Tokamak facts for kids
A tokamak is a special machine that uses strong magnets to hold super-hot gas, called plasma, in a donut shape. Scientists are building tokamaks to try and create fusion power. This is the same energy process that powers the Sun! As of 2016, tokamaks were the top choice for building a real fusion reactor.
Soviet scientists Igor Tamm and Andrei Sakharov first thought of tokamaks in the 1950s. The first working tokamak, called T-1, was built in 1958 by Natan Yavlinsky. Early fusion machines had problems keeping the plasma stable. Tokamaks solved this by making the magnetic fields twist in a special way, which helped stop the plasma from breaking apart.
By the mid-1960s, tokamaks started working much better. In 1968, Soviet scientists announced amazing results. At first, people didn't believe them. So, the Soviets invited scientists from the United Kingdom to check. In 1969, the British team confirmed the results. This led to many countries building their own tokamaks.
By the late 1970s, tokamaks had achieved all the conditions needed for fusion, but not all at once in one machine. New, larger machines like the Joint European Torus (JET) and Tokamak Fusion Test Reactor (TFTR) were built. Their goal was to reach "breakeven," where the fusion energy produced equals the energy put in.
However, these machines found new problems. Solving them would need a much bigger and more expensive machine. In 1985, Ronald Reagan and Mikhail Gorbachev agreed to work together. This led to the International Thermonuclear Experimental Reactor (ITER) project. ITER is now the main international effort to develop practical fusion power. As of 2020, JET still holds the record for fusion output.
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What Does "Tokamak" Mean?
The word tokamak comes from a Russian word. It's a short way of saying:
- toroidal chamber with magnetic coils;
- OR toroidal chamber with axial magnetic field.
The term was created in 1957 by Igor Golovin.
How Tokamaks Were Developed
Early Fusion Experiments
In 1934, scientists first made fusion happen on Earth. They shot tiny particles at a metal foil containing deuterium atoms. This showed that the deuterium–deuterium reaction needed less energy than other fusion reactions.
To get useful energy from fusion, the fuel needs to be super hot. This is why it's called "thermonuclear" fusion. In 1944, Enrico Fermi figured out that fusion could keep itself going at about 50 million degrees Celsius. At this temperature, the reactions make enough heat to keep the fuel hot.
The Idea of Magnetic Confinement
When gas gets hot enough for fusion, its electrons break away from the atoms. This creates a charged gas called plasma. Plasma can be controlled by electric or magnetic fields.
Scientists realized they could use magnetic fields to hold the plasma. If you make a magnetic field in a circle, like a donut (called a torus), the particles will circle endlessly. But there was a problem: the magnetic field is stronger on the inside of the donut than on the outside. This makes the particles drift away and hit the walls.
A solution was to twist the magnetic field lines, like stripes on a barber pole. This way, a particle would drift one way on the outside and the opposite way on the inside. Over time, these drifts would cancel out, keeping the plasma inside.
The First Tokamaks
Soviet scientists decided to build a large donut-shaped machine based on these ideas. They found that if the magnetic field lines twisted enough, the plasma would be much more stable. This idea became known as the "safety factor" (called q). If q was greater than 1, the plasma would be stable.
The T-1 reactor was designed with stronger external magnets and a smaller internal current to meet this q > 1 rule. T-1 was a success and is known as the first real tokamak.
The "Doldrums" and New Discoveries
In the late 1950s, many fusion experiments around the world faced problems. Plasma was escaping much faster than expected. This led to a period of doubt called "the doldrums."
However, tokamaks kept improving. By the mid-1960s, they started showing signs of beating the plasma escape problem. At a conference in 1968, Soviet scientists announced that their T-3 tokamak was reaching electron temperatures of 10 million degrees Celsius! This was 10 times better than any other machine.
The Culham Five
Many scientists doubted the Soviet results. So, the Soviets invited a British team, nicknamed "The Culham Five," to check their measurements using a new laser technique. In 1969, the British team confirmed that the Soviet results were correct. This news caused a "stampede" of tokamak building around the world.
Heating the Plasma
A challenge for tokamaks was heating the plasma to even higher temperatures. Scientists developed new methods:
- Magnetic compression: Squeezing the plasma with stronger magnets to make it hotter.
- Neutral-beam injection: Shooting high-energy neutral atoms into the plasma. These atoms collide with the plasma particles, heating them up.
- Radio-frequency heating: Using high-frequency waves (like microwaves) to transfer energy to the plasma.
The Princeton Large Torus (PLT) in the US was a huge success. In 1978, it reached 60 million degrees Celsius. This was a key step, as fusion reactions need temperatures between 50 and 100 million degrees Celsius to keep going.
The Race and New Problems
By the late 1970s, tokamaks had achieved all the necessary conditions for a fusion reactor. The goal was to combine these successes into one machine that could run on deuterium and tritium fuel.
Four major new tokamaks were planned: T-15 (Soviet), Joint European Torus (JET, European), JT-60 (Japan), and Tokamak Fusion Test Reactor (TFTR, US). TFTR started in 1982, followed by JET in 1983.
However, these new machines showed new problems. New instabilities appeared, and the plasma sometimes hit the reactor walls. Even when working perfectly, the plasma couldn't be held long enough for a practical reactor.
The ITER Project
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Solving these new problems would require an even bigger and more expensive machine. In 1985, Ronald Reagan and Mikhail Gorbachev agreed to work together on fusion. The next year, the US, Soviet Union, European Union, and Japan created the International Thermonuclear Experimental Reactor (ITER) organization.
ITER is the main tokamak design effort worldwide. Construction began in 2010, and it's expected to start full operation around 2035. ITER is designed to show that a practical fusion reactor is possible.
How Tokamaks Are Designed
The Basic Idea
Fusion plasma is super hot, with particles moving very fast. To keep fusion going, these particles must stay in the center. Magnetic fields make charged particles spiral along the field lines, keeping them from hitting the walls.
The simplest magnetic field is in a straight tube. But particles can still escape the ends. Bending the tube into a donut (torus) solves this, but the field is uneven. This causes particles to drift away.
The solution is to twist the magnetic field lines. This way, particles drift one way on the outside and the opposite way on the inside, canceling out the drift.
The Tokamak Solution
The tokamak is similar to earlier designs but with a key improvement: it controls plasma instabilities. The main idea is to make the magnetic fields very "twisty." This means particles orbit around the short way of the donut more often than they orbit the long way. This ratio is the "safety factor" (q). Tokamaks work with q much greater than 1, which makes them very stable.
Other Important Features
- D-shaped plasma: Early tokamaks had circular plasma. Later, scientists found that a D-shaped plasma worked better. This shape helps keep the plasma stable and reduces forces on the reactor. Most modern tokamaks, like JET and ITER, use this D-shape.
- Divertor: Hot plasma can pick up heavier elements from the reactor walls, which cool the plasma. A divertor is like a "cleaner" that uses magnetic fields to remove these heavier elements from the plasma.
- Limiter: This is a small ring of light metal that sticks into the chamber. If the plasma expands, it hits the limiter first, protecting the main walls. Lighter elements from the limiter cause less cooling than heavier wall materials.
- H-mode: Scientists discovered that under certain conditions, tokamaks can enter a "high-confinement mode" (H-mode). In this mode, the plasma is held more tightly, allowing for higher temperatures and pressures.
- Bootstrap current: Plasma can create some of its own electrical current. This "bootstrap current" helps twist the magnetic field lines without needing as much power from outside. Modern tokamaks try to use as much bootstrap current as possible.
Breakeven and Ignition
A big goal for fusion devices is to reach "breakeven." This is when the energy produced by fusion equals the energy used to keep the reaction going. The ratio of output to input energy is called Q. Breakeven means Q = 1. For a power plant, Q needs to be much higher.
If Q keeps increasing, the fusion reactions can start heating themselves. This is because some of the energy from fusion comes from alpha particles. These particles can collide with the fuel and heat it. When this self-heating is enough to keep the reaction going without any outside help, it's called "ignition." This would mean an infinite Q!
Heating the Plasma
Ohmic Heating
Plasma conducts electricity. So, you can heat it by sending an electric current through it. This is like how an electric heater works. This "ohmic heating" helps create the magnetic field that holds the plasma.
However, as plasma gets hotter, its resistance goes down. This means ohmic heating becomes less effective. It can only heat plasma up to about 20–30 million degrees Celsius. To get hotter, other heating methods are needed.
Neutral-Beam Injection
This method shoots high-energy neutral atoms into the plasma. These atoms become charged and get trapped inside the tokamak. As they collide with the plasma, they transfer their energy, heating it up. Neutral-beam injection can heat the plasma to very high temperatures.
Radio-Frequency Heating
This method uses high-frequency electromagnetic waves, like microwaves, generated outside the tokamak. If the waves have the right frequency, they can transfer their energy to the charged particles in the plasma, making them hotter.
Inside the Tokamak
The super-hot plasma inside a tokamak eventually touches the inner walls of the chamber. These walls are water-cooled to remove the heat.
Fusion reactions produce high-energy neutrons. Neutrons are not affected by magnetic fields and can pass through the chamber walls. So, tokamaks are surrounded by a special "neutron shield." This shield is made of materials like water, plastics, or concrete mixed with boron, which absorb the neutrons and their energy.
In a future fusion power plant, these neutrons would be absorbed by a liquid metal blanket. Their energy would then be used to heat water, create steam, and turn a generator to make electricity.
Tokamaks Around the World
There are many tokamaks operating or being built around the world.
Currently Operating Tokamaks
- Golem (Czech Republic) - operating since the 1960s
- T-10 (Russia) - operating since 1975
- Joint European Torus (JET) (United Kingdom) - operating since 1983
- DIII-D (United States) - operating since 1986
- STOR-M (Canada) - operating since 1987
- WEST (France) - operating since 1988 (formerly Tore Supra)
- Aditya (India) - operating since 1989
- COMPASS (Czech Republic) - operating since 1989
- FTU (Italy) - operating since 1990
- ISTTOK (Portugal) - operating since 1991
- ASDEX Upgrade (Germany) - operating since 1991
- H-1NF (Australia) - operating since 1992
- Tokamak à configuration variable (TCV) (Switzerland) - operating since 1992
- HBT-EP Tokamak (United States) - operating since 1993
- TCABR (Brazil) - operating since 1994
- HT-7 (China) - operating since 1995
- Pegasus Toroidal Experiment (United States) - operating since 1996
- NSTX (United States) - operating since 1999
- Globus-M (Russia) - operating since 1999
- ETE (Brazil) - operating since 2000
- HL-2A (China) - operating since 2002
- EAST (China) - operating since 2006
- KSTAR (South Korea) - operating since 2008
- JT-60SA (Japan) - operating since 2010 (upgraded from JT-60)
- Medusa CR (Costa Rica) - operating since 2012
- SST-1 (India) - operating since 2012
- IR-T1 (Iran) - operating since 2012
- ST25-HTS (United Kingdom) - operating since 2015
- KTM (Kazakhstan) - operating since 2017
- ST40 (United Kingdom) - operating since 2018
- HL-2M (China) - operating since 2020
- MAST Upgrade (United Kingdom) - operating since 2020
Previously Operated Tokamaks
- T-3 and T-4 (Russia) - 1960s
- LT-1 (Australia) - 1963
- Symmetric Tokamak (United States) - 1970
- Texas Turbulent Tokamak (United States) - 1971–1980
- Adiabatic Toroidal Compressor (United States) - 1972
- Tokamak de Fontenay aux Roses (France) - 1973–1976
- Alcator A (United States) - 1973–1979
- Princeton Large Torus (United States) - 1975
- Alcator C (United States) - 1978–1987
- TEXTOR (Germany) - 1978–2013
- MT-1 Tokamak (Hungary) - 1979–1998
- Tokoloshe Tokamak (South Africa) - 1980–1990
- TEXT/TEXT-U (United States) - 1980–2004
- TFTR (United States) - 1982–1997
- Novillo Tokamak (Mexico) - 1983–2000
- HL-1 Tokamak (China) - 1984–1992
- JT-60 (Japan) - 1985–2010
- Tokamak de Varennes (Canada) - 1987–1999
- T-15 (Russia) - 1988–2005
- START (United Kingdom) - 1991–1998
- COMPASS (United Kingdom) - 1990s–2001
- HL-1M Tokamak (China) - 1994–2001
- UCLA Electric Tokamak (United States) - 1999–2006
- MAST (United Kingdom) - 1999–2014
- Alcator C-Mod (United States) - 1992–2016
Planned Tokamaks
- ITER (France) - construction began in 2010, first plasma expected in 2025.
- DEMO - planned to follow ITER, aiming for continuous power generation.
- CFETR (China) - a new Chinese fusion reactor.
- K-DEMO (South Korea) - aiming for construction by 2037.
See also
In Spanish: Tokamak para niños
