Russian T-15 tokamak on postage stamp.

In 1950, while serving in the military on Sakhalin, the young Soviet scientist Oleg Alexandrovich Lavrentiev sent a proposal to the Central Committee of the CPSU outlining ideas for both a thermonuclear weapon using lithium deuteride and a controlled-fusion device based on electrostatic confinement. The fusion proposal attracted the attention of Andrei Sakharov.

Sakharov then developed the idea further with Igor Tamm, and by late 1950 they had formulated the basic concept of a magnetic thermonuclear reactor. Their proposal used a toroidal chamber with a strong magnetic field and an induced plasma current to improve confinement. In May 1951, the Soviet government formally approved a programme of research on magnetic thermonuclear reactors.

Argentina’s 1951 announcement of supposed controlled fusion at Huemul further accelerated Soviet work on magnetic confinement. Igor Golovin is generally credited with proposing the name “tokamak”, derived from the Russian expression “toroidal chamber with magnetic coils”.

Achievements of the First Tokamaks

Interior of the JET tokamak.

The earliest experimental tokamaks were small devices. One predecessor used a porcelain chamber, while the T-1, generally regarded as the first tokamak, employed a stainless-steel liner inside a copper vacuum chamber and began operation around 1958. Further development led to the T-3, which achieved electron temperatures of about 10 million K and confinement times of around 10–20 milliseconds — substantially better performance than contemporary magnetic-confinement devices.

Because the Soviet temperature measurements were indirect, a team from Culham Laboratory in the United Kingdom travelled to the Kurchatov Institute with newly developed laser Thomson-scattering diagnostics. Their measurements confirmed electron temperatures of around 1 keV, or roughly 10 million K, and the results were reported in 1969. The confirmation triggered a rapid worldwide shift towards tokamak research.

Tokamak Boom

“Planet ITER” — the ITER construction site viewed through a fisheye lens.

Tokamak construction expanded rapidly around the world. Examples included CLEO in the United Kingdom, TO-2 in the Soviet Union, JFT-2 in Japan and TFR in France. At Princeton, the Model C stellarator was converted into the ST (Symmetric Tokamak) after the T-3 results were confirmed; the first ST experiments began in 1970. Tokamaks also became important test beds for auxiliary heating methods such as radio-frequency heating and neutral beam injection. Divertors, previously explored on stellarators, were also adopted to control plasma–wall interaction and remove impurities and exhaust particles from the plasma edge.

Attempts to Ignite D–T Fusion

Although tokamaks had not reached scientific breakeven, experiments showed that larger devices generally offered improved confinement and provided access to more reactor-relevant plasma conditions. The next major step was to operate with deuterium–tritium fuel, which introduced new challenges because fusion neutrons activate components and require extensive shielding, remote handling and tritium systems.

Three large tokamaks came to dominate the next stage of research: TFTR in the United States, which began operation in 1982; JET in Europe, which achieved first plasma in 1983; and JT-60 in Japan, which began operation in 1985. JET became the first experiment to produce controlled fusion power from deuterium–tritium fuel in 1991, while TFTR became the first magnetic fusion device to operate extensively with a 50:50 deuterium–tritium mixture in 1993.

No Scientific Breakeven

TFTR and JET were designed to approach or reach scientific breakeven, although neither ultimately achieved Q = 1. JET came closest in 1997, reaching a peak fusion gain of about Q = 0.67, meaning that the fusion power was approximately 67 percent of the external power used to heat the plasma. This remains the highest directly measured fusion gain achieved by a tokamak.

Larger tokamaks also revealed new operational challenges. One of them was the formation of high-energy runaway electrons, particularly during plasma disruptions. These relativistic electrons can form intense beams capable of damaging plasma-facing components and became an important subject of tokamak safety and control research.

H-mode

Experiments had shown that increasing auxiliary heating could degrade energy confinement, largely because of turbulent transport. A major breakthrough came in 1982 on the German ASDEX tokamak, where researchers unexpectedly observed a new operating regime with much better confinement. An edge transport barrier formed, reducing turbulent particle and energy losses from the plasma. The new regime became known as H-mode (High-Confinement Mode), while the conventional regime was termed L-mode (Low-Confinement Mode).

Dangerous ELMs

The improved confinement of H-mode also introduced a new challenge: Edge Localised Modes (ELMs), repetitive instabilities at the plasma edge that produce short bursts of heat and particles towards the divertor and other plasma-facing components. ELMs can assist impurity transport, but large ELMs may cause damaging transient heat loads. Fusion experiments have therefore explored ELM mitigation and suppression techniques, as well as naturally ELM-free or small-ELM regimes such as the Quiescent H-mode demonstrated on DIII-D. Techniques such as resonant magnetic perturbations and pellet pacing can reduce ELM size or suppress large ELMs, while alternative plasma regimes aim to avoid damaging ELM activity altogether.

ITER

By the late twentieth century, tokamak research had advanced far enough for an international experiment on a much larger scale to be proposed. ITER, now under construction in France by seven international Members, is designed to test burning-plasma physics and many of the technologies required for future fusion power plants. Under the current baseline, Start of Research Operation is planned for 2034, full magnetic energy for 2036 and the beginning of deuterium–tritium operation for 2039.