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It is with the deepest sadness and a profound sense of loss that we learned that our colleague, friend and outstanding physicist, Dr. Michael Lehnen, lost his courageous and dignified struggle against a long illness on Sunday morning, 16 June 2024.  We mourn the passing of this exceptional scientist who was a towering figure in the critically important field of tokamak disruptions and their mitigation.

Michael was born on 4 October 1969 in Köln, Germany and graduated in physics from the city's university in 1997.  His PhD, conducted jointly at the University of Düsseldorf and the Forschungszentrum Jülich (FZJ), focussed on an experimental technique using beams of neutral helium atoms to measure the plasma density and temperature in the cool plasma boundary regions of the FZJ tokamak TEXTOR. After completion of his doctoral studies in 2000, Michael continued in Jülich as a research scientist, joining the plasma exhaust team working with the TEXTOR Dynamic Ergodic Divertor, a complex set of helical in-vessel magnetic coils designed to perturb the magnetic equilibrium of the tokamak and influence the plasma magnetohydrodynamic stability and transport properties. These early experiments were amongst the precursors to the modern technique of resonant magnetic perturbations, used in many tokamaks and planned in °ÄÃÅÁùºÏ²Ê¸ßÊÖ, to suppress edge localized mode activity.

It was during this period, 2000-2013, that Michael's interest in disruption physics really began and which established the foundation for him to become one of the world's preeminent experts in this key area for the success of tokamak fusion. He worked principally on the physics of runaway electron generation, suppression and interaction with materials, and on the technique of massive gas injection (MGI) as a means to mitigate the impact of disruption forces and transient heat loads. Throughout these Jülich years, he set the scene for what was to become a recurring feature in his career—collaboration with fusion scientists from across the globe, including teams at , JET, DIII-D, MAST, Tore Supra and LHD devices—in the areas of disruption physics, helium exhaust and plasma-wall interactions.

As a long-term secondee at the JET tokamak in the UK from 2011 to 2013, he took responsibility for the bolometry diagnostic and continued his role as scientific coordinator for JET MGI experiments.  He was also a prominent figure in the European fusion program, leading special working groups in transient heat loads and the development of systems for disruption and runaway electron mitigation within the European Fusion Development Agreement, the precursor to the current EUROfusion Consortium.

This formative period in Jülich was the ideal preparation for Michael's move to °ÄÃÅÁùºÏ²Ê¸ßÊÖ in 2013, assuming the challenging role of Scientific Coordinator for Disruption Physics within the Science Division, filling the position vacated through retirement by our much-respected colleague Masayoshi Sugihara, himself internationally recognized in disruption physics. This is a post on which lies so much responsibility given the importance of disruption avoidance and mitigation on a fusion reactor at the °ÄÃÅÁùºÏ²Ê¸ßÊÖ scale, with thermal and magnetic stored energies orders of magnitude higher than on current research tokamaks.

On his broad shoulders was placed the task of providing assessments and advice across the project regarding disruption electromagnetic and thermal loads, runaway electron formation and impact and physics support to the design of the °ÄÃÅÁùºÏ²Ê¸ßÊÖ disruption mitigation system—a system at a scale unprecedented in fusion research.  Matters became considerably more complex once it was realized that the baseline technique, MGI, was not going to be practically achievable on °ÄÃÅÁùºÏ²Ê¸ßÊÖ.  Instead, focus switched to the approach of cryogenic solid pellet injection (SPI) using mixtures of hydrogenic and impurity gases, then just in the early phase of physics studies on research tokamaks.

Appointed in 2018 to the leadership of the Disruption Mitigation System (DMS) Task Force, Michael embarked on a six-year crusade to establish the physics basis for SPI in support of the design of this enormously complex °ÄÃÅÁùºÏ²Ê¸ßÊÖ system which successfully passed its final design review in March this year.  He took on this daunting task with everything in his considerable armoury, marshalling the international tokamak community—both through the International Tokamak Physics Activity (ITPA), of which he was the co-Chair of the MHD Topical Group, and through °ÄÃÅÁùºÏ²Ê¸ßÊÖ Organization contracts—to exploit existing systems and, in many cases, install completely new ones (in particular on the ASDEX Upgrade and KSTAR tokamaks). Rarely, if ever, has such an extensive international mobilisation of the tokamak physics community, been witnessed.  The impact has been nothing short of remarkable.

Those present at the March DMS design review, will not forget the masterful presentation of the DMS Physics Basis which Michael delivered. Nor will we ever forget that he was able to manage the preparation of this presentation and the accompanying report whilst facing difficult medical issues.  It is a lasting testimony to the character, competence and dedication of this most exceptional scientist.  Without his contribution and that of all his many colleagues in the DMS technology and design teams, this vitally important system for the success of °ÄÃÅÁùºÏ²Ê¸ßÊÖ would simply not have reached the level of maturity required on the timescale needed.

Michael leaves behind a magnificent legacy in the field of tokamak physics.  Throughout his career, he has been a regular and familiar figure in all the major fusion science conferences, delivering many plenary and invited presentations and figuring as author or co-author on hundreds of scientific papers. He combined the very rare talents of deep physical insight and superb experimental technique. His wisdom and advice were sought by all at °ÄÃÅÁùºÏ²Ê¸ßÊÖ in his specialized area and, for his colleagues in the Science Division, he was always ready to assist and provide counsel, with the very special dry sense of humour that all of us who knew him well will always remember with great fondness.

The °ÄÃÅÁùºÏ²Ê¸ßÊÖ Organization and the entire °ÄÃÅÁùºÏ²Ê¸ßÊÖ research community has lost one of our own. He will be profoundly missed. We express our most sincere condolences to those closest to Michael and to all of his many colleagues and friends throughout the fusion world.