JOREK open source release opens up a code behind ITER disruption modelling

JOREK open source code models instabilities in spherical tokamaks such as MAST-U

(Image courtesy of JOREK Team, Photo: Siobhan Smith and Stanislas Pamela)

JOREK, the plasma simulation code that is helping design ITER’s disruption mitigation system, is now open source under an LGPLv3.0-or-later licence, and the repository sets out how teams are expected to build on it. Guidance on citation, credit and sharing developments through the central code sits alongside the licence, which gives a code with more than twenty years of history terms that are now written down for the community around it.

Why JOREK open source matters for ITER disruption heat loads

Present tokamaks can mostly tolerate the energy that MHD instabilities send to the first wall, but in ITER and future fusion power plants the heat fluxes are likely to exceed the melting limits of plasma-facing components, which is why simulating how those instabilities develop, and how to control them, carries so much weight. JOREK was already doing that work for ITER by 2021, when an overview in Nuclear Fusion reported it was testing and optimising mitigation strategies even as validation of its disruption simulations continued, and EUROfusion now reports that the results are helping to design the mitigation system itself.

That system is built around shattered pellet injection, ITER’s baseline, and JOREK has simulated the technique in ITER, JET and ASDEX Upgrade plasmas. A 2018 Nuclear Fusion study by D. Hu and colleagues modelled deuterium pellet injection into a JET plasma, for instance, and the authors said the analysis would inform further work on a configuration for the future ITER system.

From one line of code to tokamaks, stellarators and an international community

Guido Huijsmans wrote the first line of JOREK more than twenty years ago, and the code has since grown to simulate plasma in both tokamaks and stellarators. Recent papers listed on the JOREK website show how far it has travelled, from runaway electron damage to plasma-facing components to the thermal resilience of ITER’s tungsten first wall under runaway electron impact, while a 2026 overview by Hoelzl and colleagues looks ahead to predictive simulations that can help prepare design, mitigation techniques and operational scenarios for future devices.

The community around the code is international, with researchers at the ITER Organisation and from China, Japan, India and the US all playing a part. Papers listed on the website since 2024 cover EAST, HL-3, KSTAR, EHL-2 and DTT alongside European machines such as ASDEX Upgrade and JET. Matthias Hoelzl of the Max Planck Institute for Plasma Physics has helped build that community and develop the code, and he describes JOREK as “one of the leading codes worldwide” for its particular area, and the people around it as “a growing and powerful community that has made a difference”.

What the JOREK open source terms ask of contributors

The JOREK open source terms sit in the repository on the ITER Organisation’s GitHub account, where the README states LGPLv3.0-or-later and contributors sign off commits under version 1.1 of the Developer Certificate of Origin, which the README says ensures compliance with the licence requirements. Community rules sit alongside the licence. Forks are asked to keep the names JOREK or JOREK-fork, and developments built on the code should be shared through pull requests to the central repository.

Publications that use JOREK are expected to cite the 2021 overview article in Nuclear Fusion plus relevant earlier works, and the README also asks for the JOREK Team to appear as co-author, with an affiliation footnote that references the 2024 review in the same journal.

Researchers expect JOREK open source to ease collaboration

Nina Schwarz, a CEA researcher in France who has worked with JOREK for years, sees the move as a way for people to contribute to and improve the code openly, and a clearly defined licence also makes it much easier to connect JOREK with other codes. The long-term impact is difficult to predict, but Schwarz and Hoelzl are both optimistic that collaboration will improve and that the code will become more accessible. A relatively big code can make a real difference, Hoelzl says, and opening it up can pave the way for more collaboration. For a code whose results feed ITER’s disruption mitigation design, the framework for that collaboration is now in the repository.

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