OpenStar Tama Nui

Overview

OpenStar Tama Nui is a high-field spherical tokamak (ST) being developed by OpenStar Technologies in Wellington, New Zealand. The device is designed to leverage the advantages of high-temperature superconducting (HTS) magnets to explore a compact, potentially faster and more economical path to commercial fusion energy. As a spherical tokamak, Tama Nui features a low aspect ratio (the ratio of the major radius to the minor radius of the plasma), which allows for high plasma pressure and efficient magnetic field confinement. The integration of HTS technology is critical to its design, enabling strong magnetic fields in a compact volume without the need for extensive cryogenic systems associated with low-temperature superconductors. The project represents a significant national effort for New Zealand in the global fusion research landscape and is part of a growing trend of privately funded ventures aiming to accelerate fusion energy development.

Physics / Mechanism

Tama Nui operates on the principles of the spherical tokamak, a magnetic confinement concept that is a variation of the conventional tokamak. The ST's defining characteristic is its low aspect ratio, typically below 2, compared to 2.5 or higher for conventional designs like ITER. This geometry leads to a plasma shape that is nearly spherical, which has several key advantages. The compact geometry allows STs to achieve a high plasma beta (the ratio of plasma pressure to magnetic pressure), a measure of the efficiency of the magnetic field in confining the plasma. A higher beta means that for a given magnetic field strength, a more dense and hotter plasma can be confined, which is favorable for achieving the conditions required for fusion, as described by the Lawson criterion.

The core technological enabler for Tama Nui is its use of Rare Earth Barium Copper Oxide (REBCO) HTS tapes to construct its magnets. HTS materials can operate at much higher temperatures (20–77 K) and in much stronger magnetic fields than low-temperature superconductors. This allows for the design of a compact, high-field central solenoid and toroidal field coils. A strong magnetic field is crucial for plasma confinement, as it scales with the square of the field strength. The combination of the ST's high-beta characteristics and the high-field capability of HTS magnets theoretically allows a device like Tama Nui to reach fusion-relevant plasma conditions in a significantly smaller and less expensive machine than would be possible with conventional technology. The design aims for a toroidal field exceeding 3 T on-axis and a plasma current of over 1 MA, conditions intended to produce a stable, high-performance plasma.

Historical Development

The conceptual basis for Tama Nui originates from the research conducted at Victoria University of Wellington's Robinson Research Institute, a center with expertise in applied superconductivity. The project was founded by Dr. Ratu Mataira, a plasma physicist whose work focused on the potential of HTS magnets to create a viable compact fusion device. OpenStar Technologies was spun out of the university in 2020 to commercialize this research.

The initial phase of the project involved extensive modeling and design studies to validate the physics and engineering of a high-field HTS spherical tokamak. This work built upon decades of public research into spherical tokamaks, most notably at facilities like the Mega Ampere Spherical Tokamak (MAST) in the UK and the National Spherical Torus Experiment (NSTX) in the US. The key innovation pursued by OpenStar was the specific design and manufacturing process for the HTS magnets, which are critical for the device's performance and viability.

Significant funding was secured in 2022, including a NZ$7 million grant from the New Zealand Ministry of Business, Innovation and Employment (MBIE) and private venture capital, enabling the company to move from design to construction. The name "Tama Nui" was chosen, which translates from Māori as "Great Son" or "Great Child," reflecting the project's New Zealand origins and its ambitious goals. Construction of the device began in earnest in 2023 at OpenStar's facility in Wellington. The project timeline has been aggressive, characteristic of private fusion ventures, with an initial target for first plasma in the mid-2020s.

Current Status

As of early 2026, the OpenStar Tama Nui device is in the advanced stages of construction and assembly. The primary focus is on the fabrication and testing of the HTS magnet systems, which are the most complex and critical components of the machine. This includes the central solenoid, the toroidal field coils, and the poloidal field coils. OpenStar has reported successful tests of prototype HTS magnet segments, demonstrating their ability to handle the high currents and mechanical stresses expected during operation. According to company statements, the manufacturing process for the full magnet set is underway.

Simultaneously, the vacuum vessel, power supply systems, and diagnostic equipment are being procured and integrated at the Wellington facility. The team has grown to include specialists in plasma physics, magnet engineering, and vacuum systems. The project is on track to begin integrated commissioning of the subsystems in late 2026, with the goal of achieving first plasma in 2027. The initial operational phase will focus on plasma formation, stability studies, and validating the performance of the HTS magnets under plasma-facing conditions. These experiments will be crucial for benchmarking the device's performance against the physics models and for planning future upgrades.

Notable Implementations

Tama Nui itself is the sole implementation of the OpenStar design. Its key subsystems represent a specific approach to the HTS spherical tokamak concept:

• Magnet System: The core of the device is its fully HTS magnet system. The toroidal field (TF) coil assembly and the central solenoid are wound with REBCO HTS tape. The design emphasizes a demountable joint technology, which could simplify maintenance and assembly, a significant challenge in traditional tokamak designs. The magnets are designed to be cooled by a closed-loop gaseous helium system to an operating temperature of around 20 K.

• Vacuum Vessel: The device features a compact, single-walled stainless steel vacuum vessel designed to accommodate the low-aspect-ratio plasma. It includes numerous ports for diagnostics, plasma heating, and vacuum pumping systems.

• Plasma Heating: Initial operations will rely on ohmic heating, where the large plasma current heats the plasma. Plans for auxiliary heating systems, likely to include Neutral Beam Injection (NBI) or radio-frequency heating, are in place for subsequent operational phases to achieve higher plasma temperatures.

• Power Systems: A dedicated power supply system is being built to deliver the high currents required for the HTS magnets and to drive the plasma current. This includes fast-switching power electronics for plasma control.

• Diagnostics: A suite of standard plasma diagnostics is being integrated to measure key parameters like plasma density, temperature, current, and position. These instruments are essential for understanding plasma behavior and optimizing device performance.

Open Challenges

Despite the promising design, Tama Nui and the OpenStar team face significant scientific and engineering challenges common to compact, high-field fusion devices:

1. HTS Magnet Durability and Quench Protection: While HTS tapes can withstand high fields, they are susceptible to damage from mechanical stress and radiation. The magnets in Tama Nui will experience enormous Lorentz forces. Ensuring the structural integrity of the magnets and developing a reliable quench detection and protection system are critical engineering hurdles.

2. Plasma-Material Interactions: In a compact device, the plasma is in close proximity to the vessel walls and divertor components. Managing the intense heat and particle fluxes to prevent component damage and plasma contamination is a major challenge. The development of robust plasma-facing components is essential for achieving long-pulse, high-performance operation.

3. Plasma Stability and Control: Spherical tokamaks can access high-beta regimes but are also susceptible to specific magnetohydrodynamic (MHD) instabilities. Developing and implementing effective plasma control schemes in real-time will be necessary to sustain stable plasmas.

4. Tritium Breeding and Fuel Cycle: While Tama Nui is a physics experiment not intended to breed its own tritium fuel, any future power plant based on this concept must incorporate a tritium breeding blanket. Integrating a functional breeding blanket within the tight confines of a spherical tokamak remains an unsolved engineering problem for the entire field.

Outlook

The credible 5- to 15-year trajectory for the OpenStar Tama Nui project is staged. In the near term (1-3 years), the primary goal is to achieve first plasma and successfully commission the device. This phase will focus on validating the core HTS magnet technology and demonstrating stable plasma operation at the design parameters of >1 MA plasma current. Success in this phase would establish Tama Nui as a leading HTS spherical tokamak facility and de-risk the core technology.

In the medium term (3-7 years), assuming successful initial operation, the focus will shift to performance enhancement. This will likely involve the installation of auxiliary heating systems to push plasma temperatures and densities toward fusion-relevant conditions. The goal will be to achieve significant plasma sustainment times and explore high-beta physics regimes, contributing valuable data to the broader fusion community. The device could potentially achieve a plasma energy gain factor, Q_plasma, greater than 1 during this period.

Looking further ahead (7-15 years), OpenStar aims to use the results from Tama Nui to design a next-step device, potentially a net-energy-gain pilot plant. This future machine would be larger, operate with a deuterium-tritium fuel mix, and incorporate technologies for heat extraction and tritium breeding. The ultimate success of the OpenStar approach will depend on whether the operational data from Tama Nui confirms that the HTS-enabled compact spherical tokamak is a reliable, efficient, and economically viable path to a commercial fusion power plant.

References

1. Unlocking the potential of fusion energy in New Zealand — Victoria University of Wellington (2022)
2. OpenStar Technologies Aims for Fusion Energy With Compact Device — Fusion Energy News (2023)
3. Kiwi scientists get $7m boost to build machine that could unlock limitless clean energy — The New Zealand Herald (2022)
4. The New Zealand Government is funding a fusion energy start-up — Ministry of Business, Innovation and Employment (MBIE) (2022)
5. High-temperature superconductors: a new paradigm for fusion energy? — Philosophical Transactions of the Royal Society A (2022)
6. Overview of the Fusion Private Venture Ecosystem — Fusion Industry Association (2023)
7. Compact fusion energy based on the spherical tokamak — Nuclear Fusion (2019)