Thermal proton-boron fusion on spherical torus

Significant progress has been made in magnetic and inertial confinement fusion (MCF and ICF) energy development since the achievement of world record parameters on the T3 tokamak in 1968. In MCF, the triple product nτT has been elevated from 5×10¹⁷ m⁻³·s·keV to 1×10²¹ m⁻³·s·keV. At the same time, Q = P_fusion/P_heating, has increased from 1×10⁻⁹ to 0.67, with expectations to exceed 10 in the ITER experiment. In ICF, a Q value of approximately 2.4 was attained with a fusion energy output of around 5.2 MJ, indicating that the fusion-produced power was higher than the heating power or ignition was achieved. The commercialization of fusion power is projected to be realized in less than 30 years, a significant reduction from the 50 years commonly assumed previously.

Inspired by these achievements, at least 45 private enterprises in 13 countries have joined the campaign and received investments totaling $7.1 billion in recent years. Several facilities are being constructed or upgraded to find a faster route to commercialize fusion energy. Among these facilities, SPARC at Commonwealth Fusion Systems (CFS) in the USA and ST40 at Tokamak Energy in the UK stand out as notable examples.

Over the past six years, a handful of private companies in China have emerged dedicated to fusion. ENN Science and Technology Development Co., Ltd. (ENN) stands out as the first and largest. The ENN roadmap aims to build a commercial demonstration reactor by 2035, which aligns with several other companies’ most ambitious goals.

Fuel availability is a significant concern in fusion research. Among fusion reactions, D-T is currently considered as the most viable option for tokamak plasmas due to its largest fusion cross-section compared to other fusion reactions. However, tritium is a rare isotope that occurs in trace amounts and must be produced within the fusion reactor through a reaction between neutrons generated during fusion reactions and lithium in the breeding blanket. The viability of tritium breeding in fusion reactors remains to be tested, and ITER will carry out those tests.

Fuel is a long-standing and widely discussed topic in the fusion energy science community, and alternative fuel options have been explored, including deuterium-deuterium (D-D), deuterium-helium (D-³He), and proton-boron (p-¹¹B) reactions. Among these, p-¹¹B has been recognized as one of the most abundant and low-cost fuel sources. Additionally, this reaction is aneutronic. Furthermore, the products of this reaction, α-particles, have the potential to produce electricity with higher energy conversion efficiency directly.

However, as a fusion fuel, p-¹¹B plasma has extremely low reactivity and high radiation loss at temperatures of approximately 100 keV and has, therefore, been largely excluded in the past. Nevertheless, it has recently experienced renewed interest as a potential energy source, thanks to revisited cross-section data obtained through experiments and corresponding theoretical analyses. Recent simulations, with self-consistently calculated electron temperature, have demonstrated that using the recent fusion cross-section dataset, fusion power could overcome bremsstrahlung losses in plasma with an optimal ¹¹B/p ion concentration of 15%. In a recent experiment, p-¹¹B reactions triggered by interactions between energetic, intense proton beams and laser-ablated boron plasma have been investigated. A clear and notable enhancement of α-particle emission efficiency (number ratio of escaping α-particles and boron nuclei) in plasma was observed, compared with the cold boron. The proton beam transport path modulated by strong electromagnetic fields, which may cause nonlinear deposition of beam energy in the plasma, was identified to dominate the enhanced α-particle generation due to a longer collisional length. A few companies, such as ENN and TAE Technologies in MCF, HB11 Energy, and Marvel Fusion in ICF, focus on p-¹¹B fusion investigation. TAE Technologies plans to achieve p-¹¹B fusion using a Field-Reversed Configuration (FRC) and has successfully measured a significant amount of p-¹¹B fusion produced α-particles in the Large Helical Device (LHD) at NIFS in Japan. Simulation results indicate that intense negative-hydrogen-ion-neutral beam injectors created a large population of up to 160 keV (around the low-energy resonance peak of p-¹¹B fusion) energetic protons to react with the boron-injected plasma. ENN has also proposed that the synergy of neutral beam injection and ICRF could create a supper-thermal ion tail in the distribution function of proton ions and enhance fusion reactivity. These recent advancements indicate that p-¹¹B may be a plausible fusion fuel for commercialization.

One of the major challenges facing magnetic fusion energy is the iteration pace of technology development and investment efficiency. A significant obstacle is the high cost of initial devices that can generate enough electric power to attract commercial interest. The tokamak path, based on conservative, demonstrated physics performance, is progressing through large experiments like ITER (major radius R ~ 8 m, cost of about $8 billion in fiscal year 1996). However, the low-aspect-ratio tokamak or spherical torus (ST) approach offers a more executable commercialization strategy by providing a low-cost, low-power, small-size market entry vehicle and a strong economy of scale leading to power plants that are still small on an absolute scale. In this aspect, it has been estimated that the ST approach could be used to build a very small device (with an aspect ratio of A ~ 1.4 and a major radius of R ~ 1 m, similar in size to the DIII-D tokamak) that is capable of generating approximately 100–200 MW of power.

The ST approach has been studied for many years, with interest growing rapidly in recent years due to the success of the START experiment of high β and the development of new devices like STX and STEP. Some studies even project the ST approach to burning plasma devices. The key to the attractiveness of the ST approach is in the close to unity β values promised by the combination of high elongation and low aspect ratio. This results in high power density in a small device, which is beneficial for commercialization.

ENN’s program of spherical torus proton-boron fusion (STPBF) seems to be a plausible solution to address the two major challenges surrounding the commercialization of fusion energy. A key component of this program is the EHL-2 experiment platform, which is expected to be a leading ST fusion device capable of achieving several critical objectives, including:

1. To identify, study, and solve the most significant scientific and technological challenges associated with the development of STPBF.

2. To verify the scientific feasibility of p-¹¹B fusion.

3. To provide a scientific and technological foundation for subsequent construction of larger platforms to verify the engineering feasibility of p-¹¹B fusion.

4. In particular, to elevate fusion reactivity and verify the ST energy confinement scaling law, particularly in high magnetic fields and high-temperature (low collisionality) p-¹¹B plasmas.

Over the past two years, the technology routes and challenges associated with the physics design of the EHL-2 device have been identified and systematically analyzed. The results are presented in this Special Issue.

It is worth noting that due to the scarcity of spherical torus and p-¹¹B fusion experimental data, the design of devices was heavily reliant on models derived from conventional tokamak experiments, some of which may be inappropriate. This underscores the importance of a robust experimental research program like EHL-2.

We thank the editorial board, editorial office, and publisher of Plasma Science and Technology for proposing the publication of this special issue. This Special Issue features the analysis results of EHL-2 physics design by both domestic and international colleagues. Its publication marks a significant milestone for the Chinese plasma physics and nuclear fusion community, offering interested experts and colleagues a chance to provide references and comments on STPBF. We extend our special thanks to all the friends and colleagues who have dedicated their conscientious efforts to preparing, reviewing, and selecting these papers.