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JT-60U

JT-60U (Japan Torus-60 Upgrade) was a large tokamak research facility operated by the Japan Atomic Energy Research Institute (JAERI) in Naka. A major contributor to the physics basis for ITER, it held the world record for the fusion triple product in a tokamak for over two decades.

Overview

JT-60U (Japan Torus-60 Upgrade) was a large-scale tokamak experimental device that operated from 1991 to 2008 at the Naka Fusion Institute in Japan. It was a modification of the original JT-60, which began operation in 1985. As one of the three major tokamaks of its era, alongside the Joint European Torus (JET) and the Tokamak Fusion Test Reactor (TFTR), JT-60U was instrumental in advancing the physics of magnetically confined plasmas. Its primary mission was to establish the scientific and technological basis for a fusion reactor, with a particular focus on achieving high-performance, steady-state plasma operation relevant to ITER and future power plants. JT-60U was notable for its non-circular, diverted plasma configuration and its powerful heating and current drive systems, which enabled it to explore advanced tokamak operating regimes. The device achieved several world records, including the highest fusion triple product for a tokamak, a record it held for over 20 years, and the highest ion temperature ever recorded in a magnetic confinement device [1, 2].

Physics / Mechanism

JT-60U was designed to investigate plasma confinement and stability in a reactor-relevant regime. Its design incorporated several key features that distinguished it from its contemporaries and enabled its scientific achievements.

Plasma Shaping and Divertor: The device featured a D-shaped vacuum vessel and a flexible poloidal field coil system, allowing for significant plasma shaping with high elongation (κ ~ 1.8) and triangularity (δ ~ 0.5). This shaping is crucial for achieving high plasma pressure and stability. JT-60U was equipped with a single-null, pumped divertor at the bottom of the vessel. In 1997, this was upgraded to a W-shaped divertor with baffled private flux regions, which improved impurity screening, particle exhaust, and heat load management by promoting radiative detachment [3]. The divertor targets were initially graphite and later upgraded to carbon-fiber composite (CFC) to handle high heat fluxes.

Heating and Current Drive Systems: A powerful and diverse suite of auxiliary systems provided the heating and non-inductive current drive necessary for high-performance and steady-state operation.

  • Neutral Beam Injection (NBI): The primary heating system consisted of 14 beamlines, delivering up to 40 MW of power. This included both positive-ion-based NBI (P-NBI) at ~85 keV and, uniquely, a negative-ion-based NBI (N-NBI) system. The N-NBI, operating at energies up to 400 keV, was a pioneering technology designed for efficient core heating and current drive in dense plasmas, a critical function for future reactors [4].
  • Radio-Frequency (RF) Heating: JT-60U employed multiple RF systems. The Lower Hybrid Range of Frequencies (LHRF) system (1.74–2.24 GHz, ~7 MW) was used for efficient off-axis current drive. The Ion Cyclotron Range of Frequencies (ICRH) system (~110 MHz, ~6 MW) provided ion heating. The Electron Cyclotron Range of Frequencies (ECRH) system (110 GHz, ~4 MW) was used for electron heating and, critically, for precise, localized current drive to suppress magnetohydrodynamic (MHD) instabilities like Neoclassical Tearing Modes (NTMs) [5].

These systems allowed JT-60U to operate without a central solenoid for current drive, demonstrating fully non-inductive current sustainment of up to 2.6 MA, a key requirement for steady-state tokamak operation.

Historical development

The history of JT-60U is one of phased construction, upgrades, and record-setting performance.

  • 1975: The JT-60 project is formally approved as a flagship device for the Japanese fusion program.
  • 1985: The original JT-60 achieves its first plasma. It was a circular, limiter-based tokamak designed to reach breakeven plasma conditions.
  • 1989–1991: Recognizing the importance of plasma shaping and divertor operation demonstrated on other machines, JT-60 underwent a major upgrade. The original circular vacuum vessel was replaced with a larger, D-shaped vessel, and a single-null divertor was installed. The upgraded machine was renamed JT-60U.
  • 1996: The N-NBI system was installed, becoming the first of its kind on a large tokamak. The same year, JT-60U achieved a Q_DT equivalent of 1.05 using deuterium plasmas, surpassing the previous record from TFTR [1].
  • 1997: The divertor was upgraded to a W-shaped configuration to improve power handling and impurity control.
  • 1998: JT-60U achieved its highest fusion triple product (n_D(0)τ_E T_i(0)) of 1.77 × 10^21 m⁻³·s·keV, a world record for tokamaks that stood until surpassed by China's EAST tokamak in 2021. It also set the record for ion temperature at 5.2 × 10^8 K (45 keV) [2].
  • 2003: A high-power 110 GHz ECRH system was installed, enabling advanced experiments on NTM stabilization.
  • 2008: JT-60U operations were concluded after 17 years of successful experiments. The facility was then dismantled to make way for its successor, JT-60SA, a joint project between Japan and Europe under the Broader Approach agreement.

Current status

As of 2026, JT-60U is fully decommissioned. Its final operational day was August 1, 2008. The knowledge and infrastructure from the JT-60U program were directly transitioned to the construction of its successor, JT-60SA, which is built in the same torus hall. JT-60SA, which achieved its first plasma in late 2023, is a superconducting tokamak designed to support ITER operation and investigate the physics of steady-state, high-beta plasmas for DEMO-class reactors. The legacy of JT-60U continues through the analysis of its extensive experimental database and its direct influence on the research plan and design of JT-60SA and ITER.

Notable implementations

JT-60U's primary contribution was the development and integration of advanced tokamak operating scenarios, which form the basis of high-performance modes planned for ITER. Key experimental campaigns included:

  • High-β_p and Reversed Shear Scenarios: JT-60U pioneered the exploration of reversed magnetic shear plasmas. In this configuration, the safety factor (q) profile has a minimum off-axis, leading to the formation of an Internal Transport Barrier (ITB). This dramatically improves core confinement, allowing for simultaneous high temperature and density. These experiments led to the achievement of the record fusion triple product and demonstrated high bootstrap current fractions (up to 80%), a crucial element for efficient steady-state operation [6].
  • Steady-State Demonstration: Leveraging its suite of non-inductive current drive systems (N-NBI, LHCD, ECRH, and bootstrap current), JT-60U demonstrated long-pulse, fully non-inductive operation. It sustained a plasma current of 1 MA for 24 seconds, limited only by hardware constraints, providing confidence in the feasibility of steady-state tokamak scenarios [7].
  • MHD Stability and Control: The machine served as a critical testbed for understanding and controlling performance-limiting MHD instabilities. The ECRH system was used to demonstrate precise suppression of NTMs by driving current within the magnetic island, a technique now considered essential for ITER [5].
  • Divertor and Plasma-Wall Interaction: The W-shaped divertor experiments provided a wealth of data on heat flux management, detachment physics, and tritium retention in carbon-based plasma-facing components. These results directly informed the design of the ITER divertor and highlighted the challenges of using carbon in a long-pulse, high-power machine [3].

Open challenges

While highly successful, JT-60U's research also defined the key challenges that subsequent machines must solve:

  • Integrated Scenario Control: Although JT-60U demonstrated the individual components of an advanced tokamak scenario (high confinement, high bootstrap fraction, non-inductive current drive, NTM suppression), integrating them simultaneously and maintaining stability for long durations proved difficult. The control systems required to manage the interplay between transport barriers, current profiles, and plasma pressure remain an active area of research for machines like JT-60SA.
  • Edge Localized Mode (ELM) Control: Like other H-mode tokamaks, JT-60U was subject to ELMs, which expel large bursts of energy and particles that can damage plasma-facing components. While some mitigation was achieved through plasma shaping and gas puffing, robust and reactor-compatible ELM suppression or mitigation techniques were not fully developed.
  • Divertor Heat Loads in Steady State: The W-shaped divertor experiments advanced the understanding of power exhaust. However, they also confirmed that managing the extreme, localized heat fluxes expected in a reactor-scale device like DEMO remains a major engineering and physics challenge. The choice of carbon-based components, while excellent for thermal shock resistance, led to significant fuel retention, reinforcing the move towards all-metal walls (like tungsten) for ITER and future reactors [8].
  • N-NBI Reliability and Integration: JT-60U was a pioneer in N-NBI technology. While it successfully demonstrated the physics of high-energy neutral beam heating and current drive, the operational reliability and wall-plug efficiency of the system highlighted the technological maturation required before it can be deployed on a commercial power plant.

Outlook

The legacy of JT-60U is its profound impact on the design and operational strategy of ITER and the direction of the global fusion program. Its results provided the physics basis for ITER's advanced operating scenarios, which aim to achieve high fusion gain (Q > 5) in steady-state or long-pulse conditions. The data on ITBs, reversed shear plasmas, and NTM control from JT-60U were essential in building the predictive models used to forecast ITER's performance [6].

Over the next 5-15 years, the direct successor, JT-60SA, will build upon this legacy. JT-60SA is designed to resolve many of the open challenges identified on JT-60U, but with superconducting magnets enabling much longer pulse lengths (up to 100 s). It will serve as an ITER satellite facility, testing control strategies and plasma scenarios before they are implemented on ITER. The pioneering work on N-NBI on JT-60U has also directly informed the design of the 1 MeV NBI heating system for ITER. In essence, JT-60U served as a critical bridge between the physics exploration of the 1980s and the reactor-oriented science of the 21st century, and its contributions will continue to shape the path toward fusion energy.

References

  1. High performance and prospects for steady-state operation on JT-60U — Nuclear Fusion (1999)
  2. Steady state high performance in JT-60U — Physics of Plasmas (2000)
  3. Divertor characteristics of W-shaped pumped divertor in JT-60U — Journal of Nuclear Materials (1999)
  4. Recent progress of negative ion NBI for JT-60U — Fusion Engineering and Design (2000)
  5. Complete suppression of a large-sized neoclassical tearing mode and control of the sawtooth oscillation by electron cyclotron current drive in the JT-60U tokamak — Physical Review Letters (2004)
  6. Progress of integrated plasma control for steady-state operation in JT-60U — Nuclear Fusion (2007)
  7. Overview of JT-60U results towards the establishment of advanced tokamak operations — Nuclear Fusion (2005)
  8. Tritium retention in the JT-60U W-shaped divertor — Journal of Nuclear Materials (2005)