ITER

ITER (International Thermonuclear Experimental Reactor) is the world’s largest device being built to experimentally demonstrate that fusion can be a net source of energy. It is under construction at Saint-Paul-lez-Durance in southern France, through a seven-party international collaboration. On this page, we first get an intuitive grasp of what kind of machine ITER is meant to be, and then look in turn at its physics, its engineering, and the research frontier.

The Machine in One Sentence (High School)

In a single sentence, ITER is “an experimental reactor for lighting a real fire and checking whether we can generate electricity from fusion.”

Fusion is a reaction that releases a large amount of energy when light atomic nuclei stick together to form a heavier nucleus. The Sun shines because fusion is happening at its core. To make the same thing happen on the ground, we need to heat the fuel, the hydrogen family (deuterium and tritium), to the tremendous temperature of about 100 million degrees, and make the nuclei collide with one another.

But how on earth do you put something this hot into a container? The answer is a “magnetic cage.” Matter at 100 million degrees becomes plasma, an electrically charged gas, and charged particles move as if coiling around magnetic field lines. So we build a cage out of powerful doughnut-shaped magnets and float the plasma inside it, keeping it from touching the walls. This doughnut-shaped device is called a tokamak. ITER is a giant tokamak.

Let us describe ITER’s main goal with a cooking analogy. In fusion experiments so far, the gas bill for lighting the fire has been larger than the heat of the dish that came out. What ITER aims for is to start heating a plasma with a 50 MW electric heater and extract ten times that, 500 MW, of fusion heat from it. Ten times the energy you put in comes back: we write this ratio as $Q$ , and ITER’s goal is $Q \geq 10$ .

Another important goal is to create a “fire that keeps burning on its own.” The particles born in fusion (alpha particles) themselves heat the plasma from the inside, so the hot state is maintained without relying much on external heating. This state is called a burning plasma. It is the same picture as a campfire: it may borrow help from a match at first, but once the fire spreads through the wood, it keeps burning on its own. ITER’s scientific aim is to create such a burning plasma in earnest for the first time in human history and to study its behavior in detail.

The Physics and Engineering of the Machine (Undergraduate to Graduate)

Target Performance and Size

ITER’s central goal is to achieve a fusion gain of $Q \geq 10$ . $Q$ is the ratio of fusion output to heating input, defined as

Q = \frac{P_{\mathrm{fus}}}{P_{\mathrm{heat}}}

Here $P_{\mathrm{fus}}$ is the fusion output and $P_{\mathrm{heat}}$ is the externally injected heating power. ITER aims to obtain 500 MW of fusion output from 50 MW of heating, which corresponds exactly to $Q = 10$ . The scientific goal is to maintain this state through several hundred seconds of operation and to demonstrate the physics of a burning plasma.

To achieve this performance, ITER is a very large machine. Its main parameters are as follows.

Item	Value
Plasma major radius $R$	6.2 m
Plasma current $I_p$	15 MA
Fusion output	500 MW
Heating input	50 MW
Target fusion gain $Q$	10 or more

The major radius $R$ is the distance from the central axis of the doughnut to the center of the plasma cross section. The size $R = 6.2$ m was chosen because the larger the machine, the farther the center of the plasma is from the walls, and the harder it is for heat to escape.

Why Size Matters

The performance of a fusion reactor is determined by how well the plasma can retain heat. The measure of this is the energy confinement time $\tau_E$ , which represents roughly how long it takes for the stored heat energy to be lost. The condition for fusion to become self-sustaining through self-heating can be summarized in the form of the Lawson criterion,

n T \tau_E > \text{a certain fixed value}

Here $n$ is the plasma density and $T$ is the temperature. The larger this product of three quantities (the triple product), the more favorable the fusion reactor.

Making the machine larger tends to lengthen $\tau_E$ , and this is the reason ITER was made large. An empirical rule (a scaling law) for confinement performance was built from data on existing mid-sized tokamaks, and from it engineers estimated that “achieving $Q \geq 10$ requires a machine roughly this size.” ITER is also an experiment to test this scaling law at an unprecedented size and in the regime of a burning plasma.

The Role of Plasma Current and Magnetic Fields

In a tokamak, the toroidal magnetic field created by external coils (the field pointing around the doughnut) alone cannot confine the plasma. By driving a current $I_p$ through the plasma itself and superimposing the poloidal magnetic field created by that current (the field circling around the doughnut’s cross section), the magnetic field lines become twisted helices, and only then does stable confinement come about. ITER’s plasma current of 15 MA is the foundation that supports this confinement. The larger the current, the better the confinement, but at the same time the loads during a disruption, in which the current is suddenly lost, also grow larger, so controlling this becomes an important challenge.

Toroidal Field Coils

The leading player in confining the plasma is the toroidal field coil (TF coil). D-shaped coils are arranged so as to surround the doughnut, creating a strong magnetic field on the inside. ITER’s TF coils consist of 18 units and use superconducting wire of niobium-tin (Nb $_3$ Sn). Making them superconducting eliminates heat generation from resistance and allows large currents to flow continuously for long periods. For this reason the coils are cooled to about 4 K, close to absolute zero.

Enormous electromagnetic forces act on the coils because of the strong magnetic field and large current. The TF coils as a whole are pushed inward against one another, so a structure that holds them firmly in place mechanically is indispensable. Reconciling the strong field from superconductivity with a structure that can withstand it is the heart of TF coil engineering.

Heating, Vacuum Vessel, and Divertor

To heat the plasma to the 100-million-degree class, driving a current is not enough. External heating methods such as neutral beam injection and radio-frequency heating (electron and ion cyclotron heating) are combined to inject a total of several tens of MW. In ITER’s design, $Q$ is defined based on 50 MW of external heating.

The vacuum vessel that holds the plasma is a doughnut-shaped stainless-steel structure that maintains a high vacuum inside. The part on the inner side of the vessel that faces the plasma is the first wall, which absorbs the high heat and particle loads. Below the vessel is the divertor, which deliberately guides the magnetic field lines to single-handedly take on both heat exhaust from the plasma and the removal of impurities. The divertor is the part of the machine exposed to the most severe heat loads, and its surface uses tungsten, which has a high melting point and does not readily retain hydrogen.

Test Blanket Modules

Another important role of ITER is testing the technology to produce the fuel tritium on its own. Tritium hardly exists in nature, so a fusion reactor must produce it by itself. The method is tritium breeding, in which the neutrons flying out of fusion are made to strike lithium to generate tritium.

At ITER, candidate designs for the blanket that carries out this breeding are tested in an actual reactor environment by installing test blanket modules (TBMs). A TBM is attached to an opening in part of the vacuum vessel and directly measures how much tritium is produced when it receives neutrons and how the heat can be extracted. The knowledge obtained here is indispensable for the next generation of reactors that will generate electricity. There is no other device that can test a breeding blanket in a fusion neutron environment; this is an experiment unique to ITER.

The New Baseline (Announced in 2024)

The ITER Organization announced a new project baseline in 2024. Listing the key points that are known for certain, there are two.

One is that the schedule for the start of operation was revised. The staged approach to the originally planned first plasma was reworked, and the policy was changed to bring up the machine in a form closer to completion before entering full-scale operation.

The other is that the material of the first wall was fully changed to tungsten. Under the previous baseline, the plan was to use beryllium for the first wall in the initial phase, but under the new baseline the policy became to standardize on tungsten, the same as the divertor. Tungsten has a high melting point and does not readily retain the fuel hydrogen, and it is also considered a strong candidate for the wall material of future power reactors. This change is in line with the intent to run experiments under conditions closer to those of a power reactor.

Research Frontier (PhD)

ITER is not merely a large machine but an experimental platform for producing answers to a number of unsolved problems. Here we introduce research themes that also appear frequently in the literature.

Burning plasma physics is ITER’s central theme. How a plasma behaves when self-heating by alpha particles becomes dominant has not been verified in earnest at laboratory scale. The interaction of waves and turbulence driven by alpha particles, and the self-organization of heating profiles, are among the topics being studied.

Confinement degradation and improvement is also a continuing theme. The question is how to control the high-confinement mode (H-mode), in which the pressure rises steeply at the edge of the plasma, and the edge localized mode (ELM), the periodic burst phenomenon that accompanies it. In an ITER-class machine, the instantaneous heat loads from ELMs risk damaging the walls, so methods to mitigate them are being studied.

The prediction and avoidance of disruptions is also important. If a large current of 15 MA is suddenly lost, large electromagnetic forces and heat loads are applied to the machine, and high-energy electrons called runaway electrons can be generated. Early-warning detection using machine learning and safe shutdown methods using mitigation gas injection are being actively studied.

The problem of power exhaust is also on the frontier. With a fully tungsten divertor, the challenges are how far the steady-state heat flux can be lowered and how to suppress the impurity problem in which tungsten mixes into the plasma and cools it through radiation.

This research is directly connected to the design of the next stage, the prototype reactor DEMO (/future/demo/). The knowledge demonstrated at ITER, of burning plasma physics, superconducting magnets, remote maintenance, and tritium breeding via TBMs, becomes the foundation that supports the realization of a power reactor.

Check Your Understanding

Q1. ITER's target fusion gain Q of 10 or more represents what kind of state?

Q2. Why is a plasma current needed to create stable confinement in a tokamak?

Q3. What is a test blanket module (TBM) a device for testing?

Q4. In ITER's new baseline announced in 2024, how did the first wall material change?

The basics of the confinement scheme ITER adopts: Tokamak
Technologies that support the machine, such as superconducting magnets and the first wall: Reactor Engineering
The power-demonstration reactor beyond ITER: Prototype Reactor DEMO