JET DTE2 (2021) campaign

Overview

The second Deuterium-Tritium Experimental campaign (DTE2) was a landmark series of experiments conducted at the Joint European Torus (JET) facility in Culham, UK, during the latter half of 2021. As the first high-power D-T plasma experiments in a tokamak with an ITER-like first wall, DTE2's primary objective was to validate operational scenarios and physics models critical for the success of ITER. The campaign culminated in pulse #99971 on 21 December 2021, which produced a world-record 59 megajoules (MJ) of fusion energy over a sustained five-second period, surpassing the previous record of 21.7 MJ set by JET during its first D-T campaign (DTE1) in 1997. This achievement demonstrated stable, high-performance plasma operation under conditions approaching those expected in ITER and provided invaluable data on alpha particle heating, tritium retention, and neutron-induced effects on diagnostics and machine components.

Physics / Mechanism

The core physics objective of DTE2 was to study magnetically confined plasmas in a 50/50 deuterium-tritium fuel mix, the same fuel planned for commercial fusion reactors. The D-T fusion reaction, D + T → ⁴He + n, releases 17.6 MeV of energy, with 3.5 MeV carried by the alpha particle (⁴He nucleus) and 14.1 MeV by the neutron. A key goal was to investigate the behavior of these energetic alpha particles, which are expected to be the primary source of plasma self-heating in a burning plasma. DTE2 experiments were designed to confine these alpha particles effectively, allowing them to transfer their energy to the bulk plasma and validate theoretical models of alpha heating.

The campaign operated primarily in two scenarios: a high-current baseline scenario and a hybrid scenario. The record-setting 59 MJ pulse was achieved in the baseline H-mode scenario, characterized by a plasma current (Ip) of 3.8 MA and a toroidal magnetic field (Bt) of 3.7 T. This scenario maintained a high confinement state for over five seconds, a duration limited by the temperature limits of JET's copper magnetic field coils rather than plasma instabilities. The total heating power was approximately 26 MW, comprising 16 MW of neutral beam injection (NBI) and 10 MW of ion cyclotron resonance heating (ICRH). The resulting average fusion power was approximately 11 MW, corresponding to a plasma energy gain factor (Q_plasma) of 0.33. While not reaching scientific breakeven (Q_plasma = 1), this value was consistent with predictions scaled from earlier deuterium-only experiments, confirming the predictive capability of integrated modeling codes like JINTRAC.

A crucial aspect of DTE2 was operating with JET's ITER-like wall (ILW), which consists of a beryllium first wall and a tungsten divertor. This material combination, chosen for ITER due to its low tritium retention (Be) and high melting point (W), had never before been tested under the intense neutron and particle fluxes of a sustained D-T plasma. Experiments meticulously measured tritium retention in the plasma-facing components, providing critical data for the tritium breeding ratio and fuel cycle management in future devices. The ILW's performance, particularly its impact on plasma purity and confinement, was a central research question that DTE2 successfully addressed.

Historical development

The groundwork for DTE2 was laid decades earlier. JET's first D-T campaign, DTE1, took place in 1997 and was the world's first experiment to produce significant fusion power (a peak of 16.1 MW) using a 50/50 D-T mix in a tokamak. DTE1 used carbon-based plasma-facing components, which, while robust, exhibited high levels of tritium retention—a significant concern for future long-pulse reactors. This experience directly motivated the decision to upgrade JET with the ITER-like wall.

Between 2009 and 2011, JET underwent a major shutdown to install the ILW, replacing over 4,500 carbon-fiber composite tiles with beryllium and tungsten. The subsequent decade was dedicated to developing and optimizing plasma scenarios compatible with the new metallic wall, which has different plasma-material interaction properties compared to carbon. This extensive preparatory phase was essential for ensuring that the high-power D-T plasmas in DTE2 could be reliably controlled without excessive impurity influx or plasma disruptions.

Planning for DTE2 began in the mid-2010s under the EUROfusion consortium, which operates JET on behalf of the European Commission. The scientific program was meticulously designed by hundreds of scientists across Europe to maximize the scientific return for ITER. Key milestones included the re-licensing of JET's tritium handling plant, upgrading diagnostic systems to withstand the harsh 14.1 MeV neutron environment, and conducting extensive modeling and simulation to predict plasma performance. A series of experiments in pure tritium and pure deuterium in 2020 and 2021 served as final dress rehearsals, allowing for precise calibration of diagnostics and validation of plasma control strategies before the introduction of the mixed D-T fuel.

Current status

As of 2026, the DTE2 experimental campaign is complete, and JET ceased plasma operations permanently at the end of 2023. The primary focus has shifted to the comprehensive analysis of the vast dataset generated during the campaign. Over 150 scientific papers based on DTE2 results have been published, with analysis ongoing. The results have been presented at major international conferences, including the IAEA Fusion Energy Conference.

The key findings have profoundly influenced the ITER research plan. The successful demonstration of stable, long-pulse H-mode plasmas with the ILW has increased confidence in ITER's baseline operational scenario. The measured alpha heating effects were found to be consistent with classical slowing-down theory, validating a cornerstone of burning plasma physics. Data on tritium retention in the ILW confirmed that it was significantly lower than in the previous carbon wall, a critical result for ITER's fuel cycle design. The 59 MJ record remains the world benchmark for fusion energy produced in a tokamak.

Post-operational activities at JET now include a decommissioning phase, which itself is a valuable research project. This involves the remote handling and analysis of activated and tritiated components from the torus, providing unique data on material science and waste management for future fusion power plants.

Notable implementations

The DTE2 campaign was a singular effort conducted at a single facility, the Joint European Torus. However, its implementation was a massive international collaboration managed by the EUROfusion consortium, which comprises 30 research organizations from 25 European countries, plus Switzerland and Ukraine. The UK Atomic Energy Authority (UKAEA) was responsible for the operation and maintenance of the JET facility itself.

The scientific program involved contributions from hundreds of physicists and engineers across Europe and internationally, including collaborators from the US Department of Energy and the ITER Organization. The campaign's success was a direct result of this coordinated, multi-institutional effort to prepare, execute, and analyze the experiments. The data from DTE2 is now being used to refine the operational plans for ITER, representing a direct transfer of knowledge from an existing machine to a future one. The campaign serves as a model for the kind of international scientific collaboration that will be necessary to operate ITER successfully.

Open challenges

Despite its successes, DTE2 highlighted several outstanding challenges for fusion energy. The achieved Q_plasma of 0.33 is still far from the Q > 10 targeted by ITER and the requirements for a commercial power plant. The performance was limited by the engineering constraints of JET, a machine designed in the 1970s, particularly its water-cooled copper magnets which cannot sustain a high magnetic field for more than a few seconds.

Isotope effects remain an area of active research. While DTE2 provided extensive data, the physics of how plasma confinement scales with ion mass (from deuterium to a D-T mix) is not fully understood. The experiments showed that achieving high performance in D-T required careful optimization and was not a simple extrapolation from deuterium plasmas, with confinement being slightly lower than predicted by the most optimistic scaling laws. This subtle difference is critical for accurately predicting ITER's performance.

Tritium management, while shown to be manageable with the ILW, still presents challenges. Understanding the long-term dynamics of tritium migration and co-deposition with beryllium in remote areas of the vacuum vessel is crucial for developing accurate fuel cycle models and ensuring the safe and efficient operation of ITER. The post-mortem analysis of JET's tiles will provide essential data, but further research is needed.

Finally, mitigating plasma disruptions in high-power scenarios remains a critical issue. While DTE2 operated with a low disruption frequency, the consequences of a disruption in a device like ITER are far more severe. The campaign provided data on disruption precursors and loads in an ITER-like environment, but developing fully reliable avoidance and mitigation systems is an ongoing challenge for the fusion community.

Outlook

The legacy of the DTE2 campaign will shape the next 5-15 years of fusion research, primarily through its impact on ITER. The validation of key physics models and operational scenarios provides a solid foundation for ITER's initial research phases. The DTE2 dataset will serve as the primary benchmark for validating the complex simulation codes that will be used to guide ITER's experimental campaigns. This significantly de-risks ITER's mission to achieve a burning plasma.

Over the next five years, the scientific community will continue to mine the DTE2 data, leading to deeper insights into alpha particle physics, plasma-wall interactions, and transport phenomena in reactor-relevant plasmas. These findings will directly inform the detailed planning of ITER's first D-T experiments, scheduled for the mid-2030s. The experience gained in handling large quantities of tritium and operating a nuclear-licensed fusion facility is also invaluable for the ITER Organization and regulatory bodies worldwide.

Beyond ITER, DTE2's results are relevant for the design of demonstration power plants (DEMOs). The data on the performance and longevity of the beryllium-tungsten wall under high neutron fluence will guide material choices for future devices. The campaign's success has bolstered confidence in the tokamak concept as the leading path toward commercial fusion energy, providing a crucial data point that validates the multi-billion-dollar global investment in this approach.

References

1. JET breaks fusion energy record — EUROfusion (2022)
2. Overview of the JET DTE2 experimental campaign — IAEA (2023)
3. Record-breaking 59 MJ of fusion energy at JET — UK Atomic Energy Authority (2022)
4. D-T physics in preparation for ITER — Nuclear Fusion (2022)
5. Background to the record-breaking results from JET — Journal of Plasma Physics (2022)
6. Alpha-particle heating and energetic-ion transport studies in JET DTE2 plasmas — Nuclear Fusion (2023)
7. Overview of the JET results in support of ITER — Nuclear Fusion (2019)
8. Tritium fuel retention in JET with an ITER-like wall: an overview of the DTE2 campaign results — Nuclear Fusion (2024)