History of Fusion Research

The human quest to achieve fusion energy began with early 20th-century theoretical research seeking to understand the Sun’s energy source. Through the birth of quantum mechanics, secret research during the Cold War, and the era of international cooperation, to the present construction of ITER and the advent of private sector participation, we trace in detail this grand history spanning over 100 years.

Timeline

Year	Event
1920	Eddington proposes nuclear fusion as the energy source of stars
1928	Gamow discovers quantum tunneling
1929	Atkinson and Houtermans theoretically explain nuclear fusion in stars
1932	Cockcroft and Walton achieve artificial nuclear transmutation
1938	Bethe proposes the pp-chain and CNO cycle
1942	Fermi and others begin examining military applications of fusion
Early 1950s	Sakharov and Tamm conceive the tokamak (Soviet Union)
1951	Spitzer conceives the stellarator (United States)
1952	Ivy Mike test (first hydrogen bomb)
1955	First Geneva Conference “Atoms for Peace”
1958	Second Geneva Conference declassifies fusion research; Fusion Discussion Group established in Japan
1968	Novosibirsk International Conference; T-3 tokamak success confirmed
1978-1987	INTOR Project (IAEA-led international tokamak reactor design)
1982	TFTR begins operation (United States)
1983	JET begins operation (Europe)
1985	JT-60 begins operation (Japan); Gorbachev-Reagan summit proposes ITER
1988	ITER Conceptual Design Activities begin
1994	JET begins D-T experiments
1997	JET achieves world record 16 MW fusion power
1998	LHD begins operation (Japan)
2007	ITER Organization established; Broader Approach Agreement signed
2016	W7-X begins operation (Germany)
2021	SPARC design completed; JET achieves new record of 59 MJ D-T output
2022	NIF (United States) achieves fusion ignition
2023	JT-60SA achieves first plasma

Chapter 1: Establishing the Theoretical Foundation (1920s-1940s)

1.1 The Mystery of Stellar Energy

At the end of the 19th century, how the Sun could shine for billions of years was a great mystery. Chemical reactions would sustain the Sun for only a few thousand years, and gravitational contraction (the Kelvin-Helmholtz mechanism) for only tens of millions of years. Geological evidence indicated that Earth’s age was billions of years, requiring an unknown physical mechanism for the Sun’s energy source.

In 1905, Einstein’s special theory of relativity demonstrated the equivalence of mass and energy.

$$E = mc^2$$

This equation suggested the possibility that enormous energy could be released from the loss of a small amount of mass.

1.2 Eddington’s Foresight

In 1920, British astronomer Arthur Eddington boldly proposed that the energy source of stars was the process of hydrogen being converted to helium in the high-temperature, high-pressure environment of stellar interiors. In a lecture at the British Association for the Advancement of Science, he stated:

“If the temperature in the stellar interior exceeds 40 million degrees, sufficient energy could be supplied by nuclear transmutation. We should consider sub-atomic energy as the Sun’s energy source.”

When four hydrogen nuclei fuse into a helium nucleus, a mass difference occurs.

$$4 \times m_p = 4 \times 1.007825 \text{ u} = 4.031300 \text{ u}$$ $$m_{He} = 4.002603 \text{ u}$$ $$\Delta m = 0.028697 \text{ u} = 0.7\% \text{ mass defect}$$

This mass defect is converted to energy according to E = mc². From 1 kg of hydrogen, approximately 6.3 x 10^14 J of energy is released, equivalent to the heat content of about 20,000 tons of oil.

However, at the time, classical mechanics could not explain how nuclear fusion could occur, as the electrical repulsion between nuclei (Coulomb barrier) could not be overcome.

1.3 Discovery of Quantum Tunneling

In 1928, Russian-born physicist George Gamow discovered “quantum tunneling” while explaining alpha decay using quantum mechanics. He showed that energy barriers that could not be overcome in classical mechanics could be penetrated through the spread of quantum mechanical wave functions.

The probability of penetrating the Coulomb barrier is expressed as the “Gamow factor.”

$$P_G \propto \exp\left(-\frac{2\pi Z_1 Z_2 e^2}{\hbar v}\right) = \exp\left(-\pi \sqrt{\frac{E_G}{E}}\right)$$

Here, the Gamow energy $E_G$ is defined as:

$$E_G = 2m_r c^2 (\pi \alpha Z_1 Z_2)^2$$

where $\alpha = e^2/(4\pi\epsilon_0 \hbar c) \approx 1/137$ is the fine structure constant, and $m_r$ is the reduced mass.

For the D-T reaction, $E_G \approx 1.2$ MeV. Even at plasma temperatures of about 10 keV (approximately 100 million degrees), high-energy tail components can react with significant probability through tunneling.

1.4 Atkinson and Houtermans’ Theory

In 1929, British physicist Robert Atkinson and German physicist Fritz Houtermans applied Gamow’s quantum tunneling to nuclear fusion reactions in stellar interiors. Their paper “On the Possibility of Energy Production in Stars” was the first quantitative study of energy production through nuclear fusion.

They calculated that under the conditions of temperature (about 15 million degrees) and density at stellar cores, hydrogen nuclei could fuse through tunneling and produce sufficient energy.

The nuclear fusion reaction rate is expressed as the product of the Maxwell-Boltzmann distribution and the tunneling probability.

$$\langle\sigma v\rangle = \sqrt{\frac{8}{\pi m_r}} (k_B T)^{-3/2} \int_0^\infty \sigma(E) E \exp\left(-\frac{E}{k_B T}\right) dE$$

The integrand of this integral has a peak in a narrow range called the “Gamow peak.”

$$E_0 = \left(\frac{E_G (k_B T)^2}{4}\right)^{1/3}$$

For the D-T reaction at T = 10 keV, $E_0 \approx 22$ keV.

1.5 Success of Artificial Nuclear Transmutation

In 1932, at the Cavendish Laboratory of Cambridge University in England, John Cockcroft and Ernest Walton achieved the first artificial nuclear transmutation using an accelerator.

$$p + ^7\text{Li} \rightarrow 2 \, ^4\text{He} + 17.3 \text{ MeV}$$

They irradiated a lithium target with protons accelerated to 400 keV and observed the emission of alpha particles. This experiment was one of the first direct verifications of Einstein’s E = mc², confirming that the energy calculated from the mass defect matched the measured value.

In the same year, James Chadwick discovered the neutron, and Harold Urey discovered deuterium. These discoveries greatly expanded the foundation for fusion research.

1.6 Bethe’s pp-Chain and CNO Cycle

In 1938, German-born physicist Hans Bethe, who had emigrated to America, elucidated the detailed mechanism of nuclear fusion reactions in stellar interiors. For this work, he was awarded the Nobel Prize in Physics in 1967.

Bethe identified two main reaction pathways.

The pp-chain (proton-proton chain reaction) is the dominant reaction in relatively low-temperature stars like the Sun.

$$p + p \rightarrow D + e^+ + \nu_e + 0.42 \text{ MeV}$$ $$D + p \rightarrow ^3\text{He} + \gamma + 5.49 \text{ MeV}$$ $$^3\text{He} + ^3\text{He} \rightarrow ^4\text{He} + 2p + 12.86 \text{ MeV}$$

The net reaction is:

$$4p \rightarrow ^4\text{He} + 2e^+ + 2\nu_e + 26.7 \text{ MeV}$$

The CNO cycle (carbon-nitrogen-oxygen cycle) becomes dominant in stars more massive than the Sun. Carbon, nitrogen, and oxygen act as catalysts, converting four protons into helium as a net result.

$$^{12}\text{C} + p \rightarrow ^{13}\text{N} + \gamma$$ $$^{13}\text{N} \rightarrow ^{13}\text{C} + e^+ + \nu_e$$ $$^{13}\text{C} + p \rightarrow ^{14}\text{N} + \gamma$$ $$^{14}\text{N} + p \rightarrow ^{15}\text{O} + \gamma$$ $$^{15}\text{O} \rightarrow ^{15}\text{N} + e^+ + \nu_e$$ $$^{15}\text{N} + p \rightarrow ^{12}\text{C} + ^4\text{He}$$

Bethe’s research revealed that achieving fusion on Earth would require temperatures far higher than those at the Sun’s core. The Sun, with its core temperature of about 15 million degrees, sustains fusion through gravitational confinement from its enormous mass and time scales of billions of years. In terrestrial experimental devices, where confinement times are on the order of seconds, temperatures must be raised above 100 million degrees to increase the reaction rate.

Chapter 2: The Era of Secret Research (1940s-1958)

2.1 Fusion Weapon Development

During World War II, while the Manhattan Project developed fission bombs, military applications of fusion also began to be considered. In 1942, Enrico Fermi discussed the possibility of a fusion bomb with Edward Teller at the University of Chicago.

Teller had a strong interest in the idea of a fusion bomb called “Super,” which created tension with Manhattan Project leader Robert Oppenheimer. While Oppenheimer prioritized completing the fission bomb, Teller continued to be absorbed in fusion weapon research.

In 1951, Stanislaw Ulam and Edward Teller devised a staged nuclear explosion design (Teller-Ulam design). This used X-ray radiation from a fission bomb (primary) to compress and heat lithium deuteride, triggering a fusion reaction.

On November 1, 1952, the “Ivy Mike” test was conducted at Enewetak Atoll, achieving the first thermonuclear fusion in human history. The explosive yield reached 10.4 megatons (TNT equivalent), about 500 times the Hiroshima bomb. The device used liquid deuterium and weighed 62 tons as a massive installation.

The Castle Bravo test on March 1, 1954, succeeded with a practical design using solid lithium deuteride.

$$^6\text{Li} + n \rightarrow ^4\text{He} + T + 4.8 \text{ MeV}$$

The tritium produced in this reaction fuses with deuterium, releasing additional neutrons in a chain reaction.

2.2 The Beginning of Controlled Fusion Research

From the late 1940s to early 1950s, controlled fusion research began independently in the United States, Soviet Union, and Britain. All were treated as military secrets, and each country was unaware of the others’ research.

Using magnetic fields to confine fusion plasma appeared promising. Charged particles spiral along magnetic field lines and have difficulty moving perpendicular to them. The Larmor radius is expressed as:

$$r_L = \frac{m v_\perp}{|q| B}$$

For a 10 keV deuterium ion in a 5 Tesla magnetic field, $r_L \approx 2$ mm, allowing plasma to be confined in a narrow region.

2.3 Soviet Research: Invention of the Tokamak

In the Soviet Union, Igor Kurchatov initiated controlled fusion research in 1950. At the Kurchatov Institute of Atomic Energy (now Kurchatov Institute), research was led by Igor Tamm and his student Andrei Sakharov.

In 1950, Sakharov conceived a method of confining plasma in a toroidal (doughnut-shaped) vacuum vessel, combining toroidal magnetic fields with fields generated by plasma current. Tamm refined this concept, and by 1952, the basic design of the device known as the “tokamak” was complete.

The name tokamak comes from the Russian acronym “toroidal’naya kamera s magnitnymi katushkami” (toroidal chamber with magnetic coils).

The tokamak magnetic field configuration is expressed as a superposition of toroidal field $B_\phi$ and poloidal field $B_\theta$.

$$\mathbf{B} = B_\phi \hat{\phi} + B_\theta \hat{\theta}$$

The safety factor (q-value) is an important parameter representing the twist of magnetic field lines.

$$q = \frac{r B_\phi}{R B_\theta}$$

For plasma stability, q > 2-3 must be maintained at the edge.

Early tokamak devices included T-1 (1957), T-2 (1958), and T-3 (1962), with size and performance improving progressively.

Sakharov later became a dissident opposing hydrogen bomb development and was awarded the Nobel Peace Prize in 1975. Tamm received the Nobel Prize in Physics in 1958, but this was for his theoretical explanation of Cherenkov radiation.

2.4 American Research: Stellarators and Pinches

In the United States, Princeton University astrophysicist Lyman Spitzer conceived the stellarator in 1951. Legend has it that Spitzer came up with his idea during a ski trip after hearing a (later proven false) report that Argentine physicist Ronald Richter had achieved fusion.

Unlike the tokamak, the stellarator does not drive plasma current, creating complex magnetic field configurations using only external coils. Early stellarators had a “figure-8” shape, which cancelled drift caused by the difference in toroidal field strength between inner and outer regions.

$$v_{\nabla B} = \frac{v_\perp^2 + 2v_\parallel^2}{2\omega_c B} \mathbf{B} \times \nabla B$$

To counteract this ”$\nabla B$ drift” and “curvature drift,” rotational transform (twist) is applied to the magnetic field lines.

In 1951, Spitzer submitted a proposal to the Atomic Energy Commission (AEC), and secret research began as “Project Matterhorn.” The Princeton Plasma Physics Laboratory (PPPL) was established, and stellarators including Model A (1953) and Model B (1954-57) were built.

At the same time, Los Alamos National Laboratory conducted research on pinch devices as “Project Sherwood.” Pinch devices pass large currents through plasma, compressing it with the magnetic field created by the current.

$$j \times B = \nabla p$$

This force balance (pinch effect) compresses the plasma toward the center.

However, pinch devices suffered from various MHD instabilities such as sausage instabilities and kink instabilities.

2.5 British Research: The Challenge and Setback of ZETA

In Britain, the toroidal pinch device “ZETA (Zero Energy Thermonuclear Assembly)” was built at the Harwell laboratory. It was a large device for its time, with a major radius of 3 m and minor radius of 0.5 m, with construction beginning in 1954 and operation starting in 1957.

In January 1958, the British government announced that ZETA had achieved a fusion reaction. Neutron detection was reported, attracting worldwide attention as a possible achievement of controlled fusion. However, detailed analysis several months later revealed that the detected neutrons were not from thermonuclear fusion but from accelerated ions colliding with vessel walls due to plasma instabilities.

This false report became a major lesson in fusion research history. The importance of distinguishing “beam-target reactions” from instabilities versus true “thermonuclear reactions” was recognized, and subsequent experiments used more rigorous diagnostic methods such as detailed neutron spectrum analysis.

ZETA contributed to neutron measurement techniques and provided much knowledge about high-temperature plasma physics. It also became the source of new confinement concepts such as the discovery of the “reversed field pinch” (RFP).

2.6 International Conferences and Declassification

In August 1955, the “International Conference on the Peaceful Uses of Atomic Energy” (First Geneva Conference) was held in Geneva, Switzerland. Following President Eisenhower’s “Atoms for Peace” speech (1953), the main topic was peaceful use of fission reactors, but informal discussions about fusion research also took place.

Researchers from various countries were surprised to learn that they were working on similar problems. Soviet scientist Kurchatov visited the Harwell laboratory in 1956 and gave lectures about Soviet research. This was a groundbreaking event during the Cold War.

At the Second Geneva Conference in September 1958, fusion research was officially declassified. Spitzer presented on stellarators, Soviet researchers on tokamaks, and British researchers on ZETA, beginning international information exchange.

In Japan the same year, the “Fusion Discussion Group” was established with Dr. Hideki Yukawa as chairman. This organization set the direction for Japanese fusion research and later developed into the “Japan Society of Plasma Science and Nuclear Fusion Research.” In 1961, the Institute of Plasma Physics (predecessor of the current National Institute for Fusion Science) was established at Nagoya University.

Chapter 3: The Triumph of the Tokamak (1960s-1980s)

3.1 The Novosibirsk Conference and T-3 Success

In the early 1960s, plasma physics research in various countries faced numerous difficulties. “Anomalous transport,” where energy was lost at rates much faster than theoretical predictions due to plasma instabilities, was a major problem.

The situation was transformed at the 3rd IAEA Nuclear Fusion Energy Conference held in Novosibirsk, Soviet Union, in August 1968. The group led by Lev Artsimovich at the Kurchatov Institute announced that the T-3 tokamak had achieved electron temperatures of 10 million degrees (about 1 keV) and energy confinement times of several milliseconds.

This was more than 10 times the performance of other devices at the time, and Western researchers were skeptical. Particularly, the reliability of temperature measurement methods was questioned.

To verify this, a team from the UK Culham Laboratory visited Moscow in 1969 and independently measured electron temperature using the then-cutting-edge laser Thomson scattering method. The principle of Thomson scattering is that incident laser light is scattered by electrons in the plasma, and the electron temperature is determined from its Doppler broadening.

$$\frac{\Delta\lambda}{\lambda} = \frac{v_{th}}{c} = \sqrt{\frac{2k_B T_e}{m_e c^2}}$$

The measurement results completely confirmed the Soviet announcement. Electron temperatures exceeded 1 keV, and energy confinement times were as reported. This independent verification internationally established the tokamak’s superiority.

The key to the tokamak’s success was suppressing MHD instabilities by maintaining appropriate safety factor q. T-3 achieved q approximately 3-4 at the edge, realizing stable plasma confinement.

3.2 Worldwide Tokamak Construction Race

Following T-3’s success, construction of large tokamaks began worldwide in the 1970s.

At Princeton University, the stellarator research that had been the main focus was scaled back, and a transition to tokamaks occurred. The Princeton Large Torus (PLT) began operation in 1975. PLT was the first to seriously use neutral beam injection (NBI) heating, achieving plasma temperatures of 60 million degrees (about 5 keV) in 1978.

$$P_{NBI} = \frac{1}{2} m_i v_{beam}^2 \cdot I_{beam}$$

By injecting more than 2 MW of deuterium beams accelerated to 80 keV, they succeeded in creating unprecedented high-temperature plasmas.

3.3 The Era of Large Tokamaks

The 1980s became an era of competition among three large tokamaks: TFTR, JET, and JT-60.

TFTR began operation at Princeton, USA, in 1982. With a major radius of 2.4 m, minor radius of 0.8 m, maximum plasma current of 2.5 MA, and NBI heating power of 40 MW, it was one of the largest devices of its time. TFTR’s main purpose was fusion experiments using D-T fuel.

In 1994, TFTR began D-T experiments and achieved 10.7 MW fusion power in December 1994. This was the first megawatt-class fusion power in controlled fusion history. It ultimately reached 11 MW fusion power in 1997, but the Q-value (fusion power/input power) remained at about 0.27.

$$Q = \frac{P_{fusion}}{P_{input}}$$

JET began operation at Culham near Oxford, UK, in 1983. As a joint project of the European Community (EC), it was the world’s largest tokamak, with a major radius of 2.96 m and minor radius of 2.1 m x 1.25 m (D-shaped cross-section).

JET’s distinguishing feature was adopting the world’s first D-shaped plasma cross-section. This enabled higher plasma current (up to 7 MA) and stability. Additionally, divertor configuration improved impurity control at the plasma edge.

In 1997, JET achieved 16 MW fusion power in D-T experiments, breaking TFTR’s record. The Q-value was approximately 0.65. This power was pulsed, lasting about 1 second.

JT-60 began operation at Naka, Ibaraki Prefecture, Japan, in 1985. Built by the Japan Atomic Energy Research Institute (now the National Institutes for Quantum Science and Technology), it was Japan’s largest tokamak, with a major radius of 3.4 m, minor radius of 1.1 m, and maximum plasma current of 5 MA.

JT-60 operated with only deuterium (DD), but achieved world records in the fusion triple product.

$$n \tau_E T_i = 1.5 \times 10^{21} \text{ keV s m}^{-3}$$

This corresponded to “equivalent breakeven conditions” that would exceed the critical condition if D-T fuel were used (Q approximately 1.25 equivalent).

In 1996, it underwent a major upgrade (JT-60U), achieving high triangularity cross-section and fully non-inductive current drive. It achieved numerous world-first results, including long-pulse operation exceeding 28 seconds and advanced plasma modes with negative magnetic shear configuration.

3.4 Discovery of H-mode

In 1982, Friedrich Wagner and colleagues made a groundbreaking discovery at the ASDEX (Axially Symmetric Divertor Experiment) tokamak in Garching, Germany. They discovered that when heating power exceeded a certain threshold, the plasma suddenly transitioned to high-confinement mode (H-mode).

In H-mode, a “transport barrier” with steep pressure gradient forms at the plasma edge, improving energy confinement time to about twice that of normal mode (L-mode: Low-confinement mode).

$$\tau_E^{H} \approx 2 \tau_E^{L}$$

The threshold power required for H-mode transition depends on plasma density and magnetic field strength.

$$P_{th} \propto n^{0.75} B^{0.8} S$$

where S is the plasma surface area.

The discovery of H-mode had a major impact on fusion reactor design. ITER’s design assumes H-mode operation. Wagner was awarded the IAEA Nuclear Fusion Award in 2008 for this discovery.

3.5 Energy Confinement Time Scaling Laws

By integrating experimental data from many tokamaks, scaling laws for energy confinement time were established. The IPB98(y,2) scaling for H-mode is:

$$\tau_E = 0.0562 \cdot I_p^{0.93} B_T^{0.15} P^{-0.69} n^{0.41} M^{0.19} R^{1.97} \epsilon^{0.58} \kappa^{0.78}$$

where $I_p$ is plasma current (MA), $B_T$ is toroidal field (T), $P$ is heating power (MW), $n$ is electron density (10^19 m^-3), $M$ is mass number, $R$ is major radius (m), $\epsilon = a/R$ is inverse aspect ratio, and $\kappa$ is elongation.

This scaling law became an essential tool for predicting performance and designing next-generation devices. ITER’s design is also based on this scaling law.

Chapter 4: The Era of International Cooperation (Late 1980s-Present)

4.1 The INTOR Project

In the late 1970s, the “International Tokamak Reactor” (INTOR) project began under the International Atomic Energy Agency (IAEA). From 1978 to 1987, Japan, Europe, the United States, and the Soviet Union participated in conceptual design of a fusion experimental reactor.

INTOR was designed as a superconducting tokamak with major radius 5.3 m and plasma current 8 MA. It targeted 620 MW thermal output with Q = 2-5. Although INTOR was never built, its design activities became the foundation for the subsequent ITER project.

Japan contributed the “Fusion Experimental Reactor (FER)” concept, which made significant contributions to INTOR design. Through this activity, an international network of fusion engineers and scientists was established.

4.2 Birth of ITER

In November 1985, a summit meeting between Soviet General Secretary Gorbachev and US President Reagan was held in Geneva. As a symbol of Cold War detente, joint development of an international fusion experimental reactor was proposed.

In 1988, Conceptual Design Activities (CDA) for ITER (International Thermonuclear Experimental Reactor, later reinterpreted as the Latin word for “way”) began. Japan, the European Community, the United States, and the Soviet Union (later Russia) participated.

Engineering Design Activities (EDA) were conducted from 1992 to 2001. The original design (ITER-FDR) was enormous, with major radius 8.1 m and 1,500 MW thermal output, but the design was scaled down in 2001 for cost reduction. The current design has major radius 6.2 m and 500 MW thermal output.

ITER main parameters:

Parameter	Value
Major radius R	6.2 m
Minor radius a	2.0 m
Plasma current $I_p$	15 MA
Toroidal field $B_T$	5.3 T
Fusion power	500 MW
Q-value	>= 10
Pulse duration	400-3000 s
Plasma volume	830 m^3

ITER’s goal is to achieve Q >= 10 (50 MW or less heating input for 500 MW fusion output) and demonstrate “burning plasma” physics.

$$Q = \frac{P_{fusion}}{P_{heating}} \geq 10$$

A burning plasma is a state where self-heating by alpha particles (helium nuclei) exceeds external heating.

$$P_\alpha = \frac{1}{5} P_{fusion} = 100 \text{ MW} > P_{heating} = 50 \text{ MW}$$

4.3 Site Selection and Organization Establishment

ITER construction site selection was contentious. In 2003, four candidate sites were proposed (Rokkasho, Japan; Cadarache, France; Clarington, Canada; Vandelles, Spain), eventually becoming a final competition between Japan and France.

In June 2005, at a ministerial meeting in Moscow, Cadarache, France was selected as the host country. Japan would handle complementary activities through the “Broader Approach.”

In November 2006, seven parties (European Union, Japan, United States, Russia, China, South Korea, and India) signed the ITER Agreement in Paris. The ITER Organization was officially established in October 2007.

ITER construction costs are estimated at approximately 20 billion euros, shared among participating parties as follows:

Participant	Share
European Union	~45%
Japan, United States, Russia, China, South Korea, India	~9% each

Contributions are made through combinations of cash and in-kind (component manufacturing). For example, Japan is responsible for toroidal field coils, neutral beam heating systems, and blanket remote maintenance equipment.

4.4 The Broader Approach

Following the ITER site selection, Japan and the EU signed the “Broader Approach” agreement in February 2007. Three activities complementing ITER were initiated.

JT-60SA is an advanced superconducting tokamak built at the Naka Research Institute in Japan. As a successor to JT-60, it was jointly designed and built by Japan and Europe. With major radius 2.96 m, minor radius 1.18 m, and plasma current 5.5 MA, it is responsible for developing ITER operation scenarios and advanced plasma research. It achieved first plasma in October 2023.

IFMIF (International Fusion Materials Irradiation Facility) is an intense neutron source for testing structural materials for fusion reactors. To simulate 14 MeV fusion neutrons, deuteron beams are irradiated onto a lithium target.

$$D + Li \rightarrow n + X$$

Currently, construction is planned as IFMIF/DONES (DEMO Oriented Neutron Source) led by Europe.

IFERC is the International Fusion Energy Research Centre established at Rokkasho Village, Ibaraki Prefecture. It conducts fusion computational science, DEMO design activities, and remote experimentation activities.

4.5 ITER Construction Progress

ITER construction began in earnest in the 2010s.

In July 2020, tokamak assembly began in the tokamak building.

In September 2021, the first module of the central solenoid magnet was installed. The central solenoid is 18 m tall, weighs 1,000 tons, and generates a maximum field of 13 Tesla. This induces the plasma current.

Eighteen toroidal field coils (19 including spares) were manufactured, each 17 m tall, 9 m wide, and weighing 360 tons. Using Nb3Sn superconducting wire, they are cooled to 4.5 K.

As of 2025, construction is delayed from the original schedule, but manufacturing and installation of major components continues. First plasma is scheduled for the early 2030s, and D-T operation for the late 2030s.

4.6 Continued Helical/Stellarator Research

Even after tokamaks became mainstream, stellarator/helical research continued. This approach does not drive plasma current, so in principle it enables steady-state operation and eliminates the risk of disruption (sudden collapse of plasma current).

At the Max Planck Institute for Plasma Physics in Germany, following achievements at the optimized stellarator W7-AS (1988-2002), W7-X (Wendelstein 7-X) was built. W7-X is the world’s most advanced stellarator with major radius 5.5 m and plasma volume 30 m^3, achieving first plasma in 2015.

W7-X’s magnetic field configuration was designed through numerical optimization. To reduce neoclassical transport, a quasi-isodynamic configuration is employed.

$$\oint \frac{d\ell}{B} = \text{const (on each flux surface)}$$

In Japan, the Large Helical Device (LHD) at the National Institute for Fusion Science has been operating since 1998. With major radius 3.9 m and plasma volume 30 m^3, it is the world’s largest helical device.

LHD uses superconducting helical coils, achieving long-duration plasma operation. It has achieved ion temperatures of 100 million degrees (about 10 keV) and leads physics research for the helical approach.

Chapter 5: Inertial Confinement Fusion (ICF)

5.1 Principles of Laser Fusion

As a different approach from magnetic confinement, inertial confinement fusion (ICF) using lasers has also been researched.

In ICF, high-power lasers irradiate a target (fuel pellet) several millimeters in size, compressing and heating the center through the reaction of ablation (evaporation) of the outer shell.

$$\rho R > 0.3 \text{ g/cm}^2$$

When this condition is met, alpha particles remain in the fuel, and “ignition” occurs through self-heating.

Confinement time is extremely short (nanosecond order), and fuel is held only by inertia (mass), hence the name “inertial confinement.”

5.2 NIF (National Ignition Facility)

The National Ignition Facility (NIF) built at Lawrence Livermore National Laboratory in the United States is the world’s largest laser fusion facility. 192 laser beams concentrate a total of 1.9 MJ (500 TW) of energy on the target.

NIF’s main purpose is stockpile stewardship research (maintaining nuclear warhead reliability without nuclear testing), but it is also used for civilian fusion research.

In the indirect drive approach, the target pellet is placed in a metal cylinder called a “hohlraum,” and lasers irradiate the hohlraum inner wall. The generated X-rays uniformly compress the pellet.

$$\eta_{coupling} = \frac{E_{absorbed}}{E_{laser}} \times \frac{E_{compression}}{E_{absorbed}}$$

5.3 Achievement of Fusion Ignition

On December 5, 2022, NIF achieved fusion ignition for the first time in human history.

For 2.05 MJ of laser energy input, 3.15 MJ of fusion energy was released.

$$G = \frac{E_{fusion}}{E_{laser}} = \frac{3.15 \text{ MJ}}{2.05 \text{ MJ}} = 1.54$$

This is called “scientific ignition,” meaning fusion energy exceeding laser energy was released. However, considering the laser system efficiency (about 1%), output exceeding input power has not yet been achieved.

In 2023, multiple experiments reproduced ignition conditions, with a maximum of 3.88 MJ of fusion energy obtained. These achievements mark a historic milestone demonstrating the scientific feasibility of fusion.

5.4 Challenges for Laser Fusion Power Generation

Many technical challenges remain for practical laser fusion power plants.

High repetition rate is needed. NIF experiments are conducted at about one shot per day, but power plants would require about 10 target shots per second.

High-efficiency lasers are needed. Current laser efficiency is 1-2%, but practical application requires 10% or more. Diode-pumped solid-state lasers (DPSSL) are being developed.

Target manufacturing cost reduction is also needed. Current targets cost several hundred thousand yen each, but must be reduced to several yen per target.

Chapter 6: Rise of Private Fusion Ventures (2010s-Present)

6.1 Transformation Through New Technology

Since the late 2010s, private sector entry into fusion development has accelerated rapidly. A key technological factor driving this movement is advances in high-temperature superconducting (HTS) magnet technology.

Conventional Nb3Sn superconductors must be cooled to 4.5 K (-269 C), but REBCO (rare earth barium copper oxide) tape maintains superconductivity above 20 K and can generate higher magnetic fields.

$$B_{max}^{REBCO} > 20 \text{ T} \quad vs. \quad B_{max}^{Nb_3Sn} \approx 13 \text{ T}$$

Stronger magnetic fields enable the same confinement performance in smaller devices. Fusion power scales with the fourth power of magnetic field.

$$P_{fusion} \propto B^4$$

This opened the possibility of fusion reactors smaller than ITER that could be built more quickly and at lower cost.

6.2 Major Private Companies

Commonwealth Fusion Systems (CFS) spun out of MIT (Massachusetts Institute of Technology) in 2018. They are developing a compact tokamak “SPARC” using REBCO high-temperature superconducting magnets.

SPARC has major radius 1.85 m, field 12 T, targeting Q > 2. Operation was scheduled to begin in 2025 but has been delayed. CFS has raised over $2 billion in funding to date.

TAE Technologies (formerly Tri Alpha Energy), founded in 2008, has the longest history among fusion startups. They employ a unique approach using field-reversed configuration (FRC), ultimately targeting the proton-boron-11 (p-B11) reaction.

$$p + ^{11}\text{B} \rightarrow 3 \, ^4\text{He} + 8.7 \text{ MeV}$$

The p-B11 reaction produces almost no neutrons, reducing activation problems, but requires reaction temperatures above 3 billion degrees, presenting high technical hurdles.

Helion Energy, founded in 2013, is developing a pulsed fusion approach. FRC plasmas are collided from both sides to compress and induce fusion reactions. They target commercial power generation by 2028 and have signed a power purchase agreement with Microsoft.

General Fusion, a Canadian company founded in 2002, is developing magnetized target fusion (MTF). A vortex of liquid lead-lithium is mechanically compressed to heat plasma.

Kyoto Fusioneering, a Japanese company founded in 2019, develops peripheral equipment for fusion reactors (gyrotrons, blankets, plasma heating systems, etc.). Their business model supplies equipment to overseas fusion companies with global expansion.

6.3 Investment Surge

As of 2025, cumulative investment in fusion startups worldwide exceeds $9.8 billion. Major investors include Bill Gates, Jeff Bezos, Google Ventures, and Breakthrough Energy Ventures.

Behind the investment surge are several factors.

As climate change countermeasures, demand for carbon-neutral energy sources is increasing.

Technology maturation in high-temperature superconducting magnets, AI/machine learning plasma control, and advanced materials has progressed.

Policy support from various governments promoting fusion development has been enacted.

Chapter 7: National Fusion Strategies

7.1 United States

The United States pursues both magnetic confinement (Princeton, GA) and inertial confinement (NIF, NRL) research. The 2022 Department of Energy (DOE) Fusion Energy Sciences Advisory Committee (FESAC) report set a goal of a fusion pilot plant in the 2040s.

The Fusion Energy Act passed in 2024 established fusion under a separate regulatory framework from fission. This has lowered barriers to private company entry.

7.2 Europe

Europe leads fusion development as ITER’s host party. EUROfusion (European Consortium for the Development of Fusion Energy) handles JET and its successor DEMO.

JET ended operation at the end of 2024, closing over 40 years of history. The final D-T experimental campaign (2021) achieved 59 MJ of fusion energy, a record expected to stand until ITER.

DEMO (DEMOnstration Power Plant) is a prototype reactor that will actually generate electricity as the next stage after ITER. Design is progressing toward operation in the 2050s.

7.3 China

China is rapidly expanding fusion research. EAST (Experimental Advanced Superconducting Tokamak, Hefei) has broken world records for long-duration plasma operation. In 2023, it achieved 403 seconds of H-mode plasma.

China is an ITER participant and is also advancing the domestic CFETR (China Fusion Engineering Test Reactor) program. CFETR is a large tokamak targeting operation around 2035, with ITER-level performance as its goal.

7.4 Japan

Japan has consistently promoted fusion research since the establishment of the Fusion Discussion Group in 1958.

The National Institutes for Quantum Science and Technology (QST) operates JT-60SA at the Naka Fusion Research Institute. First plasma was achieved in October 2023, and full-scale operation began in 2024.

The National Institute for Fusion Science (NIFS) in Toki City, Gifu Prefecture operates LHD and conducts helical research.

In April 2023, the government formulated the “Fusion Energy Innovation Strategy.” This strategy aims for practical fusion power generation in the 2050s, accelerating development through public-private partnerships. Specifically, it calls for developing a prototype reactor, building supply chains, cultivating human resources, and strengthening international cooperation.

Chapter 8: Lessons from 100 Years of Fusion Research

8.1 Optimism and Reality

The history of fusion research is also a history of gaps between optimistic predictions and harsh reality. In the 1950s, it was said fusion would be “practical in 20 years,” and 70 years later, practical application has not yet been achieved.

However, the achievements during this period have been enormous.

Plasma temperatures have improved 500-fold, from 1 million degrees in the 1960s to over 500 million degrees today.

The fusion triple product has improved more than a million-fold since the 1970s.

$$n \tau_E T_i: \quad 10^{15} \text{ (1970s)} \rightarrow 10^{21} \text{ (current)} \text{ keV s m}^{-3}$$

In 2022, fusion ignition was achieved at NIF, demonstrating scientific feasibility.

8.2 Importance of Basic Research

Fusion research has greatly advanced basic science centered on plasma physics.

MHD theory (magnetohydrodynamics) developed to describe fusion plasma equilibrium and stability and has also been applied to solar physics and astrophysics.

Turbulent transport theory developed to understand energy and particle transport in plasmas, contributing to fluid mechanics in general.

Large-scale computational physics saw demand for plasma simulations that promoted supercomputer development.

8.3 A Model for International Cooperation

ITER is the largest international science project in human history, with participation from 7 parties (more than 35 countries). Beginning as a symbol of Cold War detente, it continues to function as a model for international cooperation.

Fusion is a challenge of a scale that no country can achieve alone, making international cooperation essential. The experience in multilateral cooperation cultivated through ITER will also be applied to future science projects.

8.4 Future Outlook

From the late 2020s through the 2030s, fusion research will enter a new phase.

ITER is scheduled for first plasma in the early 2030s and D-T operation in the late 2030s. Achieving Q >= 10 will demonstrate the technical feasibility of fusion power generation.

Private company devices will begin operation one after another, and verification of new approaches will progress.

Prototype reactor designs such as DEMO will become concrete, clarifying the path toward practical application in the 2050s.

It has been exactly 100 years since Eddington proposed fusion as the energy source of stars in the 1920s. Humanity is closer than ever to realizing “the Sun on Earth.”

Related Topics

• ITER Project - International Fusion Experimental Reactor
• JT-60SA Project - Japan-Europe Advanced Tokamak
• LHD Project - World’s Largest Helical Device
• NIF Project - US National Ignition Facility
• Tokamak Approach - Mainstream Magnetic Confinement
• Stellarator/Helical Approach - Alternative Magnetic Confinement
• Private Fusion Ventures - Private Sector Initiatives

References

1. L. Spitzer Jr., “The Stellarator Concept,” Physics of Fluids 1, 253 (1958)
2. L.A. Artsimovich, “Tokamak devices,” Nuclear Fusion 12, 215 (1972)
3. H. Bethe, “Energy Production in Stars,” Physical Review 55, 434 (1939)
4. ITER Physics Basis Editors, “ITER Physics Basis,” Nuclear Fusion 39, 2137 (1999)
5. F. Wagner et al., “Regime of Improved Confinement and High Beta in Neutral-Beam-Heated Divertor Discharges of the ASDEX Tokamak,” Physical Review Letters 49, 1408 (1982)
6. A.B. Zylstra et al., “Burning plasma achieved in inertial fusion,” Nature 601, 542 (2022)
7. H.A. Abu-Shawareb et al., “Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment,” Physical Review Letters 129, 075001 (2022)