JT-60SA

Overview

The JT-60SA (Japan Torus-60 Super Advanced) is a large, fully superconducting tokamak located at the Naka Fusion Institute in Japan. It is a joint international research project conducted by Japan and Europe under the Broader Approach (BA) Agreement, a satellite accord to the main ITER project. The primary mission of JT-60SA is to support the operation of ITER and to explore advanced plasma physics regimes necessary for the design of a demonstration fusion power plant (DEMO).

As a satellite facility, JT-60SA is designed to address key physics and engineering questions in long-pulse, high-performance scenarios. Its capabilities, including a highly shaped plasma cross-section, powerful heating and current drive systems, and superconducting magnets, allow for sustained operation at high plasma pressure. This enables the study of plasma-wall interactions, heat exhaust management with an advanced divertor, and the development of steady-state operational scenarios that approach the conditions required for net energy gain.

Physics / Mechanism

JT-60SA is an advanced magnetic confinement device based on the tokamak concept. Its design focuses on achieving and sustaining high-beta plasmas—the ratio of plasma pressure to magnetic pressure—in a configuration optimized for stability and confinement.

Magnet System: The core of the device is its fully superconducting magnet system, which enables long-pulse operation. The system consists of 18 Toroidal Field (TF) coils, a Central Solenoid (CS) with four modules, and six Equilibrium Field (EF) coils. All magnets utilize Niobium-titanium (NbTi) conductors cooled to 4.4 K by a large-scale helium refrigeration plant. The TF coils generate a magnetic field of 2.25 T at the plasma center, while the powerful CS and EF coils provide the inductive flux for plasma initiation and precise control over the plasma's shape and position. This high degree of shaping capability, with an elongation (κ) up to 1.9 and a triangularity (δ) up to 0.5, is critical for accessing high-confinement modes (H-modes) and improving plasma stability.

Heating and Current Drive: To heat the plasma to fusion-relevant temperatures and drive non-inductive current for steady-state operation, JT-60SA is equipped with a multi-faceted system capable of delivering up to 41 MW of power for 100 seconds.

Neutral Beam Injection (NBI): The system includes both positive-ion-based NBI (P-NBI) and negative-ion-based NBI (N-NBI). The P-NBI system injects neutral deuterium particles at an energy of 85 keV. The N-NBI system, a technology pioneered in Japan, injects particles at a much higher energy of 500 keV. This high energy allows the beams to penetrate to the core of the dense plasma, providing efficient core heating and a strong source of non-inductive current drive.
Electron Cyclotron Resonance Heating (ECRH): A 7 MW ECRH system uses high-frequency microwaves (110 and 138 GHz) to deposit power directly into the plasma electrons. Steerable launchers allow for precise, localized heating, which is used for core electron heating, plasma startup assist, and active control of magnetohydrodynamic (MHD) instabilities like neoclassical tearing modes (NTMs).

Divertor and Plasma-Facing Components: JT-60SA features a single-null divertor configuration designed to handle high heat and particle fluxes. The initial operational phase uses an inertial carbon-fiber-composite (CFC) divertor. A planned upgrade will replace this with a tungsten monoblock divertor, similar to the one used in ITER, to test materials and heat exhaust solutions relevant for a future power plant. This system is essential for managing impurities and exhausting helium ash from the plasma core.

Historical Development

The JT-60SA project is a modification and upgrade of the previous JT-60U (Upgrade) tokamak, which operated from 1991 to 2008 and was itself a modification of the original JT-60 (1985–1990). JT-60U was a world-leading machine, holding the record for the highest fusion triple product (n·τ·T) of 1.77 × 10^21 m^-3·s·keV achieved in 1998.

The decision to upgrade JT-60U to a fully superconducting machine was formalized through the Broader Approach Agreement, signed between Japan and Euratom in 2007. This agreement established a framework for collaborative research to accelerate the realization of fusion energy, with JT-60SA as its central project. Under this arrangement, Japan's National Institutes for Quantum Science and Technology (QST) provided the site and existing infrastructure, while Europe contributed approximately half of the components, including the TF coil structures, cryoplant, and power supplies.

Construction and assembly began in 2013, involving manufacturing contributions from numerous institutions and companies across Europe and Japan. A significant technical challenge occurred in March 2021 during integrated commissioning when a short circuit in a TF coil feeder led to a large-scale helium leak. The incident required extensive repairs and a thorough review of the commissioning procedures. After successful completion of the repairs and recommissioning, JT-60SA achieved its first plasma on October 26, 2023, a major milestone for the international fusion community.

Current Status (as of 2026)

Following the successful first plasma in late 2023, JT-60SA has been undergoing a phased commissioning and initial experimental campaign. The primary focus in 2024–2025 was on the systematic commissioning of all subsystems, including the superconducting magnets, cryogenics, plasma control systems, and heating systems. Initial experiments have concentrated on establishing reliable plasma breakdown, current ramp-up, and basic shape control in diverted configurations.

The machine has successfully demonstrated stable ohmic and ECRH-assisted plasmas with currents exceeding 1 MA. The initial NBI systems have been commissioned, allowing for the first auxiliary-heated experiments to explore plasma confinement and stability. The diagnostic systems, a comprehensive suite of over 50 individual instruments, are being progressively brought online and calibrated.

The research program is now transitioning from commissioning to the first phase of physics exploitation. This involves gradually increasing the plasma current, magnetic field, and heating power towards the machine's design targets. The immediate goals are to establish robust H-mode access and to begin exploring the physics of long-pulse operation, albeit in relatively low-power scenarios initially.

Notable Implementations

As a singular, large-scale international facility, JT-60SA's implementation is best understood through its key subsystems and programmatic contributions.

International Collaboration Model: The project is a prime example of a distributed, in-kind contribution model for constructing a major scientific facility. Components were manufactured by various European Union member states (e.g., Italy, France, Germany, Spain) and Japan, then shipped to Naka for assembly by a joint team. This model provides valuable lessons for the management and integration of complex international projects like ITER and DEMO.
Superconducting Magnet Technology: The 26 large-scale NbTi coils, weighing over 750 tonnes in total, represent a significant engineering achievement. The successful operation of the magnet system, including the complex cryogenic cooling and quench protection systems, validates the design and manufacturing processes for future superconducting tokamaks.
High-Energy Negative-Ion NBI: The 500 keV N-NBI system is a unique and critical technology for future steady-state fusion reactors. It is the most powerful system of its kind and is essential for providing the central current drive needed to sustain the plasma current non-inductively, a key requirement for achieving the Lawson criterion in a steady state.
Integrated Plasma Control: JT-60SA employs a sophisticated real-time plasma control system. This system integrates magnetic measurements, diagnostic data, and actuator commands (for magnets, fueling, and heating) to precisely manage the plasma's shape, position, and kinetic profiles. Its performance is crucial for preventing disruptions and sustaining high-performance conditions.

Open Challenges

Despite its successful start, JT-60SA faces several scientific and engineering challenges that its research program is designed to address.

Achieving and Sustaining High-Beta Scenarios: A primary goal is to operate at high normalized beta (β_N > 3) for durations of 100 seconds. This requires simultaneous optimization of plasma shape, density, and temperature profiles while actively controlling MHD instabilities like NTMs, which can degrade confinement.
Heat Exhaust and Divertor Physics: Managing the extreme heat flux to the divertor—projected to be in the range of 10–15 MW/m²—is a critical challenge for any long-pulse, high-power device. The initial carbon divertor will provide valuable data, but the eventual transition to a tungsten divertor will be essential for studying plasma-material interactions, material migration, and fuel retention in a reactor-relevant environment.
Disruption Mitigation: Plasma disruptions are a major concern for large tokamaks, as the sudden loss of confinement can induce large electromagnetic forces and thermal loads on in-vessel components. Developing and validating reliable disruption prediction and mitigation systems is a high-priority research area for both JT-60SA and ITER.
Integration of Steady-State Scenarios: The ultimate challenge is to integrate all necessary elements—high confinement, high beta, non-inductive current drive, impurity control, and heat exhaust management—into a single, stable, and reproducible operational scenario that can be sustained for long durations. This requires a deep understanding of the complex, coupled physics at play.

Outlook

The credible 5- to 15-year trajectory for JT-60SA involves a staged approach to reaching its full performance capabilities. In the near term (2026–2029), the experimental program will focus on increasing plasma performance, aiming to achieve the full plasma current of 5.5 MA and to begin high-power, long-pulse experiments. This phase will be crucial for validating ITER's baseline operational scenarios and for exploring advanced inductive regimes.

In the medium term (2030–2035), the focus will shift to fully non-inductive, steady-state scenarios. This will leverage the full capability of the N-NBI and ECRH systems to drive 100% of the plasma current. During this period, the planned upgrade to a full tungsten divertor is expected to occur, enabling experiments on heat exhaust and plasma-material interactions under reactor-like conditions. The results from this phase will directly inform the operational plan for ITER's high-fusion-power campaigns and provide critical data for the engineering design of DEMO.

By the end of its operational life in the late 2030s, JT-60SA is expected to have provided a comprehensive database on the physics and technology of long-pulse, high-beta tokamak operation, solidifying the scientific basis for a commercially viable fusion power plant.

References

JT-60SA: A pivotal step on the roadmap to fusion energy — Fusion Engineering and Design (2023)
Initial operation of the JT-60SA tokamak — Nuclear Fusion (2024)
The JT-60SA Research Plan — National Institutes for Quantum Science and Technology (QST) (2020)
JT-60SA Project — JT-60SA Official Website
Broader Approach Activities Annual Report 2022 — Broader Approach (2023)
First plasma in JT-60SA — ITER Organization (2023)
Design and key features of the JT-60SA device — Fusion Engineering and Design (2015)