JT-60SA

JT-60SA is one of the world’s largest superconducting tokamak fusion experimental devices, located in Naka City, Ibaraki Prefecture, Japan. Built jointly by Japan and Europe, it achieved first plasma (the first ignition of a plasma) in October 2023. This page starts with a high-school-level picture of what the device is, then works through its physics and engineering, and finally its role in supporting ITER and developing the DEMO reactor.

What This Device Is, in One Sentence (High School)

Nuclear fusion is a reaction in which light atomic nuclei join together to form heavier nuclei, releasing a large amount of energy in the process. The Sun shines because fusion is happening at its core. To make the same thing happen on the ground, you have to heat the fuel to over 100 million degrees, creating a state in which atomic nuclei fly around freely, that is, a plasma.

The problem is that nothing this hot can be held in any container. The moment it touches the wall, the container melts. This is where magnetic fields come in. A plasma is a collection of electrically charged particles, so it can be confined by magnetic force (a magnetic field). A device that builds a doughnut-shaped magnetic cage to float the plasma and confine it without letting it touch the walls is called a tokamak. The workings of a tokamak are explained in detail on the tokamak page.

JT-60SA is one kind of tokamak. The “SA” in its name stands for Super Advanced, reflecting that it is an advanced device using superconductivity. Superconductivity is a phenomenon in which a metal’s electrical resistance drops to zero when it is cooled to a very low temperature. When resistance is zero, the wire does not heat up even when current flows through it continuously. Ordinary electromagnets heat up as current keeps flowing, so they cannot be used for long. By using superconducting electromagnets, JT-60SA can sustain a strong magnetic field for a long time.

Let’s get a sense of its size. JT-60SA is about 15 m tall and weighs about 2,600 tonnes in total. It is a huge device, several stories high. With this device, experiments can sustain a plasma for as long as 100 seconds. Realizing fusion power generation requires the technology to keep a plasma stable for a long time. It helps to think of JT-60SA as a “place for practice and research” toward that goal.

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

From here, we look a little more closely at the physics and engineering that make JT-60SA work.

Basic Device Parameters

The plasma in a tokamak has a doughnut shape. The distance from the center of the doughnut to the center of the plasma cross-section is called the major radius $R$, and the radius of the plasma cross-section is called the minor radius $a$. In JT-60SA, $R \approx 2.96$ m and $a \approx 1.18$ m. The ratio of the two, $A = R/a$, is called the aspect ratio, and for JT-60SA $A \approx 2.5$. This value is somewhat smaller than for a standard tokamak, giving it a fatter doughnut shape. A fatter shape has the advantage of making it easier to raise the plasma’s performance.

The current flowing through the plasma is called the plasma current $I_p$. In JT-60SA, $I_p$ reaches up to 5.5 MA (megaamperes). This current generates a magnetic field and helps confine the plasma itself. The magnetic field applied in the direction that circles the doughnut is called the toroidal field $B_t$, and in JT-60SA $B_t \approx 2.25$ T (teslas). Since the Earth’s magnetic field is about $5 \times 10^{-5}$ T, this is a strong field roughly 40,000 times as large.

Superconducting Magnets and Long-Pulse Operation

To sustain a strong magnetic field for a long time, JT-60SA uses superconducting electromagnets. The magnet system consists of 18 TF coils (toroidal field coils) that create the toroidal field, 6 EF coils (equilibrium field coils) that control the shape and position of the plasma, and a central solenoid of 4 modules that induces the plasma current. The TF coils draw on the design philosophy inherited from the predecessor JT-60U and were manufactured by Europe.

Let’s use a little math to see why superconductivity enables long-pulse operation. In an ordinary copper electromagnet, when a current $I$ flows through a conductor of resistance $Rn$, heat is generated per unit time equal to

$$P = Rn I^2$$

Here $P$ is the heat generated (in watts) and $Rn$ is the electrical resistance. Creating a strong magnetic field requires a large current, but since the heat generated increases in proportion to $I^2$, the coil overheats and operation must stop quickly. Because the predecessor JT-60U used copper coils, a single run was limited to about 65 seconds.

In a superconductor, $Rn = 0$, so in principle $P = 0$, meaning no heat is generated. By cooling superconducting wire (such as niobium-titanium, NbTi) to near liquid helium temperature, JT-60SA avoids this heating problem and enables long-pulse operation lasting 100 seconds. In fusion power generation, keeping the plasma steady for as long as possible is essential, so this long-pulse capability is decisively important.

Heating Systems

To heat the plasma to the temperature at which fusion occurs, powerful energy must be injected from outside. JT-60SA’s heating systems have a combined output of about 41 MW (megawatts). The centerpiece is neutral beam injection (NBI). This is a method that fires a beam of fast neutral atoms into the plasma, heating it through collisions. JT-60SA has a relatively low-energy positive-ion NBI and a high-energy negative-ion NBI at 500 keV. The higher the beam energy, the deeper into the plasma it reaches, all the way to the core, heating that region efficiently. This negative-ion NBI technology is an area of Japanese expertise honed since the JT-60U era.

In addition, electron cyclotron heating (ECH) is used. This is a method that shines microwaves onto the plasma and delivers energy resonantly, matched to the frequency at which electrons orbit the magnetic field. Because it can heat a precisely targeted location, it can also be used to shape the plasma and to suppress the instabilities described later.

Metrics for Measuring Plasma Performance

When discussing plasma performance, an index called beta $\beta$ is often used. It is the ratio of the plasma pressure to the magnetic pressure, written as

$$\beta = \frac{p}{B^2 / 2\mu_0}$$

Here $p$ is the plasma pressure, $B$ is the magnetic field strength, and $\mu_0$ is the permeability of vacuum. The larger $\beta$ is, the higher the pressure of plasma that can be confined with the same magnetic field, which means a higher density of fusion output, in other words, closer to an economical power reactor. However, raising $\beta$ too high makes the plasma unstable and prone to breaking down, so how high a $\beta$ can be kept stably is a research topic. Studying high-beta operation is one of JT-60SA’s main goals.

Research Frontier (PhD)

JT-60SA is not a device that aims to generate power on its own; it is designed to contribute to two major goals. One is supporting ITER, now under construction in France, and the other is cultivating the human resources and know-how needed for the prototype reactor DEMO beyond it.

Research on High-Beta Steady-State Operation

To make fusion power generation economically viable, high-performance plasma must be sustained steadily (without interruption) rather than in pulses. The plasma current in a tokamak is usually driven by induction from the central solenoid (the principle of a transformer), but this method limits how long the current can be maintained. This is where non-inductive current drive becomes key.

An important component of non-inductive current is the bootstrap current. This is a current that flows spontaneously when there is a pressure gradient in the plasma, and it arises without injecting energy from outside. In a high-beta state the fraction of bootstrap current is higher, so high-beta operation and steady-state operation are closely linked. By combining bootstrap current with current drive from NBI and ECH, JT-60SA is conducting research aimed at fully non-inductive operation on the 100-second scale while maintaining a high normalized beta $\beta_N$ of 4 or more.

Research Supporting ITER

Because JT-60SA’s plasma is about half the size of ITER’s and has high physical similarity, results obtained here can be directly applied to ITER. There are several representative topics worth investigating in advance. One is the transition conditions to the H-mode (high-confinement mode). The H-mode is an operating state in which the plasma’s confinement suddenly improves once a certain heating power is exceeded, and it is assumed to be the standard operating mode for ITER and power reactors. Determining under what conditions the H-mode is reached is an important challenge.

Another is the control of edge localized modes (ELMs). At the edge of an H-mode plasma, an instability occurs in which pressure is expelled periodically, and this damages the device’s walls. How to keep these ELMs as small as possible is unavoidable for operating a power reactor over long periods. Furthermore, predicting and mitigating disruptions, in which the plasma suddenly collapses, is also a major research topic. Because a disruption, in which a high-current plasma vanishes abruptly, exerts large forces on the device, technology to catch its precursors and reduce the damage is needed.

A Bridge to DEMO

This research is being carried out under a cooperative framework called the Broader Approach (BA) between Japan and Europe. The BA is a cooperative activity that Japan and Europe, as ITER participants, agreed on to complement ITER and accelerate the realization of the prototype reactor DEMO, when the ITER construction site was decided to be in France in 2005. JT-60SA is its core device, and in addition to the results from the device itself, it also serves as a place to train the next generation of fusion researchers and engineers. As a bridge connecting ITER and DEMO, the knowledge JT-60SA accumulates will become the foundation for realizing fusion power generation.

Check Your Understanding

Q1. Why does JT-60SA use superconducting electromagnets, compared with ordinary copper electromagnets?

A Because superconductors can create a strong magnetic field only for an instant, more strongly than copper B Because their electrical resistance is zero and they generate no heat, allowing a strong magnetic field to be sustained for a long time on the 100-second scale C Because superconductivity eliminates the need for cooling and simplifies the device D Because copper does not transmit magnetic fields, so confinement is impossible without superconductivity

Q2. Why is a high beta value desirable for fusion power generation?

A Because the higher beta is, the more stable the plasma becomes and the less it breaks down B Because the higher beta is, the weaker the required magnetic field can be C Because it can hold a higher-pressure plasma with the same magnetic field, raising the density of fusion output and getting closer to an economical power reactor D Because the higher beta is, the less energy is needed for heating

Q3. Why is JT-60SA suitable as a device supporting ITER?

A Because its plasma is about half the size of ITER's with high physical similarity, so research results can be directly applied to ITER B Because JT-60SA is built in France, the same as ITER C Because JT-60SA is larger than ITER and can test more extreme conditions D Because JT-60SA uses the same D-T reaction as ITER and even generates power

Q4. Why is the bootstrap current important for achieving steady-state operation?

A Because it is the only current that can heat the plasma without an external power source B Because it is a non-inductive current that flows spontaneously due to a pressure gradient, and its fraction is higher in a high-beta state, making it the key to combining high-beta operation with steady-state operation C Because it strengthens the inductive current of the central solenoid and extends the operating time D Because it completely prevents disruptions

Related Topics

• Tokamak (Principles of the Tokamak): Learn in detail how the tokamak scheme adopted by JT-60SA works.
• ITER: The world’s largest fusion experimental reactor, which JT-60SA supports through its research.
• DEMO (Prototype Reactor): The next-generation reactor that will demonstrate power generation, which JT-60SA aims to bridge toward in terms of human resource development and know-how.