The spherical tokamak path to fusion power: Opportunities and challenges for development via public–private partnerships Open Access

The concept of the spherical, or low aspect ratio, tokamak (aspect ratio defined as the ratio of the torus major radius, R, to the minor radius, a, A = R/a) has been discussed by tokamak scientists since the mid-1970s.^1,32 As a possible option for a fusion reactor, it was first introduced by Jassby in 1977 in “SMARTOR—A Small-Aspect-Ratio Torus for Demonstrating Thermonuclear Fusion.”¹ At the same time, Sykes et al. (UKAEA at the time, later one of the founders of Tokamak Energy) predicted better stability at low aspect ratios,² extending previous studies.^4,32 The motivation to start experimental spherical tokamak (ST) research is often associated with the publication of an article by Peng and Strickler,³ although several STs were already built and some were operational by that time, e.g., GUTTA,⁴ ASPERATOR T-3,²⁴ etc. (Ref. 4 and see Google for specific devices). Since then, more than 20 STs have been built around the world. Several overviews on the status of the ST research have been published, e.g., Refs. 23 and 24.

The spherical tokamak path to commercial fusion was proposed and analyzed in detail by Stambaugh et al.⁵ Recent progress in technology development and the results of the experimental studies on several STs have already demonstrated the feasibility of this approach. The ST concept allows a significant reduction in development costs and timescales of construction of the intermedium devices and allows the use of advanced technologies and innovations while still utilizing applicable fusion technologies developed for tokamaks over the last 50 years. This has already been confirmed by the short timescale of the construction of ST40 and by achieving the desired fusion relevant plasma temperatures.²⁸ Advances in high temperature superconductor (HTS) technology^6–8 allow a significant increase in the toroidal field (B_t), which we now know will increase energy confinement times and performance in STs, as already demonstrated on ST40.^9,10 The combination of the high β (ratio of the plasma pressure to magnetic pressure) that has been achieved in STs¹¹ and the high toroidal field (B_t) that can be produced by the HTS toroidal field magnets opens a path to lower-volume fusion reactors, as the fusion power is proportional to β²B_t⁴V.⁵ 

Experiments on the medium size spherical tokamaks MAST¹² and NSTX¹³ have confirmed a much stronger dependence of the plasma confinement time on B_t in STs compared with conventional aspect ratio tokamaks. ST40 demonstrated improvements in performance at B_t > 1 T in ohmic heated discharges⁹ and ion temperatures close to 10 keV in neutral beam heated hot ion mode discharges.^10,28 Furthermore, we know that the fraction of the plasma current (bootstrap current) can be very high, approaching 100%, in an ST.³³ (Bootstrap is the self-driven current due to high plasma pressure gradients that are more easily achieved in STs.) This is vitally important as it dramatically reduces the need for the external current drive, which is both costly and inefficient with high energy consumption. Gryaznevich and Chuyanov showed that high beta and high bootstrap current could be achieved simultaneously in an ST and proposed the idea of modular fusion power plants based on comparatively small size STs.¹⁴ All these results have provided sufficient evidence for Tokamak Energy to raise private investment for the construction and operation of three ST prototypes so far, including the record-breaking ST40.

As described in the Introduction, the spherical tokamak is a promising approach to achieving fusion power. Fusion power production from spherical tokamaks involves several key steps.

To achieve practical fusion power, a tokamak reactor must be designed and built to maintain stable plasma for a long period of time while also withstanding the high temperatures and radiation produced by the fusion reactions. The spherical tokamak design has several advantages in this regard,^5,17,18 including a more compact size and improved plasma stability. One disadvantage is that the space around the central stack in an ST is limited, so it is challenging to use this space for a blanket or efficient protection of the Toroidal Field (TF) magnet from neutrons. Unless new shielding materials are found, this may constrain the minimum size of an ST reactor, which may still be smaller than conventional aspect ratio designs.^17,18

The first step is to create and confine hot plasma within the ST. Heating can be achieved through resistive heating from the plasma current, by injection of neutral beams, by RF [e.g., Electron Cyclotron Resonant Heating (ECRH) and Ion Cyclotron Resonant Heating (ICRH)] heating, or by a combination of these methods, and by the alpha heating produced by fusion reactions at sufficient conditions.

Prototype ST devices, such as ST40 (Fig. 1), currently operate with deuterium. Deuterium–deuterium fusion releases neutrons with less energy than D-T fusion. D-T fusion is optimum for energy production because the cross section for the D-T fusion reaction is high enough (and far higher than alternative fusion reactions; see Fig. 3 later in the text) at temperatures of 10–20 keV that are optimal for the D-T fusion^15,16 and that have already been approached on ST40. Thus, as predicted by Peng and Stambaugh,^3,5 the physics in STs is, indeed, good.

An important step in the spherical tokamak path to fusion power is to scale up the technology to larger and more powerful reactors that can produce significant amounts of electricity. This may require further advancements in plasma physics, control, and heating systems but mainly in materials science and engineering. The scale-up may be achieved in part by a modular approach,¹⁴ though, as mentioned above, the minimum size of a module is likely to be determined by the thickness of the neutron shield protecting the high temperature superconductor in the center column.

Overall, the spherical tokamak path to fusion power holds great promise for providing a safe, clean, and virtually limitless source of clean, carbon-free energy for humanity's future needs. This promise comes at a time when the need for the rapid development of clean, carbon-free fusion energy is becoming ever more apparent.

Over the last few years, it has become clear that privately funded companies can make a decisive contribution to the development of fusion power. This development was anticipated by Clery in an article in EuroFusion News in December 2014 entitled “Does fusion research lack diversity?”.²⁵ He made the point that “The energy and initiative of the private companies could give fusion a much-needed shot in the arm, and for them to take the next step toward viability they will need much more money and could use support rather than disdain.”

In 2016, Dr. Hsu, now Fusion Energy Coordinator at U.S. Department of Energy, gave testimony to the Energy Subcommittee of the House Committee on Science, Space, and Technology, Hearing on Fusion Energy Science in the United States.²⁶ Dr. Hsu made several important points in his evidence, including that there is a wide range of scientifically credible “innovative Fusion energy concepts” and that lowering the cost of Fusion energy development (which we see as one of the advantages of the ST path) could have several benefits, the most important of which is enabling, due to reduced development costs and timescales, a healthy public–private partnership for development, as is done in many other technological fields.

Dr. Hsu defined “innovative Fusion energy concept” as any concept that has a pathway toward economically competitive power production and could potentially result in a demonstration fusion reactor for total development costs of less than a few billion dollars and in less than 20 years. He also referred to the spherical tokamak (ST) and the tokamak using high temperature superconducting magnets as being more mature concepts that could also potentially accelerate the timeline and lower the costs to a demonstration reactor.

At the same hearing, Dr. Prager, then director of the Princeton Plasma Physics Laboratory, made some prescient comments about the ST:²⁷ The ST concept investigated by NSTX-U can operate at high plasma pressure (which provides more Fusion power) and at relatively weak magnetic field (which reduces cost) compared to conventional tokamaks. The practical impact is that this offers the possibility, for example, of designing a fusion pilot plant or Fusion nuclear science facility of a size significantly reduced from that based on conventional tokamaks. A fusion pilot plant would generate net electricity and perform an integrated test of a full fusion energy system, including testing materials components in the presence of copious fluxes of neutrons that are produced in the fusion reaction. A nuclear fusion science facility provides an integrated test but does not aim for net electricity production. He emphasized that the pilot plant would employ high temperature superconducting magnets because they offer reduced magnet size (but require technological development).

By that time, Tokamak Energy Ltd. (TE.Ltd.) was already established, starting as a spin-off from CCFE in 2009 and based at the Culham Innovation Center. The initial funding came from the UK Rainbow Innovation and Science Seed Fund, Oxford Instruments, and several private investors. At present, TE.Ltd. has a site at Milton Park, near Abingdon, UK. TE.Ltd employs about 280 people and has raised funding from private and corporate investors and from a number of UK and US DoE grants for around 350 million USD. From the very beginning, TE.Ltd. has chosen the innovative path to fusion based on spherical tokamak with high temperature superconducting magnets (described in detail below).

Private companies^22,26,27 can bring fresh ideas and approaches to the development of fusion technology, helping to overcome technical and economic barriers that may have previously hindered progress.

Private companies can collaborate with academic and government research institutions to leverage their expertise and resources.^20,22,29,30 Such partnerships can help to accelerate the pace of research and development and foster knowledge exchange. These partnerships are often welcomed by scientists in academic and government research institutions who want to see the science and technology they are working on turned into products that can deliver major benefits to humankind.

Private companies can help accelerate the commercialization of fusion technology by investing in the development of the technology and then in the construction of fusion power plants and developing and marketing the technology needed to support them.

Private companies can take a market-driven approach to the development of fusion energy, focusing on the commercial needs of customers and end-users. This can help to ensure that the technology is developed in a way that is cost-effective and efficient—and is designed to meet the needs of the energy market and energy users. This is different from, and complementary to, the scientific-progress-driven approach of publicly funded fusion projects.

Overall, private companies have the potential to accelerate the development of fusion energy based on the exciting scientific properties of devices, such as the spherical tokamak or any other alternative concept. Their investment, innovation, and market-driven approach can help to bring this promising technology to fruition in a faster and more effective way than is possible otherwise.

As mentioned above and in Refs. 6–8, 34, and 35, high-temperature superconductors (HTS) have the potential to significantly advance the development of commercial fusion power. We note that they have been developed rapidly by private companies, notably Tokamak Energy and Commonwealth Fusion Systems, while major new fusion devices, such as ITER, were committed to the use of the more mature, but lower performance, low-temperature superconductors (LTS). Tokamak Energy and Commonwealth Fusion Systems have both focused on rare-earth barium copper oxide (REBCO) HTS material because of its remarkable property of carrying very high current density at temperatures of 20–30 K and in magnetic fields of 20 T or more.

Superconducting magnets are a crucial component of tokamak fusion reactors, as they generate the magnetic fields needed to confine the plasma without dissipating power in the magnets, except for keeping the magnets cold. REBCO HTS materials have the proven potential to produce stronger magnetic fields than conventional, low temperature superconductors. In fact, HTS magnets can deliver about twice the magnetic field strength of conventional, low temperature superconductors—and they can do so in a compact system requiring much less space. This enables the development of more compact and efficient tokamaks. One of Tokamak Energy's most significant achievements has been to build an HTS magnet that delivered 26.2T,³⁶ which is another example of a successful public–private partnership. This peak field was achieved in liquid helium, but in a separate test at the Fusion power module-relevant temperature of 20 K, a peak field on the coil of 24 T was achieved.³⁶ Several HTS magnets have been constructed, different technologies have been tested, and sophisticated numerical analysis tools have been developed by TE.Ltd.³⁸ TE's ground-breaking Demo4 magnet system³⁷ is under construction in a purpose-built test facility. It is a complete balanced set of HTS magnets shaped in a tokamak configuration, allowing us to create substantial magnetic forces and test them in fusion power plant-relevant scenarios for the first time.

As REBCO HTS superconductors can operate at much higher temperatures than conventional low temperature superconductors, up to 93 K (but optimal at 20–30 K), rather than 4 K, which is required for LTS, the energy (and financial) cost of cooling is hugely reduced. In addition, HTS magnets in the center column of the ST need not be totally protected from any heat load related to the neutron flux, as is the case with LTS magnets. It is possible to deal with kW scale heat loads at temperatures of 20–30 K, which means that the thickness of the neutron and secondary gamma shielding can be reduced compared to the LTS magnets. This will also significantly reduce the costs of constructing and operating ST fusion reactors.

Overall, REBCO HTS superconductors have the potential to significantly improve the performance and reduce the costs of tokamak fusion reactors, which will help to accelerate the development of commercial fusion power.

There are many plasma physics devices that are candidates for a fusion reactor. The most mature is the conventional tokamak, but this has known constraints of efficiency (low beta) and engineering challenges.^16,17 The steady state conventional tokamak has requirements for auxiliary systems to sustain plasma current, which can be most easily overcome by reducing the aspect ratio and, therefore, increasing the bootstrap fraction.^17,18,33

The opportunity and challenge for the ST are to prove first that plasma ion temperatures of ∼10 keV, the threshold necessary for commercial fusion energy can be achieved, and then that the triple product of density, temperature, and energy confinement time can approach the range necessary for breakeven, ignition, and fusion energy gain.¹⁹ 

At some point, all plasma physics devices that are candidates for fusion power will need to pass these thresholds—and our view is that these parameters, such as temperature and triple product, are the most important, independently verifiable, measures of progress. The idea of a series of device independent thresholds to measure progress in candidates for fusion power devices was introduced in the BETHE call for proposals under the ARPA-E fusion program.²⁰ 

At Tokamak Energy, we set out to build a high field ST, the ST40, Fig. 1, to demonstrate high performance in a compact device.⁹ ST40 was planned to be the cheapest, quickest way to achieve the plasma ion temperature of ∼10 keV, and a triple product of density, temperature, and energy confinement time, in the 10¹⁹ keV s m⁻³ range, which, if DT is used, will satisfy or be close to the ignition criteria according to Ref. 19 and Fig. 2.

The ST40 performance point is shown in Fig. 2, where the triple product is plotted vs plasma temperature for tokamaks, stellarators, other magnetic fusion devices, and inertial fusion devices. ST40 has a higher plasma temperature and a similar triple product compared to many larger devices and shows performance well above that achieved in devices designed, financed, and built by other private fusion companies.

The path for the commercialization of fusion is not simply a matter of the development of technologies and continued research. Private, corporate, and sovereign investors need incentives and structures that allow them to benefit from any progress that is achieved. Protection of intellectual property accumulated during this development process will attract further investment and support faster progress in the development of a strong fusion industry supply chain—as this will enable commercial relations among industries, investors, and R&D companies.

Tokamak Energy owns valuable intellectual property, particularly for the HTS magnets. This intellectual property is not just published patents but includes trade secrets, such as manufacturing methods. Patents provide a way of publishing valuable new ideas, so that others can see and understand the ideas, while protecting the ownership of the invention for up to 20 years. This 20-year horizon provides a good incentive for fusion energy companies to innovate and aim to deploy their technology very rapidly. Many fusion companies will anticipate further inventions and patents during their development path, so that the 20-year ownership and competitive advantage horizon will keep extending into the future to protect the inventions during the rapid deployment phase for fusion energy.

The process of radical technological innovation in private companies can be complex and multifaceted but generally involves the following stages:

The first stage of the innovation process in fusion is usually idea generation. This involves brainstorming and exploring different concepts and approaches to solving the identified problems. Often, this happens before a company is formally launched or financed. Companies may involve employees, external consultants, and other stakeholders in this process. Several fusion companies have been formed to try to avoid the known challenges of D-T fusion, while others, such as Tokamak Energy, aim to take advantage of the proven demonstration of MW scale fusion power from D-T reactions in tokamaks and then tackle the known challenges, either directly or through collaboration.

Once the idea or concept for a fusion device is determined, the company usually moves on to the research and development phases. In this phase, the company invests resources into developing new technologies or improving the existing ones. This may involve numerical simulations, construction, and use of several test beds, conducting market research to determine needs, exploring new materials or processes, and experimenting with different approaches to problem-solving.

The company will then typically test the concept through prototyping and other proof-of-concept activities. This involves developing a preliminary design and testing it in a controlled environment to see if it can deliver the desired performance.

If the proof-of-concept is successful, the next stage is scaling up the technology. This may involve further development of the design, sourcing of materials, and building manufacturing capacity. Companies may also look for partnerships or collaborations to help scale up technology.

Once the technology has been successfully scaled up, the final stage is commercialization. This involves introducing technology to the market and finding customers who are willing to pay for the product or service. Companies may use various marketing strategies and partnerships to help them achieve this goal.

It is worth noting that the process of radical technological innovation in private companies is usually not linear and may involve iterating between these stages multiple times. Furthermore, the process can be affected by a wide range of internal and external factors, such as regulatory requirements, competition, and market conditions. Successful innovation requires a combination of technical expertise, entrepreneurial spirit, and strategic planning.

As the privately funded fusion industry develops, we expect to see more and more industry-to-industry collaboration, enabled by clear ownership of intellectual property.

The optimum path forward for the development of fusion energy is now seen to be through some form of public–private partnership to combine the best attributes of the public and private sectors as innovation in private companies and research in government laboratories differ in several ways.

However, the primary goal of innovation in private companies is typically to create new products, services, or intellectual property rights that can be sold in the market and generate revenue. In contrast, research in government laboratories often focuses on advancing scientific knowledge that will bring broader societal benefits and create, develop, and sustain expertise. Companies will often refer to a “technology readiness level” (TRL) to measure their progress. In the public research, an increase in the TRL is often balanced by extending research to new and low-TRL areas, as this is efficient for increasing the number of publications, which is often the measure of the efficiency of the research. In the private approach, R&D aimed at increasing the TRL level is crucial, while publications are complementary.

Private companies typically rely on funding from investors and customers to support their innovation efforts. They may also access government grants or loans. Government laboratories, on the other hand, are often funded by taxpayer money and may have more stable and long-term funding sources.

Private companies are often more agile and can move faster than government laboratories. They can make decisions and pivot quickly to take advantage of emerging technological developments and market opportunities. In contrast, government laboratories may be subject to longer bureaucratic processes, which can slow down the pace of research and development.

Private companies have a strong incentive to protect their intellectual property and may hold patents or trade secrets to maintain a competitive advantage. Government laboratories, on the other hand, may prioritize sharing their research findings with the scientific community and making them available to the public, and for this, the number of publications is often very important.

Private companies may collaborate with other companies, universities, or research institutions to develop new technologies and bring them to market. Government laboratories may also collaborate with other organizations but may prioritize collaborations that align with their mission and goals, e.g., in the broadening of the research areas.

Overall, private companies and government laboratories play different but complementary roles in advancing scientific knowledge and developing new technologies. While private companies focus on commercialization and profitability, government laboratories have a broader mandate to address societal challenges and advance scientific knowledge.

In the UK, the Spherical Tokamak for Energy Production (STEP) program is a form of public–private partnership with the government as the customer, the spherical tokamak as the selected plasma confinement device, and high temperature superconductors as the chosen material for the magnets. It is, thus, closely aligned with the technology developed by the private companies, in particular Tokamak Energy. Overall, the UK government supports the development of commercial fusion energy through a combination of research and development programs, public–private partnerships, and international collaboration. The government's focus on the spherical tokamak design and the establishment of the STEP program demonstrate its commitment to developing a commercial Fusion power plant based on this innovative technology.

The U.S. Department of Energy Milestone-Driven Fusion Energy Development Program,²² is a different form of public–private partnership with the government as a co-founder but with the choice of devices left deliberately to private companies. This program is particularly exciting as it is modeled on the COTS program³¹ used by NASA almost 20 years ago to bring forward a new generation of space launchers. The COTS program was hugely successful as it directly supported Space-X and stimulated private investment and competition. The result has been a 100-fold reduction in the cost per kilogram of payload for space launches in 20 years.

The DOE has several programs and initiatives to support the development of commercial fusion energy.²² The DOE milestone program has now selected eight companies, including Tokamak Energy, for the first 18 months. One of the most powerful aspects of the program is this 10-year goal, or “decade vision.” Another powerful aspect of the program is the way DOE will manage it on the basis of objective milestones. Overall, the DOE supports the development of commercial fusion energy through a combination of fundamental research, public–private partnerships, and support for private companies and entrepreneurs. We also appreciate the DOE support for R&D on ST40 through INFUSE²⁹ and the Cooperative Research and Development Agreement (CRADA)³⁰ with the U.S. National Laboratories as an encouraging example of the public–private partnership.

The U.S. government has funded the collaboration on ST40 for quite a few years. It was realized via participation of PPPL scientists in ST40 experiments, assistance with diagnostics and data analysis and resulted in several joint publications and presentations at international conferences. In this case, the public partner (PPPL) benefited from advancing ST physics by using experimental results from ST40, and the private partner (TE.Ltd) was able to use the results of the data analysis to advance and optimize the operations of ST40.

We would be delighted to extend the existing public–private partnership projects and to discuss longer term options for collaborative R&D on ST40 and beyond.

We present a leading concept for the development of commercial fusion energy based on a compact high field spherical tokamak (ST) as an example of a public-private partnership. This concept differs from the mainstream path to fusion power via huge conventional tokamak devices, such as ITER and DEMO. It allows a significant reduction in development costs and timescales and allows use of the most advanced technologies and innovations while still utilizing Fusion technologies developed for Tokamaks over the last 50 years, which was partly already confirmed by the timescale of construction of ST40 and achieving desired Fusion relevant plasma performance. Such an approach offers the potential to advance the Fusion program and provide a unique integrated system that will help to establish the expertise and techniques for designing and building future Fusion power plants, as well as help to prepare industry for the challenges of building them. The ST with HTS magnets is already a leading contender for public-private partnerships in US and UK approaches to the accelerated development of fusion energy.

The authors have no conflicts to disclose.

David Kingham: Writing – original draft (equal). Mikhail P. Gryaznevich: Writing – original draft (equal).

The data that support the findings of this study are available within the article.