, distributed cooling to manage heat from AC losses, neutron heating, and thermal radiation, ensuring the superconductor remains below its critical temperature. It contrasts with architectures that rely solely on conduction cooling from the magnet's exterior.

  • Pancake-stacked: Individual conductor lengths are wound into flat, circular coils known as 'pancakes'. These pancakes are then stacked axially to form the complete toroidal or poloidal field magnet. This modular approach simplifies manufacturing, testing, and assembly. Electrical connections, or 'joints', are made between pancakes, which must be superconducting to minimize resistive losses and heat generation.

  • Armored: Each HTS tape is co-wound with a structural material, typically a high-strength steel or copper-alloy tape. This 'armor' provides essential reinforcement, helping to distribute mechanical stress and prevent strain-induced degradation of the brittle REBCO ceramic layer. The combination of vertical alignment and armor allows the conductor to withstand tensile strains that would otherwise fracture the superconductor.

The conductor is designed to operate at temperatures around 20 K, which provides a substantial thermal margin above the boiling point of liquid hydrogen, reducing the complexity and cost of the cryogenic system compared to the liquid helium (4.2 K) systems required for LTS magnets used in devices like ITER.

Historical development

The development of VIPA technology is intrinsically linked to the history of Tokamak Energy. The company was spun out of Culham Laboratory in 2009 with the goal of accelerating fusion energy development through the spherical tokamak pathway, enabled by HTS magnets.

Early work focused on characterizing REBCO tapes and developing conductor concepts. The first patents for the core VIPA design principles were filed in the mid-2010s. A significant milestone was achieved in 2015 with the construction and operation of the ST25 HTS, the world's first tokamak to feature an all-HTS set of toroidal field coils. While a small-scale experiment, it demonstrated the successful fabrication and operation of HTS magnets in a tokamak environment.

This was followed by the ST40 device, which began operation in 2017. While its initial magnets were conventional copper, the device was designed as a platform for HTS upgrades and to explore high-field spherical tokamak physics. In 2022, Tokamak Energy used a test rig containing a full-scale toroidal field limb magnet, built with VIPA conductors, to demonstrate a magnetic field of 24.4 T. This was a critical validation of the technology's high-field capability. Later that year, the company announced that its ST40 device had achieved a plasma temperature of 100 million K, a first for a privately funded spherical tokamak, though this was achieved with copper magnets.

The development of VIPA has been an iterative process, with continuous improvements in conductor design, joint technology, and manufacturing techniques. This work has been supported by a series of private funding rounds and UK government grants.

Current status

As of 2026, VIPA technology is considered mature enough for deployment in next-generation, high-performance fusion devices. Tokamak Energy has successfully manufactured and tested full-scale prototype magnets for its planned ST80-HTS device. These tests have validated the conductor's performance under operational conditions, including high magnetic fields, large currents, and cryogenic temperatures. The company has established a dedicated manufacturing facility in the UK capable of producing kilometers of the specialized VIPA conductor.

In 2024, Tokamak Energy announced the completion of its ST80-HTS magnet system, which consists of 14 toroidal field (TF) coils and a set of poloidal field (PF) coils. The TF magnets, built with VIPA conductors, are designed to operate at fields exceeding 20 T. The entire magnet system has undergone extensive cryogenic and power testing in preparation for integration into the ST80-HTS device. This represents the most advanced application of the VIPA technology to date and is a critical step towards demonstrating net energy gain in a compact spherical tokamak.

Notable implementations

The primary and sole implementation of VIPA technology is within the research and development program of its creator, Tokamak Energy Ltd. Key devices and projects include:

  • ST25 HTS: A small-scale proof-of-concept device that first demonstrated the use of HTS magnets in a tokamak configuration. It validated the basic principles of pancake coil winding and operation.
  • ST40: A larger spherical tokamak that has served as a physics platform. While it operates with copper magnets, it has provided crucial data for the design of its HTS-based successors.
  • ST80-HTS: The company's next-generation device, currently under construction. It is designed to be the first spherical tokamak to demonstrate the core physics of a fusion power plant, enabled entirely by a magnet system using VIPA conductors. Its goal is to achieve plasma conditions relevant to the Lawson criterion for sustained fusion.
  • ST-E1: A planned pilot plant intended to demonstrate commercially relevant net electricity generation (Q_engineering > 1). This future device will rely on a scaled-up version of the VIPA magnet technology demonstrated in ST80-HTS.

Open challenges

Despite significant progress, several challenges remain for the widespread deployment of VIPA technology in a commercial fusion power plant.

  • Neutron Damage: The long-term performance of VIPA conductors in a high-neutron-flux environment is a critical uncertainty. Neutron irradiation can create defects in the REBCO crystal lattice, degrading its superconducting properties. While HTS materials are generally more resilient to neutron damage than LTS, extensive testing is required to quantify the magnet's operational lifetime and inform shielding requirements for a power plant like ST-E1. This is a key area of research for all fusion magnet technologies.

  • Manufacturing at Scale: While Tokamak Energy has established a production line, scaling up to the quantities required for a fleet of commercial power plants presents a significant industrial challenge. This involves securing the REBCO tape supply chain, automating the complex co-winding process, and ensuring high-yield, quality-controlled production of many large-scale magnets.

  • Joint and Quench Technology: The electrical joints between pancake coils must be robust, low-resistance, and capable of withstanding thermal and mechanical cycles. Furthermore, developing a reliable quench detection and protection system for HTS magnets is more complex than for LTS magnets. The slow propagation speed of a normal (non-superconducting) zone in HTS conductors makes early detection difficult, requiring sophisticated monitoring systems to prevent magnet damage.

  • Cost: REBCO tape remains expensive, and the complex manufacturing process for VIPA conductors contributes significantly to the overall cost of the magnet system. Reducing these costs through manufacturing improvements and supply chain development is essential for the economic viability of a VIPA-based fusion power plant.

Outlook

The 5-15 year trajectory for VIPA technology is focused on demonstrating its capability in integrated, high-performance fusion systems. The immediate goal is the successful operation of the ST80-HTS device, expected in the late 2020s. Achieving its target plasma performance would be a major validation of the entire high-field spherical tokamak approach and the enabling VIPA magnet technology.

Success with ST80-HTS will shift the focus towards the engineering and materials science challenges of a power-plant-scale device, ST-E1, planned for the 2030s. This will involve scaling the magnet size and performance, refining the design for improved maintainability, and conducting extensive materials testing to validate the lifetime of components in a fusion environment. The development of a robust supply chain for HTS tape and other critical components will be a parallel priority. If these technical and industrial hurdles are overcome, VIPA technology could become a leading magnet solution for a class of compact, commercially viable fusion power plants.

References

  1. Tokamak Energy achieves 100 million C temperature in ST40 spherical tokamak — Tokamak Energy Ltd. (2022)
  2. Tokamak Energy achieves 24.4 T in HTS magnets for fusion energy — Tokamak Energy Ltd. (2022)
  3. Tokamak Energy completes world-class fusion magnets — Tokamak Energy Ltd. (2024)
  4. Development of high field spherical tokamaks — Philosophical Transactions of the Royal Society A (2019)
  5. The ST80-HTS project: a spherical tokamak with high-temperature superconducting magnets — IEEE Transactions on Applied Superconductivity (2023)
  6. Design and analysis of the ST40 high field spherical tokamak — Fusion Engineering and Design (2019)
  7. Development of high temperature superconducting magnets for the ST-E1 spherical tokamak fusion pilot plant — IEEE Transactions on Applied Superconductivity (2024)