A tokamak uses powerful magnetic fields to hold extremely hot, electrically charged gas—plasma—in a doughnut-shaped chamber long enough for some atomic nuclei to fuse. In the deuterium–tritium reaction commonly used in fusion research, the fusion products carry energy away in different ways: charged helium nuclei can help heat the plasma, while uncharged neutrons escape the magnetic field. Understanding that energy flow also explains why plasma gain is not the same as a power plant producing net electricity.
What is a tokamak?
A tokamak is an experimental magnetic-confinement machine. Its central idea is to create a hot plasma and use magnetic fields to keep most of it from striking the walls of a doughnut-shaped vacuum chamber. ITER, the international experimental tokamak, describes its machine as a way to test fusion operation and reactor-scale technologies; it is not equipped to generate electricity.
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The magnetic field does not make the particles still or trap every one perfectly. Rather, it guides the motion of charged particles and reduces their movement across the field, helping retain the hot plasma for a useful period.
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How a tokamak creates and confines plasma
1. Prepare the chamber and introduce fuel
The process begins in a toroidal, or doughnut-shaped, vacuum vessel. Air and other impurities are removed, then a small amount of fuel gas is introduced. Electricity and the machine’s magnetic system ionize the gas: electrons separate from atomic nuclei, leaving a mixture of charged particles called plasma. Because those particles carry electric charge, magnetic fields can influence their motion.
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2. Form a helical magnetic field
External magnetic coils produce field components around the torus, while an electric current driven through the plasma contributes another component. Together, the toroidal and poloidal fields form a helical field that helps shape the plasma and limit its contact with the chamber walls. ITER’s educational material describes this field geometry as the basis of tokamak confinement.
3. Heat the plasma
Fusion requires nuclei to collide with enough energy to overcome their electrical repulsion. A tokamak therefore heats its plasma using the current and additional heating systems. ITER describes a target plasma temperature of about 150 million °C for its machine, and auxiliary heating in the range of 150–300 million °C in its explanation of fusion conditions. These are ITER-specific descriptions, not a single universal temperature threshold for every fusion design.
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Why temperature alone is not enough
Successful laboratory fusion depends on a combination of three conditions:
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- Density: there must be enough nuclei in the plasma for collisions to occur frequently.
- Energy-confinement time: the plasma must retain its energy long enough for fusion reactions to happen before it cools or escapes.
ITER’s engineering handbook gives a deuterium–tritium triple-product criterion greater than 3 × 1021 keV·s·m−3 in the 10–20 keV temperature range. The triple product combines plasma density, temperature, and energy-confinement time; the stated threshold applies to that reaction and temperature range, not as a stand-alone temperature target.
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What happens in a deuterium–tritium reaction?
Deuterium and tritium are hydrogen isotopes. When their nuclei fuse, they produce a helium nucleus—also called an alpha particle—and a neutron. ITER identifies this as the most achievable and efficient fusion reaction for laboratory experiments.
- The alpha particle is positively charged, so the magnetic field can confine it. Its energy helps heat the plasma, contributing to self-heating.
- The neutron has no electric charge, so the magnetic field does not confine it. ITER says approximately 80% of the reaction energy is carried away by the neutron.
That neutron energy is a potential heat source for a future power plant. In the familiar proposed conversion route, surrounding structures absorb the energy as heat, that heat makes steam, and the steam drives a turbine-generator. Building such a system requires more than producing fusion reactions: it also depends on handling the heat and developing the technologies and fuel systems a plant would need.
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What do ITER’s fusion-power targets mean?
ITER’s stated design target is about 500 MW of fusion power in the plasma from 50 MW of external plasma-heating input, corresponding to a plasma gain ratio, Q, of 10. This is a target, not an achieved operating result. It compares fusion power with the heating power supplied to the plasma; it does not mean ITER would deliver ten times as much electricity as the entire facility consumes.
Three different questions should be kept separate when interpreting fusion claims:
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| Measure | What it compares or describes | What it does not establish |
|---|---|---|
| Plasma gain (Q) | Fusion power produced in the plasma relative to external power used to heat it. | Net energy for the whole facility or electricity sent to a grid. |
| Whole-facility energy balance | The energy consumed by the complete experimental or power-plant system against its useful energy output. | A result that can be inferred from a plasma Q target alone. |
| Grid electricity | Electric power delivered after conversion and after a facility accounts for its own needs. | A capability of ITER: its tokamak is not equipped to produce electricity. |
ITER’s FAQ also reports a historical magnetic-confinement gain record of Q=0.67 for Europe’s JET tokamak in the 1990s. That is a historical figure attributed to ITER, not a claim here that it is an independently verified current record.
What remains between a tokamak experiment and a power plant?
Beyond reaching a plasma-gain target, a future plant would have to integrate technologies for sustained operation, heat extraction, materials and component protection, and fuel supply. In a deuterium–tritium system, tritium breeding modules are a concept to test; tritium self-sufficiency remains a future requirement, not something ITER has demonstrated as a closed commercial fuel cycle.
The plasma also generates exhaust and impurities that must be managed. ITER’s divertor is designed to handle waste gas and impurities while withstanding the machine’s highest surface heat loads. These are demanding engineering functions, distinct from the question of whether nuclei fuse in the plasma.
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Sources and further reading
- ITER Organization: What is a tokamak?
- ITER Organization: Making fusion happen
- ITER Organization: Fusion fuel and the deuterium–tritium reaction
- ITER Organization: Fusion basics and energy gain
- ITER Organization: Educational introduction to magnetic confinement
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