Tokamak
A tokamak (; Russian: токамáк) is a machine which uses a powerful magnetic field generated by external magnets to confine plasma in the shape of an axially symmetrical torus. The tokamak is the leading candidate of magnetic confinement fusion designs being developed to produce controlled thermonuclear fusion power.
Tokamaks use a combination of a central solenoid and toroidal and poloidal magnets to shape a ring of plasma. This is heated by a range of methods, including neutral-beam injection, electron and ion cyclotron resonance, lower hybrid resonance. Nuclear fusion may be achieved, measured by neutron detectors. Due to requiring a continuously changing magnetic field, modern tokamaks sustain "plasma discharges" on the timescales of seconds or minutes.
The world's largest tokamak by radius and plasma current is the JT-60SA in Japan. Other operational experiments include WEST in France and EAST in China. The International Thermonuclear Experimental Reactor (ITER) tokamak is the primary global effort to research fusion power, under construction in France, and aimed for completion by 2034. Many smaller designs, and offshoots like the spherical tokamak, continue to be used to investigate performance parameters and other issues. Commercial fusion companies have also proposed tokamak construction in recent years, including Commonwealth Fusion Systems' SPARC design.
Tokamaks were first conceptualized by Soviet physicists Andrei Sakharov and Igor Tamm. Experiments were constructed from 1951 at Kurchatov Institute in Moscow led by Lev Artsimovich. Their 1958 T-1 device is sometimes considered the first tokamak. The same year, the USSR, US, and UK began sharing fusion research.
A "tokamak stampede" occurred after 1968, when British Culham Laboratory scientists verified high performance results of the Kurchatov Institute's T-3 tokamak. It had been demonstrated that a stable plasma equilibrium requires magnetic field lines that wind around the torus in a helix shape. The z-pinch and stellarator concepts of magnetic confinement had attempted this, but demonstrated serious instabilities. The safety factor (labelled q in mathematical notation) guided tokamak development: tokamaks with q > 1 strongly suppressed these instabilities.
By the mid-1970s, dozens of tokamaks were in use around the world. The Princeton Plasma Physics Laboratory (PPPL) in the US became another center of research, beginning with the 1975 Princeton Large Torus. In the late 1970s, the Kurchatov Institute tested the first superconducting magnets and divertors in tokamaks, ubiquitous in modern large designs.
The Tokamak Fusion Test Reactor (TFTR),
and the Joint European Torus (JET)
were the first to run experiments with an admixture of rare tritium to deuterium, previous reactors demonstrated fusion in a deuterium plasma.
Both reactors developed understanding of alpha particles' role in heating fusion plasma. JET also researched the high-confinement mode, and set magnetic confinement fusion power records standing as of 2025, for energy gain factor (Q = 0.67), total energy output (69 MJ), and fusion power (16 MW).
These machines demonstrated new problems that limited their performance. Solving these would require a much larger and more expensive machine, beyond the abilities of any one country. After an initial agreement between Ronald Reagan and Mikhail Gorbachev in November 1985, the International Thermonuclear Experimental Reactor (ITER) project emerged; construction of the complex began in Cadarache, France in 2013, and tokamak assembly began in 2020.
The Egyptian
- 2001-01-01T00:00:00.000000Z
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