History of Fusion Research

The history of fusion research began with a simple question: “Why does the Sun keep shining?” The journey to find the answer widens into the birth of quantum mechanics, classified Cold War research, and international collaboration that crosses national borders. On this page, we follow more than a century of fusion research through the ages in an approachable way. Knowing this history makes it clear just how far we have come today. The principle of fusion itself is explained in What Is Fusion.

Solving the Mystery of the Sun (High School)

At the end of the 19th century, scientists faced a great mystery. The Sun has been putting out an enormous amount of heat and light for billions of years. If the Sun were merely burning like coal, the calculations showed it would burn out in a few thousand years. The Sun’s energy source could not be explained by the science of the time.

The person who shed light on this mystery was the British astronomer Arthur Eddington. In 1920, he proposed that “in the core of the Sun, hydrogen is being turned into helium, and the energy produced in the process makes the Sun shine.” When four hydrogen nuclei fuse into a single helium nucleus, the mass becomes just a tiny bit lighter. The idea is that this lost mass turns into an enormous amount of energy, following Einstein’s $E = mc^2$ . This is precisely fusion, and we live thanks to the Sun, a gigantic fusion reactor.

From here, the dream was born of “doing on Earth the same thing the Sun does.” If we could extract energy from hydrogen, the fuel is contained almost inexhaustibly in the water of the sea. The challenge to make this dream come true is exactly the history of fusion research that we are about to tell.

From Theory to Experiment (Undergraduate)

Eddington’s proposal faced a major theoretical wall. Every atomic nucleus carries a positive charge, so when you try to bring them close together they strongly repel one another. This repulsion is called the electrical barrier (the Coulomb barrier). The core of the Sun is hot, at about 15 million degrees, but even that is not enough to overcome the Coulomb barrier by classical physics. Eddington’s theory was stuck for a while.

The breakthrough came from quantum mechanics. In 1928, George Gamow discovered quantum tunneling. This is a phenomenon in which a particle probabilistically slips through a barrier that it classically should not be able to cross. Thanks to tunneling, it could be explained that even at temperatures around the core of the Sun, atomic nuclei occasionally penetrate the barrier and fusion can occur.

Once the theory was in place, experiments followed. In 1932, John Cockcroft and Ernest Walton fired accelerated protons at lithium and succeeded for the first time in artificially transmuting atomic nuclei. Then, in 1939, Hans Bethe systematically worked out the pathways of fusion reactions inside stars. He clarified the proton-proton chain reaction (pp chain), which works mainly in light stars, and the CNO cycle, which works mainly in heavy stars. For this achievement, Bethe received the Nobel Prize in Physics in 1967. At this point, the mechanism by which the Sun shines was complete as a theory.

Classified Research and the Geneva Conference (Undergraduate to Graduate)

In the early 1950s, after World War II, the United States, the Soviet Union, and the United Kingdom each independently began research into controlled fusion. At the time, fusion was seen as close to military technology, and much of the research was classified. The nations walked separate paths without knowing one another’s results.

During this period, the prototypes of the two devices that would later become mainstream were born. In the Soviet Union, Andrei Sakharov and Igor Tamm devised the tokamak, which confines plasma with a doughnut-shaped magnetic field. In the United States, Lyman Spitzer invented the stellarator, which confines plasma by twisting the magnetic field lines. Both are magnetic confinement schemes that try to hold high-temperature plasma away from the walls of the vessel using the force of a magnetic field. Meanwhile, in the 1952 Ivy Mike test, uncontrolled fusion, that is, a hydrogen bomb, succeeded for the first time. The difficulty of extracting energy lay not in exploding it, but in sustaining it gently.

The turning point came in 1958. At the Second International Conference on the Peaceful Uses of Atomic Energy, held in Geneva, the nations made their fusion research results public. The veil of secrecy was lifted, and researchers could compare one another’s data for the first time. From here, fusion research stepped into an era of internationally open collaboration.

The Shock of the Tokamak and the Age of Large Devices (Graduate)

In the 1960s, many devices were tried, but plasma temperature and confinement time did not rise easily. The plasma was unstable, quickly becoming turbulent and letting heat escape. A sense of stagnation hung over the researchers.

What completely changed the mood was the Soviet tokamak T-3. In 1968, at an international conference held in Novosibirsk, the Soviet team reported that T-3 had achieved an electron temperature of about 10 million degrees, an order of magnitude beyond anything at the time. The result was so good that Western researchers at first did not believe it. So a team from the UK’s Culham Laboratory brought in a measurement apparatus based on laser scattering and carried out an independent verification. The results backed up the Soviet report. This verification led to the international recognition of the tokamak’s superiority, and research around the world made a major turn toward the tokamak.

After this, nations set out to build large tokamaks. In the 1980s, the United States’ TFTR, Europe’s JET (Joint European Torus), and Japan’s JT-60 competed in performance. On the confinement side, the discovery of the high-confinement mode (H-mode) at Germany’s ASDEX in 1982 was decisive. It is a phenomenon in which, once a certain heating condition is exceeded, a transport barrier forms at the plasma edge and confinement performance jumps up in steps, and it has become the foundation of present-day tokamak design.

Results also accumulated in actual fusion output. In 1994, the United States’ TFTR recorded fusion output in a D-T experiment using deuterium and tritium as fuel, and in 1997 JET achieved a fusion output of 16.1 MW. This JET record long remained the highest value of fusion output in the laboratory. Japan’s JT-60 did not use D-T fuel, but from experiments with deuterium plasma it demonstrated equivalent performance in which the energy gain factor (Q value, fusion gain), if converted to D-T equivalent, would exceed 1, and it greatly contributed to raising plasma performance. The knowledge from JT-60 has been passed on to the design of its successor, JT-60SA, and of ITER.

From International Collaboration to Ignition, and Then to Private Industry (PhD)

The success of the large devices made the next challenge clear: a device on a scale that works as an energy source becomes so huge that a single nation cannot bear the cost or the technology alone. Here the idea of international collaboration took on reality. Prompted by the 1985 summit between Gorbachev and Reagan, the ITER (International Thermonuclear Experimental Reactor) concept, in which nations join forces to build a single experimental reactor, got moving. After long negotiations, the ITER Organization was formally established in 2007, becoming one of the largest international scientific projects in human history, with seven parties participating: Japan, Europe, the United States, Russia, China, South Korea, and India. ITER aims to achieve a fusion output of 500 MW and $Q \geq 10$ , producing more than ten times the heating power put in. The details of the plan are explained in The ITER Project.

A path different from magnetic confinement also advanced. It is the inertial confinement scheme, in which a small fuel pellet is compressed and heated in an instant with powerful lasers. In December 2022, the United States’ National Ignition Facility (NIF) succeeded in obtaining 3.15 MJ of fusion energy from 2.05 MJ of laser energy put in. It was a historic moment in which humanity demonstrated, for the first time in the laboratory, ignition, in which the energy produced by fusion exceeds the energy poured into the fuel. The detailed workings of NIF are covered in NIF (National Ignition Facility).

Then, in the late 2010s, a new character joined the history of fusion: private companies. Backed by advances in enabling technologies, such as powerful magnets using high-temperature superconductors, private capital began flowing into fusion, which had until then been a state enterprise. In the United States, Commonwealth Fusion Systems, TAE Technologies, and Helion Energy, among others, are each aiming to realize a reactor with their own distinct scheme. In Japan, too, companies such as Kyoto Fusioneering are developing their business worldwide. This move toward private entry is introduced in detail in Private Fusion Ventures. The 100-year journey that began with the mystery of the Sun has now entered a new stage with practical application in sight.

Check Your Understanding

Q1. Who proposed in 1920 that the Sun's energy source is fusion?

Q2. Why can atomic nuclei overcome the Coulomb barrier and fuse even at the temperature of the Sun's core?

Q3. Which device, reported in 1968 and later backed up by an independent verification from a UK team, turned the world's research toward the tokamak?

Q4. About how many MW of fusion output did JET achieve in 1997?

Q5. What state does the ignition that NIF achieved in December 2022 refer to?