ENN Science and Technology Development Co., Ltd. (ENN) is committed to generating fusion energy in an environmentally friendly and cost-effective manner, which requires abundant aneutronic fuel. Proton-boron (p-11B or p-B) fusion is considered an ideal choice for this purpose. Recent studies have suggested that p-B fusion, although challenging, is feasible based on new cross section data, provided that a hot ion mode and high wall reflection can be achieved to reduce electron radiation loss. The high beta and good confinement of the spherical torus (ST) make it an ideal candidate for p-B fusion. By utilizing the new spherical torus energy confinement scaling law, a reactor with a major radius R0=4 m, central magnetic field B0=6 T, central temperature Ti0=150 keV, plasma current Ip=30 MA, and hot ion mode Ti/Te=4 can yield p-B fusion with Q>10. A roadmap for p-B fusion has been developed, with the next-generation device named EHL-2. EHL stands for ENN He-Long, which literally means “peaceful Chinese Loong.” The main target parameters include R01.05 m, A1.85, B03 T, Ti030 keV, Ip3 MA, and Ti/Te2. The existing ST device EXL-50 was simultaneously upgraded to provide experimental support for the new roadmap, involving the installation and upgrading of the central solenoid, vacuum chamber, and magnetic systems. The construction of the upgraded ST fusion device, EXL-50U, was completed at the end of 2023, and it achieved its first plasma in January 2024. The construction of EHL-2 is estimated to be completed by 2026.

The quest for controlled fusion energy has been a long-standing dream for humanity since the 1950s, primarily due to its carbon-free nature, high unit energy, and abundance of resources. However, the challenges associated with achieving this goal are both scientific and engineering-related. Several books1 and papers2 review the progress in this field.

The development of a fusion energy reactor involves three essential steps: (1) selection of fusion fuels, (2) selection of a confinement approach, and (3) selection of a method to utilize or convert the energy produced. Over the past 70 years, much attention has been focused on the second step, and the feasibility or progress of a confinement approach can be described by the product of three physical parameters: ion density (ni), ion temperature (Ti), and energy confinement time (τE), commonly referred to as the Lawson criteria.2,3 To confine plasmas efficiently, various approaches have been considered. Among these, magnetic confinement, which uses a magnetic field to confine the plasma, and inertial confinement, which initiates nuclear fusion reactions by compressing and heating fuel targets, are considered the two most promising techniques. Significant progress has been made in both magnetic confinement fusion and inertial fusion. For example, in JET's 1997 Deuterium–Tritium (DT) experiments, a Q value, defined as the ratio between the output fusion power and the input power, of 0.67 was achieved.4,5 In the latest DT experiments in 2021, a value of 0.33 was achieved for 5 s, setting a new record for magnetic confined fusion energy output with 59 MJ.6 In recent experiments at NIF, a Q value of approximately 1.5 was achieved with fusion energy output around 3.15 MJ,7 indicating that more fusion power was produced than input power, or ignition was achieved in inertial confinement fusion. Though there are differences in the definition of ignition2 between the inertia confinement fusion and that of magnetic confinement fusion, there is little doubt about the scientific feasibility of D–T fusion.

Considerable effort has been dedicated to magnetic confinement fusion. Various configurations have been proposed to confine plasmas, including tokamaks,8 stellarators,9 field reversed configuration (FRC),10 and magnetic mirrors.11 Tokamaks and stellarators are toroidal devices that utilize twisted magnetic fields to confine high-temperature plasma. Both configurations have demonstrated much higher plasma parameters than other magnetic confinement configurations to date. In stellarators, twisted magnetic field lines are created using external coils, while in tokamaks, field lines are twisted by a combination of the toroidal magnetic field generated by toroidal field coils (TF) and the poloidal magnetic field generated by poloidal field coils (PF) and the plasma current. Tokamaks are generally easier to construct than stellarators, and over one hundred tokamaks have been built and extensively studied both experimentally and theoretically. This has provided a solid foundation for the design of next-generation tokamaks such as ITER12 and CFETR.13 

In addition to conventional tokamaks with a large aspect ratio, defined as A=R0/a where R0 and a denote the major and minor radius, respectively, the spherical tokamak/torus (ST) with a small aspect ratio has been proposed.14 The small aspect ratio significantly influences magnetohydrodynamic (MHD) stabilities, allowing ST to operate with high βT, the ratio of plasma pressure to toroidal magnetic pressure. This enables a more efficient use of the magnetic field, benefiting compact devices. Experiments have been conducted on START,15 MAST,16 NSTX,17 and Globus-M.18 Recent experimental results from the Globus-M2 device have demonstrated that energy confinement in ST can be significantly increased by doubling the toroidal magnetic field.19 Furthermore, a new scaling law for ST has been developed, showing promising energy confinement time results at high toroidal magnetic fields, i.e., τEIp0.53BT1.05.19 One of the main drawbacks of ST is the limited space within the central solenoid (CS) coil, making it challenging to increase the toroidal field. However, recent advances in high-temperature superconductor (HTS) technology have made it possible to achieve high magnetic fields in compact devices.20,21 ST40, designed with a magnetic field of 3 T, has operated at a toroidal field (TF) up to 2 T at R0 = 40 cm, although the TF magnet is made from Cu and is liquid nitrogen (LN2) cooled.22 Recently, ion temperature plasmas of around 9.6 keV (>100 × 106 K) were achieved in ST40,23 representing a significant milestone for compact fusion devices. The UK has also initiated the ambitious Spherical Tokamak for Energy Production (STEP) program (https://step.ukaea.uk/), which aims to develop a compact prototype reactor by 2040 capable of delivering a net electric power of Pel> 100 MW to the grid.24 

The p-11B fusion has been studied both theoretically25–28 and experimentally.29–31 On the experimental side, research has been conducted using laser-produced plasmas29 and particle accelerators employing beam-target fusion.30 Recently, clear experimental measurements of α particles produced by p-11B fusion in the large helical device (LHD) have been achieved.31 This involved using high-energy neutral beams and boron powder injection in high-temperature fusion plasma. These findings represent the first experimental demonstration of p-11B fusion in a magnetically confined plasma.

The significant accomplishments mentioned above have significantly enhanced confidence in the potential of commercial fusion energy. However, for fusion energy to become more competitive with other sources, such as fission and solar energy, considerations beyond the physics aspects are necessary. Achieving commercial fusion energy in the near future requires careful consideration of various options. In this work, we describe ENN's chosen roadmap for fusion energy, encompassing the selection of fuels, confinement approaches, and a research plan for the next 10–15 years.

This paper is organized as follows: In Sec. II, we discuss the choice of fuels. In Sec. III, we describe a newly built system code that demonstrates the feasibility of a proton-Boron fusion energy reactor based on a spherical torus. In Sec. IV, we summarize the key technologies that need resolution. In Sec. V, we present the ENN fusion roadmap. Finally, in Sec. VI, we provide a summary and discuss the findings.

In order to achieve commercial fusion energy, it is essential to utilize eco-friendly fusion sources. This implies a need for low neutron yield or aneutronic fusion. Additionally, the cost per unit of fusion power must be sufficiently low to compete with other energy sources such as wind, photovoltaic (PV), and fission. To fulfill these requirements, the fuel must be abundant and accessible, energy conversion must be high, and the technology must be suitable for the distributed energy market.

A comparison of the advantages and disadvantages of different fusion fuels is presented in Table I. D–T fusion stands out as the easiest reaction to achieve fusion energy gain due to its high reaction cross section at relatively low temperatures, as well as its high energy yield for each reaction. Currently, the mainstream roadmap for commercial fusion is based on D–T fusion.

TABLE I.

The comparison between different fusion fuels. Note that accurate fuel prices and reserves are difficult to obtain and may change over time; therefore, the data provided here are rough estimates for reference purposes. See  Appendix B for more details.

Nuclear reaction Advantages Disadvantages Rough fuel price (CNY/g) Output energy 20TW · year Rough reserves
D–T  Easiest way to fusion  High energy neutron wall material protection, Tritium breeding  T: 0.2 × 106  T: 1000 tons  T: 25 kg 
6Li: 2000 tons  6Li: 2 × 106 tons 
D–D  Relatively easy  High energy neutrons, peak reaction cross section low  D: 30  D: 7000 tons  45 × 1012 tons 
D–3He  Few neutrons, low neutron energy  3He expensive, D–D side effects  3He: 0.1 × 106  3He: 1000 tons  Produced by nuclear reactions, mined on the moon 
p-11 With few neutrons and cheap raw fuels, α particles direct energy conversion  Extremely high reaction threshold  11B: 30  11B: 8000 tons  1 × 109 tons 
Nuclear reaction Advantages Disadvantages Rough fuel price (CNY/g) Output energy 20TW · year Rough reserves
D–T  Easiest way to fusion  High energy neutron wall material protection, Tritium breeding  T: 0.2 × 106  T: 1000 tons  T: 25 kg 
6Li: 2000 tons  6Li: 2 × 106 tons 
D–D  Relatively easy  High energy neutrons, peak reaction cross section low  D: 30  D: 7000 tons  45 × 1012 tons 
D–3He  Few neutrons, low neutron energy  3He expensive, D–D side effects  3He: 0.1 × 106  3He: 1000 tons  Produced by nuclear reactions, mined on the moon 
p-11 With few neutrons and cheap raw fuels, α particles direct energy conversion  Extremely high reaction threshold  11B: 30  11B: 8000 tons  1 × 109 tons 

However, D–T fusion poses significant challenges, particularly concerning energetic neutron shielding and tritium breeding. Currently, there are no materials available for effective neutron shielding, and achieving a tritium breeding rate (TBR) larger than 1.2, which is necessary, may be difficult in the short term.5 The proven reserves of 6Li, which is less than 10% in natural lithium, for tritium breeding is also limited. Concerns about D–T fusion for energy have been raised from an engineering perspective.5,32 For example, fusion energy can only be extracted from the surface, which limits the power output per unit volume, and thus affects the unit cost. Even if neutron shielding and tritium breeding are well addressed, the two major drawbacks of fission—radioactivity safety or environment5 concerns and limited resources—hold similarly to D–T fusion, though at a much less severe level since fusion reactors are inherently safer than fission reactors, and lithium resources are abundant enough to support D–T fusion as a first-generation fusion fuel for about one thousand years. These concerns, combined with the challenges mentioned above, make D–T fusion challenging to compete with fission32 or fossil fuels.33 

D–D fusion, on the other hand, is relatively easy to achieve due to its large reaction cross-section and abundance of fuel. However, it also produces high-energy neutrons. D–3He fusion produces fewer neutrons and has a relatively large reaction cross section, making it an attractive choice for fusion. Nevertheless, the disadvantages of D–3He fusion include the side effects of D–D fusion and the scarcity of 3He storage on Earth. If 3He can be mined from the moon, it may make D–3He fusion more feasible, although the reserves and cost of 3He remain open questions in the short term.

The p-11B fusion is an attractive fusion choice for several reasons. From the fuel perspective, abundant boron is readily available on Earth. Regarding fusion products, p-11B fusion exclusively generates charged α particles, which have the potential for active control and direct electricity generation through charge separation. It is important to note that side reactions, such as p+11B n+11C, producing low-energy neutrons with an energy of 2.765 MeV, exist. The fraction of this fusion reaction is approximately 105, and the low-energy neutrons are more manageable. These properties align with the requirements for commercial fusion. However, it should be acknowledged that the reaction threshold for p-11B fusion is exceptionally high due to its relatively low fusion cross section. The criteria for ignition and fusion reactivity for various fusion reactions are illustrated in Fig. 1. Notably, the required criteria for p-11B fusion are three orders of magnitude higher than those for D–T fusion. The fusion reactivity, σv, significantly improves when the effective temperature, Tr=(m1T2+m2T1)/(m1+m2), exceeds 200 keV. Two sets of different cross-sectional data are utilized in p-11B fusion reactivity calculations.34,35 Despite these challenges, it is believed that high ion temperatures or energies can be achieved by utilizing high-energy negative ion-based neutral beam injection (NNBI)36 and effective ion cyclotron range of frequency (ICRF) heating.37 Notably, both TFTR38 and JT-6039 tokamaks have achieved ion temperatures Ti45 keV.

FIG. 1.

The triple products (a) and the thermal fusion reactivity (b) for D–T, D–D, D–3He, and p-11B fusion, and the strategy to make p-11B fusion possible (c). The data are from Refs. 2, 34, 35, 43, 44, and 54.

FIG. 1.

The triple products (a) and the thermal fusion reactivity (b) for D–T, D–D, D–3He, and p-11B fusion, and the strategy to make p-11B fusion possible (c). The data are from Refs. 2, 34, 35, 43, 44, and 54.

Close modal

After this analysis, choosing p-11B as the fuel for commercial fusion appears reasonable. The primary reason is not that it is easy, but rather that we have limited choices. Due to the scarcity of fuels, options like D–T and D–3He are not viable. Additionally, to avoid dealing with neutrons, D–D fusion is not a preferred choice.

System codes utilizing scaling laws from experiments have been developed and employed.40,41 However, to design a spherical tokamak using p-11B as fuel, a new system code is required. This section will introduce our new system code based on the updated spherical tokamak scaling.

The new code is developed and streamlined from our original system code.42 The flow chart of the code is illustrated in Fig. 2.

FIG. 2.

The flow chart of the system code. A major difference from other system codes is that certain scaling law-relevant parameters, such as τE, are set as input parameters to avoid sensitivity.

FIG. 2.

The flow chart of the system code. A major difference from other system codes is that certain scaling law-relevant parameters, such as τE, are set as input parameters to avoid sensitivity.

Close modal
The input parameters encompass the size of the vacuum vessel or the shape of the plasma (explained in detail in Subsection III B), density and temperature profiles, toroidal magnetic field (B0), plasma current (Ip), and energy confinement time (τE). The profiles are expressed as
n(x)=n0(1x2)Sn,
(1)
T(x)=T0(1x2)ST,
(2)
where n0 and T0 are the density and temperature on the magnetic axis and Sn ST are the shape factors of density and temperature, respectively. Here, x=r/a represents the normalized radial displacement, where r is the displacement from the magnetic axis along the radial direction, and a is the minor radius. The profiles can exhibit more H-mode-like behavior when decreasing the shape factors, i.e., having a steep gradient in the edge region. By using these expressions, the volume-averaged and line-averaged plasma density and temperature can be calculated as follows:
n=01n0(1x2)Sn2xdx=n01+Sn,
(3)
T=01T0(1x2)ST2xdx=T01+ST,
(4)
nl=01n0(1x2)Sndx=π2Γ(Sn+1)Γ(Sn+1.5)n0,
(5)
Tl=01T0(1x2)STdx=π2Γ(ST+1)Γ(ST+1.5)T0.
(6)
Here, Γ(z) is Euler's gamma function.

The energy confinement time is also an input parameter in the new code, enabling the calculation of heating power and fusion power.

The output parameters of the new code include plasma size, averaged density and temperature (navg and Tavg), stored energy (Wth), fusion, α particle power, and radiation power (Pfus, Pα, and Prad). Combining the fusion power and energy confinement time allows the determination of the required heating power (Pheat) from the power balance equation, and the fusion energy gain factor (Qfus) can be calculated. The plasma βT can be determined using profiles and toroidal magnetic field. Finally, scaling factors such as Greenwald density fraction (nG), H factors (H98 and HST), L-H mode transition power (PLH), safety factor (q), and normalized beta (βN) can be computed.

The geometry of the plasmas is illustrated in Fig. 3. The shape of the plasma can be fixed when parameters such as the major radius (R0), aspect ratio (A=R0/a), triangularity (δ), elongation (κ), and gap between the plasma and the vacuum wall (g) are specified. The initial design of the vacuum vessel can be determined based on these parameters. The plasma volume can be calculated using
Vp=2π2κ(R0δa)a2+163δπκa3.
(7)
The volume of the device is calculated by using
Vd=2π2κ(R0δ(a+g))(a+g)2+163δπκ(a+g)3.
(8)
FIG. 3.

The plasma shape.

FIG. 3.

The plasma shape.

Close modal
The surface area of the plasma, Sp, and the surface area of the device, Sw, are
Sp=(4π2R0aκ0.654κδ)a2,
(9)
Sw=(4π2R0a+gκ0.654κδ)(a+g)2.
(10)

It should be noted that we have set the major radius and the magnetic axis at the center of the plasma, i.e., R0=(Rmax+Rmin)/2. In realistic plasma equilibrium, the magnetic axis would experience a shift outside due to the Shafranov shift. We omit this difference for simplification.

There are multiple species inside the plasma, including two ions for fusion reactions, helium particles, and other impurities, with their densities represented by n1, n2, nHe, and nimp, respectively, and the charge numbers represented by Z1, Z2, ZHe, and Zimp. The ion and electron density can be determined according to the quasi-neutral condition, i.e.,
ni=n1+n21+δ12+nHe+nimp,
(11)
ne=n1Z1+n2Z21+δ12+nHeZHe+nimpZimp,
(12)
where δ12=1 when fusion particles 1 and 2 are identical, while δ12=0 when they are different. The average and effective charge numbers can be calculated by
Zi=neni,
(13)
Zeff=n1Z12+n2Z221+δ12+nHeZHe2+nimpZimp2ne.
(14)
The effective density of the two fusion ions is presented by
n12=n1+n21+δ12,
(15)
and the ratio of the fusion ions is f12=n12/ni. The ratios of the helium ions and impurities are represented by fHe=nHe/ni and fimp=nimp/ni, respectively. Furthermore, the ratios of the two separate fusion ions can be calculated by x1=n1/n12 and x2=n2/n12. Under this definition, and together with plasma quasi-neutrality, the following relations need to be satisfied:
f12+fHe+fimp=1,
(16)
f12(x1Z1+x2Z2)1+δ12+fHeZHe+fimpZimp=1.
(17)
Assuming that the temperature of the ions is the same, while the temperature ratio of the electrons and ions is fixed at fT, i.e., Te=fTTi, the effect of the ion and electron temperature ratio can be evaluated.
The fusion power of the two fusion ions can be calculated by
Pfus=11+δ12n1n2σvY=Y1+δ12n10n20ΦVp,
(18)
where Y is the total energy released from the fusion reaction and σ represents the cross-section of the two fusion ions. Here, n10 and n20 are the densities at the magnetic axis for the two fusion ions. Considering the radial distribution of the ion temperature, the fusion reactivity can be calculated as follows:
Φ=201(1x2)2Snσvxdx,
(19)
where σv is related to the effective ion temperature Ti(x). Assuming there is a potential enhancement factor fσ,43 for non-Maxwellian distributed ions or from other effects, the fusion rate can be presented by
σv=fσσvM,
(20)
where σvM can be used to calculate the fusion rate for a Maxwellian ion temperature distribution. In this way, the effect of the fusion rate enhancement can be evaluated.
The fusion equation for D–T fusion is
D+T24He(3.5MeV)+01n(14.1MeV).
(21)
Here, Y=17.6 MeV, and the formulas for D–T fusion cross section are used.44 
The fusion equation for p- 11B fusion is
p+11B324He(8.7MeV).
(22)
Here, Y=8.7 MeV, and we use the new fusion cross-sectional data35 instead of the old one.34 
After calculating the fusion power, the fusion gain factor can be obtained by
Q=PfusPaux,
(23)
where Paux is the auxiliary power used to sustain the plasma.
The power balance equation is given as
dWthdt=Pα+PauxPbremPcyclWthτE=0,
(24)
where Wth is the plasma thermal energy and Pα is the heating power by α particles. Pbrem and Pcycl are the bremsstrahlung and cyclotron radiation loss power, respectively.
In the non-relativistic case, the bremsstrahlung power can be evaluated using the following expression:26 
Pbrem
(25)
=CBne02kBTe0Vp{Zeff[11+2Sn+0.5ST+0.79361+2Sn+1.5STkBTe0mec2+1.8741+2Sn+2.5ST(kBTe0mec2)2]+32(1+2Sn+1.5ST)kBTe0mec2}(MW),
(26)
where CB=5.34×1043, kBTe0 and mec2 are in keV, and the electron density in m3. Here, kB and me are the Boltzmann constant and electron rest mass, respectively.
The cyclotron power can be obtained from45 
Pcycl=4.14×107BT02.5(1Rw)0.5Vpaeff0.5×neff0.5Teff2.5(1+2.5Teff511)(MW).
(27)
Here, Rw denotes the reflection rate of the wall, and it is important to reuse the cyclotron power by reflecting it back to the plasma. The effective density, neff, the effective minor radius, aeff, and the effective temperature, Teff, are defined as
neff=ne=ne01+Sn,
(28)
aeff=aκ0.5,Teff=Te001(1x2)STdx.
(29)
Note that the definition of Teff is different from Te.
The energy confinement time, τE, is a critical parameter in magnetic confinement fusion and serves as an input parameter in the new system code. The scaling law for the H-mode energy confinement time in conventional tokamaks is described by the IPB98(y,2) model,40 expressed as
τECT=0.0562Ip0.93BT0.15Ploss0.69ne0.41M0.19R01.97ϵ0.58κ0.78,
(30)
where M is the average mass number and ϵ=1/A.
For spherical tokamaks, the energy confinement time can be described by the following scaling law:19 
τEST=0.066Ip0.53BT1.05Ploss0.58ne0.65R02.66κ0.78.
(31)
Both of these scaling laws are used in the new system code to compare the different confinement factors between conventional tokamaks and spherical tokamaks. The H-factor, defined as the ratio of the required energy confinement time to the scaling, is utilized to evaluate the confinement of the tokamak. It can be calculated as follows:
H98=τEτECT,
(32)
HST=τEτEST.
(33)

Note that there is not much difference between the L-mode and H-mode for ST scaling law; that is, Eq. (31) includes the data for both.

The toroidal beta can be calculated using
βT=2μ0kB(niTi+neTe)dVBT2=2μ0kB(niTi+neTe)(1+Sn+ST)BT2,
(34)
where μ0 is the permeability of free space.
The safety factor is calculated by default using41 
q=2πκBTa2κμ0R0Ip=5BTa2κR0Ip,
(35)
and also with ST correction
q*=π(1+κ2)BTa2κμ0R0Ip.
(36)
Generally, the safety factor should be larger than 2 to avoid disruption. The normalized beta of the plasma can be calculated using
βN=100βTaBTIp,
(37)
and it is required that βN<12/A to stabilize instabilities.

The ITER design parameters are employed to benchmark the new system code, and the comparison results are presented in Table II. The outcomes obtained from the new simplified system code closely align with the results reported by Costley41 for the same input parameters.

TABLE II.

Comparison of the new system code with Costley's results for ITER parameters.

ITER parameters Costley15 ENN model
Central ion temperature, Ti0 (keV)  25  25 
Central density, ne0 (m−3 0.77×1020  0.77×1020 
Confinement time, τE (s)  2.0  2.0 
Beta, β  0.02  0.02 
Magnetic field, B0 (T)  5.18  5.18 
Major radius, R0 (m)  6.35  6.35 
Aspect ratio, A  3.43  3.43 
Plasma current, Ip (MA)  9.2  9.2 
Heating power, Pheat (MW)  70  70.4 
Fusion power, Pfus (MW)  350  356 
Fusion gain, Q  5.1 
ITER parameters Costley15 ENN model
Central ion temperature, Ti0 (keV)  25  25 
Central density, ne0 (m−3 0.77×1020  0.77×1020 
Confinement time, τE (s)  2.0  2.0 
Beta, β  0.02  0.02 
Magnetic field, B0 (T)  5.18  5.18 
Major radius, R0 (m)  6.35  6.35 
Aspect ratio, A  3.43  3.43 
Plasma current, Ip (MA)  9.2  9.2 
Heating power, Pheat (MW)  70  70.4 
Fusion power, Pfus (MW)  350  356 
Fusion gain, Q  5.1 

The general strategy to make p-11B fusion possible is shown in Fig. 1(c), i.e., reduce the requirement by increasing the reactivity and reducing the radiation, and increasing the triple products. In pursuit of p-11B fusion, it is imperative to enhance the triple product, particularly by elevating the ion temperature and lowering the reaction threshold conditions. While high Ti is essential for increased fusion reactivity, it is crucial to manage electron bremsstrahlung loss power, which can surpass fusion power if Te matches Ti. Hence, hot ion modes with Ti/Te=4 are deemed necessary.42 It is encouraging to note that in p-11B fusion, the fusion product α particles primarily heat ions rather than electrons,53,54 contrasting with the dynamics in D–T fusion. This phenomenon can be attributed to the significantly closer velocities between the MeV α particles and the hundred keV ions compared to electrons, facilitating more efficient energy exchange, specifically heating, between ions and α particles.

Augmenting the fusion reactivity becomes feasible with a larger fusion reaction cross section.27,35 Additional physical mechanisms, such as nonthermalized distribution43,46,47 and potential avalanche processes48 and other nonlinear effects49 in p-11B fusion reactions, hold promise for further boosting fusion reactivity. Recent JET D–T experiments have shown promising results. These include observations of synergistic effects on fusion rates with both NBI and ICRF,37 as well as significant non-thermal beam-target and beam-beam fusion reaction.50 The primary scientific requirements for realizing p-11B fusion are succinctly outlined in Fig. 4.

FIG. 4.

Key issues for spherical torus p-B fusion. Primary factors can interact with each other. If one technological breakthrough is achieved, other technical requirements may be lowered accordingly.

FIG. 4.

Key issues for spherical torus p-B fusion. Primary factors can interact with each other. If one technological breakthrough is achieved, other technical requirements may be lowered accordingly.

Close modal

Using the system code described in Sec. III, the optimal parameters for achieving p-11B fusion gain (Q>1) are as follows: magnetic field strength (B0) in the range of 47 T, ion temperature to electron temperature ratio (Ti/Te) of 3, ion temperature (Ti0) between 100and300 keV, electron density (ne0) of 13×1020 m−3, and energy confinement time (τE) ranging from 2050 s. Additionally, if the potential fusion enhancement factor fσ is high (e.g., fσ>2), the other requirements can be significantly reduced. By utilizing the new ST energy confinement scaling law, a reactor with the major radius (R0) of 4 m, central magnetic field (B0) of 6 T, central temperature (Ti0) of 150 keV, plasma current (Ip) of 30 MA, and hot ion mode (Ti/Te) of 4 can yield p-11B fusion with Q>10. More detailed discussions of p-B reactor parameters can be found in  Appendix A. One should be careful that different assumptions may yield different results.

The attainment of an extremely high p-11B fusion gain is challenging. To illustrate the difficulty, we compare the cases of Conventional Tokamaks (CT) and ST in Table III with a set of highly optimized conditions for p-11B fusion, specifically fσ=5 and Ti/Te=4. The primary distinctions between CT and ST lie in the confinement scaling law and plasma beta. As a result, we observe that CT can only achieve Q3 with B0=12 T, necessitating an even higher H factor of H98=3.41. In contrast, ST can achieve Q=30 with HST1.

TABLE III.

The comparison of parameters of a CT and ST from the system code.

Parameters CT ST
Average ion temperature, Ti,avg(keV)  33  33 
Average electron density, ne,avg(1020m3)  1.65  0.66 
Confinement time, τE(s)  19.3 
Beta, βT  0.038  0.32 
Central Magnetic field, B0(T)  12  2.6 
Major radius, R0(m)  3.2 
Fusion Gain, Q  3.2  30 
Aspect ratio, A  3.5  1.7 
n0/nG  0.76 
Safety factor, q  3.27  2.16 
Elongation, κ  2.5  3.3 
Fusion power, Pfus(MW)  241  107 
Heating power, Pheat(MW)  74.5  3.56 
Plasma current, Ip(MA)  15  22 
H factor  (H98) 3.41  (HST)0.98 
Ti/Te 
p-11B Fusion enhancement factor fσ 
Parameters CT ST
Average ion temperature, Ti,avg(keV)  33  33 
Average electron density, ne,avg(1020m3)  1.65  0.66 
Confinement time, τE(s)  19.3 
Beta, βT  0.038  0.32 
Central Magnetic field, B0(T)  12  2.6 
Major radius, R0(m)  3.2 
Fusion Gain, Q  3.2  30 
Aspect ratio, A  3.5  1.7 
n0/nG  0.76 
Safety factor, q  3.27  2.16 
Elongation, κ  2.5  3.3 
Fusion power, Pfus(MW)  241  107 
Heating power, Pheat(MW)  74.5  3.56 
Plasma current, Ip(MA)  15  22 
H factor  (H98) 3.41  (HST)0.98 
Ti/Te 
p-11B Fusion enhancement factor fσ 

Benefiting from the high βT and favorable energy confinement time of ST, the required magnetic field and heating power are significantly lower compared to conventional tokamaks. Consequently, spherical tokamaks emerge as more suitable candidates for p-11B fusion. However, it is crucial to note that currently, the plasma current, temperature, and energy confinement time of spherical tokamaks are considerably lower than those of conventional tokamaks and need validation at high parameters. Verifying the confinement properties at elevated plasma parameters constitutes a crucial task in ENN's fusion roadmap. Additionally, it is emphasized that other magnetic confinement approaches typically exhibit even lower confinement time scaling than tokamaks and, therefore, may not be a prioritized option in ENN's strategy.

After summarizing the key scientific issues, the key technological challenges for ST p-11B fusion can also be outlined accordingly:

  • High heat load materials: The compact size of ST is advantageous for the commercialization of fusion energy but results in a higher particle and energy flux on the plasma facing components, especially the divertor. Effective management of these challenging conditions requires the development of new materials or the implementation of innovative design strategies.

  • High power supply: In addition to the power supply for the toroidal and poloidal magnetic coils, increased power demands for heating and current drive actuators are anticipated.

  • High efficient ion heating: The ion temperature required for p-11B fusion is substantially higher than that for D–T fusion, necessitating advanced ion heating methods. Promising approaches include neutral beam injection (NNBI) at high energies with long-duration operation or ion cyclotron resonance heating (ICRF). However, mature technologies for these methods are yet to be fully developed.

  • High-temperature superconductivity: Although the inherent properties of ST reduce the demand for a strong toroidal magnetic field, advancements in high-temperature superconducting materials hold the potential to optimize the engineering design of ST, making them more efficient for fusion applications.

  • Low aspect ratio engineering: The use of a low aspect ratio in fusion reactors offers benefits such as improved plasma confinement and reduced tokamak size. However, it imposes limitations on central solenoid space, affecting available volt-seconds for ohmic heating and plasma feedback control. Addressing this challenge requires the development of new technologies for effective heating and control in low aspect ratio fusion systems.

  • Advanced divertor: Controlled production of α particles in p-11B fusion enables direct electric power generation. Realizing this potential requires new conceptual designs and experimental validations to ensure the feasibility and practicality of this approach.

In this section, we will outline a new commercial fusion roadmap for p-11B utilizing spherical torus.

The roadmap for ENN fusion research is depicted in Fig. 5. A noteworthy milestone in this progression is the establishment of the central solenoid-free spherical torus, EXL-50, which commenced operations in 2018 (detailed in Ref. 51). EXL-50's primary focus is the exploration of current drive techniques through electron cyclotron resonance heating (ECRH). Some initial results have been obtained, such as driving a plasma current exceeding 170 kA without a CS and achieving a maximum driving efficiency around 1 A/W for ECRH. However, these results still need further development to achieve higher plasma density and current. A major upgrade is on the horizon for the EXL-50 device, involving the incorporation of a central solenoid and a doubling of the toroidal magnetic field strength. This enhancement plan also encompasses improvements to NBI, ICRF, low hybrid wave heating, and ECRH. These enhancements are anticipated to facilitate the achievement of hot ion mode discharges with ion temperatures surpassing 1.0 keV. Furthermore, the subsequent project, EXL-50U (Fig. 6), is designed to develop crucial technologies for ST p-11B fusion. It also aims to conduct feasibility studies covering both the physics and engineering aspects for EHL-2 (Fig. 7), which is slated to be the next-generation machine in the ongoing fusion research initiative.

FIG. 5.

The ENN's roadmap for ST p-11B fusion.

FIG. 5.

The ENN's roadmap for ST p-11B fusion.

Close modal
FIG. 6.

The upgrade of EXL-50 to EXL-50U was completed in 2023. EXL-50U achieved its first plasma in January 2024.

FIG. 6.

The upgrade of EXL-50 to EXL-50U was completed in 2023. EXL-50U achieved its first plasma in January 2024.

Close modal
FIG. 7.

Preliminary design of EHL-2.

FIG. 7.

Preliminary design of EHL-2.

Close modal

In the initial phase of the commercial fusion roadmap for p-11B utilizing a spherical torus, a dedicated research and development platform, EHL-2, will be constructed. The primary objectives of Phase I include achieving the p-11B thermal fusion reaction and validating the existence of hot ion modes characterized by a high ion-to-electron temperature ratio (Ti/Te2) at elevated ion temperatures. Additionally, the platform will focus on experimental validation, specifically measuring and confirming the presence of α particles resulting from the fusion reaction. The research effort will explore novel physics phenomena influenced by a high magnetic field (BT 3 T), conduct in-depth studies of energetic particle physics, and develop control methods for managing these energetic particles. Diagnostics related to energetic particles will be established and refined. The platform will also serve as a testing ground for a new divertor design that facilitates direct energy conversion, aiming for improved efficiency in harnessing fusion energy. Furthermore, phase I will involve fine-tuning and optimizing strategies for achieving ion heating and current drive with maximum efficiency. Through these targeted objectives, phase I aims to lay the groundwork for subsequent phases in the pursuit of realizing commercial p-11B fusion using a spherical torus.

Phase II of the commercial fusion roadmap for p-11B using a spherical torus will concentrate on elevating plasma parameters to closely emulate the conditions required for practical fusion. The construction of a new spherical torus, EXL-3, will be a key aspect of this phase, aiming to achieve a significant level of p-11B fusion power generation. The primary objective during phase II is to increase the ion temperature, with specific targets set at 70 keV for EHL-3A and 140 keV for EHL-3B. These targets bring the ion temperature closer to the required levels (Ti) for p-11B fusion. Additionally, phase II will involve a detailed analysis of engineering constraints that arise under fusion-related conditions, ensuring the safe and efficient operation of the fusion reactor. A critical aspect of this analysis will be the testing and selection of first-wall materials capable of withstanding the harsh nuclear environment associated with fusion reactions.

Phase III represents the apex of efforts toward achieving commercial p-11B fusion. The primary emphasis during this phase will be on optimizing the fusion process for practical and economically viable energy generation. As part of phase III, a comprehensive strategy for cost reduction will be explored and implemented to enhance the commercial viability of p-11B fusion technology.

The preliminary design parameters for each device during each phase can be found in Table IV. The realization of commercial p-11B fusion is anticipated to begin around 2035 after the successful construction and operation of these devices. The goal of EHL-2 is to verify the thermal reaction rate of ST p-B fusion, establish experimental scaling laws, and provide a design basis for future reactors. The construction of EHL-2 is also intended to verify whether the ST scaling law still holds for high parameters. Details of the EHL-2 physics and engineering designs will be published in a series of papers over the next 1–2 years. The construction cost of EHL-2 is estimated to be around 5 × 109 CNY, with the aim of obtaining the first plasma around 2027.

TABLE IV.

The preliminary target parameters for ENN ST devices. For EHL-3B, the fusion-produced alpha particles (133 MW) will also heat the plasma. In other devices listed in the table, ion heating will mainly be through NBI, with the possibility of additional heating via ICRF.

Parameters EXL-50 EXL-50U EHL-2 EHL-3A EHL-3B
Average/Central ion temperature, Ti(keV)  ⋯/1.0  ⋯/6.0  ⋯/30  ⋯/70  46.2/140 
Average/Central electron density, ne(1020m3)  ⋯/0.2  ⋯/1.0  ⋯/1.3  ⋯/1.5  0.83/2.5 
Confinement time, τE(s)  ⋯  0.07  0.5  25 
Beta, β  ⋯  0.1  0.11  0.18  0.24 
Central Magnetic field, B0(T)  0.5  1.2@0.6m 
Major radius, R0(m)  0.5  0.6–0.8  1.05  3.2 
Aspect ratio, A  1.5  1.5–1.85  1.85  1.8  1.7 
Heating power, Pheat(MW)  > 3  17  60  6.83 + 133 
Plasma current, Ip(MA)  0.5  0.9  3.0  10  25 
Ti/Te  ⋯  1.5 
Parameters EXL-50 EXL-50U EHL-2 EHL-3A EHL-3B
Average/Central ion temperature, Ti(keV)  ⋯/1.0  ⋯/6.0  ⋯/30  ⋯/70  46.2/140 
Average/Central electron density, ne(1020m3)  ⋯/0.2  ⋯/1.0  ⋯/1.3  ⋯/1.5  0.83/2.5 
Confinement time, τE(s)  ⋯  0.07  0.5  25 
Beta, β  ⋯  0.1  0.11  0.18  0.24 
Central Magnetic field, B0(T)  0.5  1.2@0.6m 
Major radius, R0(m)  0.5  0.6–0.8  1.05  3.2 
Aspect ratio, A  1.5  1.5–1.85  1.85  1.8  1.7 
Heating power, Pheat(MW)  > 3  17  60  6.83 + 133 
Plasma current, Ip(MA)  0.5  0.9  3.0  10  25 
Ti/Te  ⋯  1.5 

ENN adopts a rapid iterative approach to project development, demonstrating the capability to swiftly construct devices. For instance, EXL-50, a medium-sized ST, was built in less than 10 months (October 2018–July 2019). The upgraded device, EXL-50U, was completed in just 6 months (July 2023–Dec. 2023). The next significant milestone and potential alternatives in the roadmap will depend on the experimental outcomes of EHL-2, expected around the year 2028.

For ion heating, ENN is primarily developing NNBI technology and investigating the feasibility of using ICRF in STs.52 The p-B reaction produces three α particles, posing a three-body problem where the energies of each output particle cannot be determined solely by momentum and energy conservation, unlike in D–T, D–D, and D–He3 fusion. This feature presents challenges in obtaining accurate p-B fusion cross-section data. Given the critical importance of precise cross-section data for our roadmap, ENN is also dedicating effort to measuring more accurate p-B cross-section data, with results to be published in the near future.

It is worth mentioning that the rapid advancement of artificial intelligence (AI) capabilities is reshaping scientific and technological research, undoubtedly accelerating progress in fusion energy research.55 ENN is actively involved in the development of digital technology, recognizing its crucial role in accelerating progress along the ST p-B fusion energy roadmap.

In this paper, we present ENN's roadmap for achieving p-11B fusion utilizing spherical torus technology. The roadmap does not present all the solutions but rather identifies the technologies we need to develop. We conduct a comprehensive comparison of the advantages and disadvantages of different fusion reactions, ultimately selecting p-11B fusion due to its exceptional potential for commercialization. Our analysis highlights several key factors that make p-11B fusion a promising candidate. First, the abundance of boron fuel on Earth is expected to significantly lower the cost of p-11B fusion compared to other fusion reactions. Additionally, the absence of a breeding blanket requirement, which is necessary for D–T fusion, simplifies the design and reduces the overall size of the fusion device. Moreover, the aneutronic nature of the p-11B fusion reaction alleviates the need for a dense screen blanket, further contributing to size reduction. This opens up the possibility of direct energy conversion or direct electric power generation, contingent upon the controlled production of only charged α particles.

A simplified system code has been successfully developed, enabling the evaluation of fusion reactor performance. This code takes various input parameters into account, such as plasma shape, density and temperature profiles, magnetic field strength, plasma current, and energy confinement time. In return, it provides crucial output parameters, including plasma size, average density and temperature, fusion power, radiation loss power, heating power, fusion gain, βT (the ratio of plasma pressure to magnetic pressure), and scaling factors. To assess plasma confinement and performance, scaling laws for energy confinement time have been applied to both conventional tokamak and ST. A specific comparison between CT and ST has been conducted. The results of this comparison demonstrate that ST offer certain advantages over CT. Specifically, ST requires lower magnetic field strengths and reduced heating power, making them more feasible and potentially easier to construct for achieving the desired fusion conditions.

Fusion reactivity indeed increases with higher ion temperatures, which is advantageous for achieving successful fusion reactions. However, it is crucial to note that as temperatures rise, so does the electron bremsstrahlung power loss. When the electron bremsstrahlung power loss surpasses the fusion power at the same ion temperature, it becomes essential to operate in hot ion mode. Both CT and ST face the requirement of maintaining ion temperatures higher than electron temperatures (Ti/Te4) to realize p-11B fusion. This condition is essential to achieve the necessary conditions for successful fusion energy gain. To make p-11B fusion more attainable, methods that enhance fusion reactivity, such as maintaining a non-thermalized ion distribution, can prove highly beneficial.

The technologies needed for p-11B fusion can be summarized after the physical requirements have been reviewed. These include high heat load materials, high-power supply, highly efficient ion heating, high-temperature superconductivity technology, low aspect ratio engineering, and advanced divertor design.

The roadmap for ENN's p-11B fusion using ST is outlined. Two STs, namely, EXL-50 (a central solenoid-free ST) and EXL-50U (an upgraded version of EXL-50), are crucial in establishing the foundation for ST p-11B fusion. Phase I of the roadmap has been initiated, focusing on evaluating key scientific and technological aspects, including hot ion mode experiments and validating ST confinement scaling laws at high plasma parameters. A new spherical torus, EHL-2, is being constructed during this phase, with the goal of realizing the thermal p-11B fusion reaction and establishing the physical basis for p-11B fusion. Following the completion of phase I, phase II will concentrate on improving parameters and overcoming engineering constraints to scale up the fusion reactor. EHL-3, a new ST with plasma parameters closer to those required for p-11B fusion, will be built. In phase III, preparations for commercial fusion will include exploring methods to reduce costs.

The roadmap for ENN's pursuit of p-11B fusion using ST is detailed and consists of several key phases. Here is a summary of the roadmap:

  • Phase I—Establishing the scientific and technological basis:

    • Operation of two STs, EXL-50 (a central solenoid-free ST) and EXL-50U (an upgrade of EXL-50).

    • Evaluation of the key focuses on scientific and technological aspects.

    • Conducting hot ion mode experiments to validate their feasibility.

    • Verification of ST confinement scaling laws at high plasma parameters.

    • Construction of a spherical torus, EHL-2, dedicated to realizing the p-11B fusion reaction.

    • Establishing the physical foundation for p-11B fusion.

  • Phase II—Parameter improvement and engineering overcoming:

    • Concentration on enhancing plasma parameters and addressing engineering challenges.

    • Building EHL-3 (in two steps with EHL-3A and EHL-3B), another ST with plasma parameters approaching those required for p-11B fusion.

    • Preparing the groundwork for scaling up the fusion reactor.

  • Phase III—Preparing for commercial fusion:

    • Focusing on cost reduction strategies and preparations for commercial fusion.

    • Exploring methods to make p-11B fusion economically viable.

This comprehensive roadmap outlines the stepwise progression from scientific exploration to technological development and, ultimately, the pursuit of commercial p-11B fusion, showcasing ENN's commitment to advancing fusion energy research.

In the past five years, ENN has explored various fusion approaches, considering almost all types of fusion technologies. The findings and key challenges of commercializing fusion energy have been summarized in a report.54 The major conclusion drawn is that achieving commercial fusion energy is highly challenging, and the success of fusion energy depends on various factors, ranging from the choice of fuel to the confinement approach. As a private company, ENN's choices are further constrained. For instance, ENN prefers not to deal with neutron-related challenges. If, in the next stages of ENN's roadmap, p-B fusion proves to be impractical, the alternative choice would be D–D–He3 fusion. This fusion process involves the catalyzed fusion of deuterium–deuterium (D–D) with a small amount of 3He to sustain the reactor, thereby reducing the Lawson criterion requirements. Additionally, handling neutrons in this fusion process are considered to be less challenging. Hence, the research and development efforts on spherical torus technology at ENN remain relevant, as D–D–He3 fusion is being explored as a potential alternative to p-B fusion. The focus is on achieving radiation control, high beta, and good confinement in pursuit of viable and sustainable fusion energy solutions.

The authors express their gratitude to Mr. Yu-suo Wang, the chairman of ENN Group, for supporting this work and fusion energy research. They also acknowledge the contributions of the ENN fusion team and collaborators in supporting this endeavor. Additionally, they would like to express their appreciation for the valuable comments and suggestions from the two anonymous referees.

The authors have no conflicts to disclose.

M.-S. Liu contributed to the commercialization roadmap, while H.-S. Xie contributed to the scientific foundation. M.-S. Liu and H.-S. Xie contributed equally to this work.

Min-sheng Liu: Conceptualization (equal); Funding acquisition (lead); Investigation (supporting); Project administration (equal); Resources (lead); Supervision (equal); Writing – review & editing (supporting). Hua-sheng Xie: Conceptualization (lead); Formal analysis (lead); Investigation (lead); Methodology (lead); Project administration (equal); Software (lead); Supervision (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (lead). Yuan-ming Yang: Investigation (equal); Visualization (equal). Jia-qi Dong: Methodology (equal); Writing – review & editing (supporting). Kai-ming Feng: Methodology (equal); Writing – review & editing (supporting). Xiang Gu: Investigation (equal). Xian-li Huang: Investigation (equal). Xin-chen Jiang: Investigation (equal). Ying-ying Li: Investigation (equal). Zhi Li: Investigation (equal). Bing Liu: Investigation (equal). Wen-jun Liu: Investigation (equal). Di Luo: Investigation (equal). Yueng-Kay Martin Peng: Methodology (supporting); Writing – review & editing (supporting). Yue-jiang Shi: Investigation (equal). Shao-dong Song: Investigation (equal). Xian-ming Song: Investigation (equal). Tian-tian sun: Investigation (equal). Mu-zhi Tan: Investigation (equal). Xue-yun Wang: Investigation (equal). Yu-min Wang: Investigation (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (supporting). Gang Yin: Investigation (equal). Han-yue Zhao: Investigation (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

It is straightforward to outline the rough operational parameter space for a p-B fusion reactor.54 For instance, the optimal ion temperature falls within the range of 100–300 keV to maximize fusion power while minimizing heating requirements and radiation losses. Considering the economic and thermal loading constraints of the wall, the plasma density cannot be too high or too low, approximately Pfus0.1100MW/m3, which limits ne0120×1020m3. Furthermore, based on the Lawson criteria, the confinement time is constrained to 10–50 s.

For magnetic confinement fusion (MCF), since β<1, the minimum magnetic field is determined. Additionally, the magnetic field should not be excessively high to minimize cyclotron radiation losses. In the case of a ST, with β20%, the optimal magnetic field strength is approximately B058 T.

To minimize bremsstrahlung and cyclotron radiation losses, the electron temperature should be kept low, ideally with Ti/Te4. Moreover, a high wall reflectance, preferably 90%, is required to reduce cyclotron radiation.

Predicting the realizability of the fusion enhancement factor is challenging, although a higher value is desirable. Even for fσ=1, achieving p-B fusion gain is possible,27,42 especially under high hot ion mode conditions (Ti/Te4).

In Fig. 8, we present a typical POPCON (Plasma Operating Contours) of a p-B ST reactor, assuming no fusion reactivity enhancement (fσ=1), which still yields a fusion gain of Qfus=11. It should be noted that different assumptions will affect the final design parameters. Therefore, the parameters provided here are for reference only. Feasibility and changes will depend on technological progress in the future. For example, if fσ=2, the required temperature Ti0 can be reduced from 250 to 150 keV, as stated in the abstract of this work.

FIG. 8.

Typical ST p-B fusion reactor parameters, where the fusion reactivity enhancement factor fσ=1, yields a fusion gain Qfus=11. The results will change when the assumption is changed. For example, if fσ=2, the required temperature Ti0 can be reduced. We provide all the important parameters in the figure for those who are interested, although this may make the figure more complicated.

FIG. 8.

Typical ST p-B fusion reactor parameters, where the fusion reactivity enhancement factor fσ=1, yields a fusion gain Qfus=11. The results will change when the assumption is changed. For example, if fσ=2, the required temperature Ti0 can be reduced. We provide all the important parameters in the figure for those who are interested, although this may make the figure more complicated.

Close modal

The export and use of tritium (T) and 3He fuels are controlled by the government, making accurate fuel prices and reserves difficult to obtain and subject to change over time. Here, we gather some data from various sources. We assume an exchange rate of 1 USD  7 CNY. Tritium, with a half-life of 12.3 years, is reported to be priced around $30 000 per gram (approximately CNY 0.2 × 106/g),56 and commercially available tritium is estimated to be around 25 kg. For 3He, the price in 2010 was reported to be around $2000/l (approximately CNY 0.1 × 106/g),57 with an available quantity of around 10 kg.

Deuterium and boron are readily available, with the latest query prices for both being around 30 CNY/g. However, pure 11B is significantly more expensive at around 1500 CNY/g compared to natural boron powder. Global proven boron mineral mining reserves exceed one × 109 metric tons, with a natural abundance of 11B at 80.3%.58 

There are approximately 30 × 106 tons of proven lithium reserves globally, corresponding to about 2 × 106 tons of 6Li. The total lithium content in seawater is estimated to be 230 × 109 tons, with a natural abundance of 6Li at 4.85%.59 

Currently, global energy consumption is approximately 20 TW.60 To produce 20 TW · year of energy, equivalent to 6.3 × 1020 J, we can calculate the required amount of fuel using the energy yield per gram for various fusion reactions. Take the D–T reaction, for example, where 3 g of tritium yields 1.7 × 1015 J of energy. Therefore, to produce 20 TW · year of energy, 1.1  ×103 kg of tritium is required. The data in Table I are calculated accordingly.

1.
F. F.
Chen
,
An Indispensable Truth: How Fusion Power Can save the Planet
(
Springer
,
2011
).
2.
S. E.
Wurzel
and
S. C.
Hsu
, “
Progress toward fusion energy breakeven and gain as measured against the Lawson criterion
,”
Phys. Plasmas
29
,
062103
(
2022
).
3.
J. D.
Lawson
, “
Some criteria for a useful thermonuclear reactor
,”
Technical Report No. GP/R 1807
(
Atomic Energy Research Establishment
,
1955
);
J. D.
Lawson
, “
Some criteria for a power producing thermonuclear reactor
,”
Proc. Phys. Soc. Sect. B
70
,
6
(
1957
).
4.
A.
Gibson
and
JET Team
, “
Deuterium–tritium plasmas in the Joint European Torus (JET): Behavior and implications
,”
Phys. Plasmas
5
,
1839
(
1998
).
5.
L. J.
Reinders
,
The Fairy Tale of Nuclear Fusion
(
Springer
,
2021
).
6.
E.
Gibney
, “
Nuclear-fusion reactor smashes energy record
,”
Nature
602
,
371
(
2022
).
7.
J.
Tollefson
and
E.
Gibney
, “
Nuclear-fusion lab achieves ‘ignition’: What does it mean?
,”
Nature
612
,
597
(
2022
).
8.
J.
Wesson
,
Tokamak
, 3rd ed. (
Oxford Press
,
2004
).
9.
F.
Nespoli
,
S.
Masuzaki
,
K.
Tanaka
,
N.
Ashikawa
,
M.
Shoji
,
E. P.
Gilson
,
R.
Lunsford
,
T.
Oishi
,
K.
Ida
,
M.
Yoshinuma
,
Y.
Takemura
,
T.
Kinoshita
,
G.
Motojima
,
N.
Kenmochi
,
G.
Kawamura
,
C.
Suzuki
,
A.
Nagy
,
A.
Bortolon
,
N. A.
Pablant
,
A.
Mollen
,
N.
Tamura
,
D. A.
Gates
, and
T.
Morisaki
, “
Observation of a reduced-turbulence regime with boron powder injection in a stellarator
,”
Nat. Phys.
18
,
350
(
2022
).
10.
L. C.
Stainhauer
, “
Review of field-reversed configurations
,”
Phys. Plasmas
18
,
070501
(
2011
).
11.
R. F.
Post
, “
The magnetic mirror approach to fusion
,”
Nucl. Fusion
27
,
1579
(
1987
).
12.
K.
Ikeda
, “
ITER on the road to fusion energy
,”
Nucl. Fusion
50
,
014002
(
2010
).
13.
V. S.
Chan
,
A. E.
Costley
,
B. N.
Wan
,
A. M.
Garofalo
, and
J. A.
Leuer
, “
Evaluation of CFETR as a Fusion Nuclear Science Facility using multiple system codes
,”
Nucl. Fusion
55
,
023017
(
2015
).
14.
Y.-K. M.
Peng
and
D. J.
Strickler
,
Nucl. Fusion
26
,
769
(
1986
).
15.
M.
Gryaznevich
,
R.
Akers
,
P. G.
Carolan
,
N. J.
Conway
,
D.
Gates
,
A. R.
Field
,
T. C.
Hender
,
I.
Jenkins
,
R.
Martin
,
M. P. S.
Nightingale
,
C.
Ribeiro
,
D. C.
Robinson
,
A.
Sykes
,
M.
Tournianski
,
M.
Valovic
, and
M. J.
Walsh
, “
Achievement of record beta in the START spherical tokamak
,”
Phys. Rev. Lett.
80
,
3972
(
1998
).
16.
A.
Sykes
,
J.-W.
Ahn
,
R.
Akers
,
E.
Arends
,
P. G.
Carolan
,
G. F.
Counsell
,
S. J.
Fielding
,
M.
Gryaznevich
,
R.
Martin
,
M.
Price
,
C.
Roach
,
V.
Shevchenko
,
M.
Tournianski
,
M.
Valovic
,
M. J.
Walsh
,
H. R.
Wilson
, and
MAST Team
, “
First physics results from the MAST Mega-Amp spherical tokamak
,”
Phys. Plasmas
8
,
2101
(
2001
).
17.
J. E.
Menard
,
M. G.
Bell
,
R. E.
Bell
,
E. D.
Fredrickson
,
D. A.
Gates
,
S. M.
Kaye
,
B. P.
LeBlanc
,
R.
Maingi
,
D.
Mueller
,
S. A.
Sabbagh
,
D.
Stutman
,
C. E.
Bush
,
D. W.
Johnson
,
R.
Kaita
,
H. W.
Kugel
,
R. J.
Maqueda
,
F.
Paoletti
,
S. F.
Paul
,
M.
Ono
,
Y.-K. M.
Peng
,
C. H.
Skinner
,
E. J.
Synakowski
, and
the NSTX Research Team,
Beta-limitting MHD instabilities in improved-performance NSTX spherical torus plasma
,”
Nucl. Fusion
43
,
330
(
2003
).
18.
V. K.
Gusev
,
S. E.
Aleksandrov
,
V. K.
Alimov
,
I. I.
Arkhipov
,
B. B.
Ayushin
,
A. G.
Barsukov
,
B. Y.
Ber
,
F. V.
Chernyshev
,
IN.
Chugunov
,
A. V.
Dech
et al, “
Overview of results obtained at the Globus-M spherical tokamak
,”
Nucl. Fusion
49
,
104021
(
2009
).
19.
G. S.
Kurskiev
,
V. K.
Gusev
,
N. V.
Sakharov
,
Y.
Petrov
,
N. N.
Bakharev
,
I. M.
Balachenkov
,
A. N.
Bazhenov
,
F. V.
Chernyshev
,
N. A.
Khromov
,
E. O.
Kiselev
,
S. V.
Krikunov
,
V. B.
Minaev
et al, “
Energy confinement in the spherical tokamak Globus-M2 with toroidal magnetic field reaching 0.8T
,”
Nucl. Fusion
62
,
016011
(
2022
).
20.
A. J.
Creely
,
D.
Brunner
,
R. T.
Mumgaard
,
M. L.
Reinke
,
M.
Segal
,
B. N.
Sorbom
, and
M. J.
Greenwald
, “
SPARC as a platform to advance tokamak science
,”
Phys. Plasmas
30
,
090601
(
2023
).
21.
Z. S.
Hartwig
,
R. F.
Vieira
,
D.
Dunn
,
T.
Golfinopoulos
,
B.
LaBombard
,
C. J.
Lammi
,
P. C.
Michael
,
S.
Agabian
,
D.
Arsenault
,
R.
Barnett
,
M.
Barry
,
L.
Bartoszek
,
W. K.
Beck
,
D.
Bellofatto
,
D.
Brunner
,
W.
Burke
,
J.
Burrows
,
W.
Byford
,
C.
Cauley
,
S.
Chamberlain
,
D.
Chavarria
,
J. L.
Cheng
,
J.
Chicarello
,
V.
Diep
,
E.
Dombrowski
,
J.
Doody
,
R.
Doos
,
B.
Eberlin
,
J.
Estrada
,
V.
Fry
,
M.
Fulton
,
S.
Garberg
,
R.
Granetz
,
A.
Greenberg
,
M.
Greenwald
,
S.
Heller
,
A. E.
Hubbard
,
E.
Ihloff
,
J. H.
Irby
,
M.
Iverson
,
P.
Jardin
,
D.
Korsun
,
S.
Kuznetsov
,
S.
Lane-Walsh
,
R.
Landry
,
R.
Lations
,
R.
Leccacorvi
,
M.
Levine
,
G.
Mackay
,
K.
Metcalfe
,
K.
Moazeni
,
J.
Mota
,
T.
Mouratidis
,
R.
Mumgaard
,
J. P.
Muncks
,
R. A.
Murray
,
D.
Nash
,
B.
Nottingham
,
C.
O'Shea
,
A. T.
Pfeiffer
,
S. Z.
Pierson
,
C.
Purdy
,
A.
Radovinsky
,
D. K.
Ravikumar
,
V.
Reyes
,
N.
Riva
,
R.
Rosati
,
M.
Rowell
,
E. E.
Salazar
,
F.
Santoro
,
A.
Sattarov
,
W.
Saunders
,
P.
Schweiger
,
S.
Schweiger
,
M.
Shepard
,
S.
Shiraiwa
,
M.
Silveira
,
F. T.
Snowman
,
B. N.
Sorbom
,
P.
Stahle
,
K.
Stevens
,
J.
Stillerman
,
D.
Tammana
,
T. L.
Toland
,
D.
Tracey
,
R.
Turcotte
,
K.
Uppalapati
,
M.
Vernacchia
,
C.
Vidal
,
E.
Voirin
,
A.
Warner
,
A.
Watterson
,
D. G.
Whyte
,
S.
Wilcox
,
M.
Wolf
,
B.
Wood
,
L.
Zhou
, and
A.
Zhukovsky
, “
The SPARC toroidal field model coil program (overview)
,”
IEEE Trans. Appl. Supercond.
34
(
2
),
0600316
(
2024
).
22.
M.
Gryaznevich
,
TE Ltd. Physics Team, and TE Ltd HTS Team for Tokamak Energy Ltd
., “
Experiments on ST40 at high magnetic field
,”
Nucl. Fusion
62
,
042008
(
2022
).
23.
S. A. M.
McNamara
,
O.
Asunta
,
J.
Bland
,
P. F.
Buxton
,
C.
Colgan
,
A.
Dnestrovskii
,
M.
Gemmell
,
M.
Gryaznevich
,
D.
Hoffman
,
F.
Janky
et al, “
Achievement of ion temperatures in excess of 100 million degrees Kelvin in the compact high-field spherical tokamak ST40
,”
Nucl. Fusion
63
,
054002
(
2023
).
24.
H. Meyer for the STEP Plasma Control, Heating, Current Drive Team and Contributors
, “
The physics of the preferred plasma scenario for STEP
,” in
Proceedings of the 48th EPS Conference on Plasma Physics
,
2022
.
25.
F. A.
Geser
and
M.
Valente
, “
A theoretical model for the cross section of the proton-boron fusion nuclear reaction
,”
Radiat. Phys. Chem.
167
,
108224
(
2020
).
26.
W. M.
Nevins
, “
A review of confinement requirements for advanced fuels
,”
J. Fusion Energy
17
(
1
),
25
(
1998
).
27.
S. V.
Putvinski
,
D. D.
Ryutov
, and
P. N.
Yushmanov
, “
Fusion reactivity of the pB11 plasma revisited
,”
Nucl. Fusion
59
,
076018
(
2019
).
28.
I. E.
Ochs
and
N. J.
Fisch
, “
Lowering the reactor breakeven requirements for proton–boron 11 fusion
,”
Phys. Plasmas
31
,
012503
(
2024
).
29.
D.
Margarone
,
J.
Bonvalet
,
L.
Giuffrida
,
A.
Morace
,
V.
Kantarelou
,
M.
Tosca
,
D.
Raffestin
,
P.
Nicolai
,
A.
Picciotto
,
Y.
Abe
et al, “
In-target proton-boron nuclear fusion using a PW-class leaser
,”
Appl. Sci.
12
,
1444
(
2022
).
30.
S.
Starve
,
M. W.
Ahmed
,
R. H.
France
III
,
S. S.
Henshaw
,
B.
Müller
,
B. A.
Perdue
,
R. M.
Prior
,
M. C.
Spraker
, and
H. R.
Weller
, “
Understanding the 11B(p, α) αα reaction at the 0.675 MeV resonance
,”
Phys. Lett. B
686
,
26
(
2011
).
31.
R. M.
Magee
,
K.
Ogawa
,
T.
Tajima
,
I.
Allfrey
,
H.
Gota
,
P.
McCarroll
,
S.
Ohdachi
,
M.
Isobe
,
S.
Kamio
,
V.
Klumper
,
H.
Nuga
,
M.
Shoji
,
S.
Ziaei
,
M. W.
Binderbauer
, and
M.
Osakabe
, “
First measurements of p11B fusion in a magnetically confined plasms
,”
Nat. Commun.
14
,
955
(
2023
).
32.
L. M.
Lidsky
, “
The trouble with fusion
,”
MIT Technol. Rev.
1983
,
1
12.
33.
E. J.
Lerner
,
S. M.
Hassan
,
I.
Karamitsos-Zivkovic
, and
R.
Fritsch
, “
What are the fastest routes to fusion energy?
,”
Phys. Plasmas
30
,
120602
(
2023
).
34.
W. M.
Nevins
and
R.
Swain
, “
The thermonuclear fusion rate coefficient for p-11 B reactions
,”
Nucl. Fusion
40
(
4
),
865
(
2000
).
35.
M. H.
Sikora
and
H. R.
Weller
, “
A new evaluation of the 11B(p,α)αα reaction rates
,”
J. Fusion Energy
35
(
3
),
538
(
2016
).
36.
Y.
Ikeda
,
N.
Umeda
,
N.
Akino
,
N.
Ebisawa
,
L.
Grisham
,
M.
Hanada
,
A.
Honda
,
T.
Inoue
,
M.
Kawai
,
M.
Kazawa
et al, “
Present status of the negative ion based NBI system for long pulse operation on JT-60U
,”
Nucl. Fusion
46
,
S211
(
2006
).
37.
K. K.
Kirov
,
C. D.
Challis
,
E.
De la Luna
,
J.
Eriksson
,
D.
Gallart
,
J.
Garcia
,
M.
Gorelenkova
,
J.
Hobirk
,
P.
Jacquet
,
A.
Kappatou
,
Y. O.
Kazakov
,
D.
Keeling
,
D.
King
,
E.
Lerche
,
C.
Maggi
,
J.
Mailloux
,
P.
Mantica
,
M.
Mantsinen
,
M.
Maslov
,
S.
Menmuir
,
P.
Siren
,
Z.
Stancar
,
D.
Van Eester
, and
JET Contributors
,
Nucl. Fusion
64
,
016026
(
2024
).
38.
R. J.
Hawryluk
,
S.
Batha
,
W.
Blanchard
,
M.
Beer
,
M. G.
Bell
,
R. E.
Bell
,
H.
Berk
,
S.
Bernabei
,
M.
Bitter
,
B.
Breizman
,
N. L.
Bretz
et al, “
Fusion plasma experiments on TFTR: A 20 year retrospective
,”
Phys. Plasmas
5
,
1577
(
1998
).
39.
S.
Ishida
,
Y.
Neyatani
,
Y.
Kamada
, and
JT-60 Team
, “
High performance experiments in JT-60U high current divertor discharges
,” in
Proceedings of the 16th International Conference on Fusion Energy, Montreal
(
International Atomic Energy Agency
,
1996
), Vol.
1
, p.
315
.
40.
M.
Shimada
,
D. J.
Campbell
,
V.
Mukhovatov
,
M.
Fujiwara
,
N.
Kirneva
,
K.
Lackner
,
M.
Nagami
,
V. D.
Pustovitov
,
N.
Uckan
,
J.
Wesley
et al, “
Progress in the ITER physics basis—Chapter 1: Overview and summary
,”
Nucl. Fusion
47
,
S1
(
2007
).
41.
A. E.
Costley
,
J.
Hugill
, and
P. F.
Buxton
, “
On the power and size of tokamak fusion pilot plants and reactors
,”
Nucl. Fusion
55
,
033001
(
2015
).
42.
J. Q.
Cai
,
H. S.
Xie
,
Y.
Li
,
M.
Tuszewski
,
H. B.
Zhou
, and
P. P.
Chen
, “
A study of the requirements of p-B11 fusion reactor by tokamak system, code
,”
Fusion Sci. Technol.
78
(
2
),
149
163
(
2022
).
43.
H. S.
Xie
,
M. Z.
Tan
,
D.
Luo
,
Z.
Li
, and
B.
Liu
, “
Fusion reactivities with drift bi-Maxwellian ion velocity distributions
,”
Plasma Phys. Controlled Fusion
65
,
055019
(
2023
).
44.
H. S.
Bosch
and
G. M.
Hale
, “
Improved formulas for fusion cross-sections and thermal reactivities
,”
Nucl. Fusion
32
(
4
),
611
(
1992
).
45.
A. B.
Kukushikin
and
P. V.
Minashin
, “
Generalization of Trubnikov formula for electron cyclotron total power loss
,” in
Tokamak-Reactors XXXVI International Conference on Plasma Physics and CF
, Zvenigorod, February 9–13,
2009
.
46.
H. Z.
Kong
,
H. S.
Xie
,
B.
Liu
,
M. Z.
Tan
,
D.
Luo
,
Z.
Li
, and
J. Z.
Sun
, “
Enhancement of fusion reactivity under non-Maxwellian distributions: Effects of drift-ring-beam, slowing-down, and kappa super-thermal distributions
,”
Plasma Phys. Controlled Fusion
66
,
015009
(
2024
).
47.
H. S.
Xie
and
X. Y.
Wang
, “
On the upper bound of non-thermal fusion reactivity with fixed total energy
,”
Plasma Phys. Controlled Fusion
66
,
065009
(
2024
).
48.
H.
Hora
,
S.
Eliezer
,
G. J.
Kirchhoff
,
N.
Nissim
,
J. X.
Wang
,
P.
Lalousis
,
Y. X.
Xu
,
G. H.
Miley
,
J. M.
Martinez-Val
,
W.
McKenzie
, and
J.
Kirchhoff
, “
Road map to clean energy using laser beam ignition of boron-hydrogen fusion
,”
Laser Part. Beams
35
(
4
),
730
(
2017
).
49.
W. Q.
Wei
,
S. Z.
Zhang
,
Z. G.
Deng
,
W.
Qi
,
H.
Xu
,
L. R.
Liu
,
J. L.
Zhang
,
F. F.
Li
,
X.
Xu
,
Z. M.
Hu
,
B. Z.
Chen
,
B. B.
Ma
,
J. X.
Li
,
X. G.
Ren
,
Z. F.
Xu
,
D. H. H.
Hoffmann
,
Q. P.
Fan
,
W. W.
Wang
,
S. Y.
Wang
,
J.
Teng
,
B.
Cui
,
F.
Lu
,
L.
Yang
,
Y. Q.
Gu
,
Z. Q.
Zhao
,
R.
Cheng
,
Z.
Wang
,
Y.
Lei
,
G. Q.
Xiao
,
H. W.
Zhao
,
B.
Liu
,
G. C.
Zhao
,
M. S.
Liu
,
H. S.
Xie
,
L. F.
Cao
,
J. R.
Ren
,
W. M.
Zhou
, and
Y. T.
Zhao
, “
Proton-boron fusion yield increased by orders of magnitude with foam targets
,” (
2023
).
50.
V. G.
Kiptily
,
R.
Dumont
,
M.
Fitzgerald
,
D.
Keeling
,
S. E.
Sharapov
,
M.
Poradzinski
,
Z.
Stancar
,
P. J.
Bonofiglo
,
E.
Delabie
,
Z.
Ghani
,
V.
Goloborodko
,
S.
Menmuir
,
E.
Kowalska-Strzeciwilk
,
M.
Podesta
,
H.
Sun
,
D. M.
Taylor
,
J.
Bernardo
,
I. S.
Carvalho
,
D.
Douai
,
J.
Garcia
,
M.
Lennholm
,
C. F.
Maggi
, and
J.
Mailloux
, “
Evidence of electron heating by alpha particles in JET deuterium-tritium plasmas
,”
Phys. Rev. Lett.
131
,
075101
(
2023
).
51.
Y. J.
Shi
,
B.
Liu
,
S. D.
Song
,
Y. Y.
Song
,
X. M.
Song
,
B. W.
Tong
,
S. K.
Cheng
,
W. J.
Liu
,
M. Y.
Wang
,
T. T.
Sun
,
D.
Guo
,
S. J.
Li
,
Y. Y.
Li
,
B.
Chen
,
X.
Gu
,
J. Q.
Cai
,
D.
Luo
,
D.
Banerjee
,
X.
Zhao
,
Y. M.
Yang
,
W. W.
Luo
,
P. H.
Zhou
,
Y.
Wang
,
A.
Ishida
,
T.
Maekawa
,
M. S.
Liu
,
B. S
,
Yuan
,
Y.-K. M.
Peng
, and
the EXL-50 Team
, “
Solenoid-free current drive via ECRH in EXL-50 spherical torus plasma
,”
Nucl. Fusion
62
,
086047
(
2022
).
52.
H. J.
Ma
,
H. S.
Xie
, and
B.
Li
, “
Simulation of ion cyclotron wave heating in the EXL-50U spherical tokamak based on dispersion relations
,”
Plasma Sci. Technol.
26
,
025105
(
2024
).
53.
J. M.
Dawson
, “
Advanced fusion reactors
,” in
Fusion, Part B
, edited by
E.
Teller
(
Academic Press
,
1981
), p.
465
.
54.
H. S.
Xie
,
Introduction to Fusion Ignition Principles: Zeroth Order Factors of Fusion Energy Research (in Chinese)
(
USTC Press
,
Hefei
,
2023
).
55.
J.
Degrave
,
F.
Felici
,
J.
Buchli
,
M.
Neunert
,
B.
Tracey
,
F.
Carpanese
,
T.
Ewalds
,
R.
Hafner
,
A.
Abdolmaleki
,
D.
de las Casas
,
C.
Donner
,
L.
Fritz
,
C.
Galperti
,
A.
Huber
,
J.
Keeling
,
M.
Tsimpoukelli
,
J.
Kay
,
A.
Merle
,
J.-M.
Moret
,
S.
Noury
,
F.
Pesamosca
,
D.
Pfau
,
O.
Sauter
,
C.
Sommariva
,
S.
Coda
,
B.
Duval
,
A.
Fasoli
,
P.
Kohli
,
K.
Kavukcuoglu
,
D.
Hassabis
, and
M.
Riedmiller
, “
Magnetic control of tokamak plasmas through deep reinforcement learning
,”
Nature
602
(
7897
),
414
(
2022
).
56.
D.
Clery
, “
Out of gas: A shortage of tritium fuel may leave fusion energy with an empty tank
,”
Science
376
,
1372
(
2022
).
57.
S.
Adee
, “
Physics projects deflate for lack of helium-3
,”
IEEE Spectrum
47
(
9
),
16
17
(
2010
).
58.
See https://en.wikipedia.org/wiki/Boron for reserve of boron (last accessed May 23
2024
).
59.
See https://en.wikipedia.org/wiki/Lithium for reserve of lithium (last accessed May 23
2024
).
60.
See https://en.wikipedia.org/wiki/World_energy_supply_and_consumption for world energy consumption (last accessed May 23
2024
).