German plasma physicist at the Max Planck Institute for Plasma Physics who has shaped the physics understanding of ASDEX Upgrade and led European studies on the path from ITER to a DEMO fusion power plant.
Hartmut Zohm has spent his career at the Max Planck Institute for Plasma Physics (IPP) in Garching, Germany, becoming one of Europe's most influential tokamak physicists. His primary experimental platform has been ASDEX Upgrade, the institute's medium-sized divertor tokamak that has served as a workhorse for plasma physics research since 1991. As head of the tokamak physics division and a leading figure in the ASDEX Upgrade program, Zohm has overseen experiments and analyses that have fundamentally shaped understanding of tokamak operating regimes, MHD stability, and the physics basis for reactor design.
ASDEX Upgrade occupies a strategic position in the global tokamak landscape: large enough to achieve reactor-relevant plasma conditions but flexible enough for rapid experimental iteration. Under Zohm's scientific direction, the machine has been systematically upgraded — most notably with a full tungsten first wall — to maximize its relevance to ITER and DEMO. Zohm's ability to connect experimental observations to reactor implications has made him a key interpreter of ASDEX Upgrade results for the broader fusion community.[1]
Zohm's name is closely associated with the study of edge-localized modes (ELMs), the quasi-periodic plasma instabilities that occur in H-mode tokamak operation. ELMs expel bursts of energy and particles from the plasma edge, and in a reactor-scale device, unmitigated ELMs could deposit sufficient energy on plasma-facing components to cause rapid erosion or even melting. Understanding and controlling ELMs is therefore one of the most practically important problems in fusion physics.
Zohm contributed to the characterization and classification of different ELM types (Type I, Type II, Type III) based on their frequency, energy content, and triggering conditions. His analyses connected ELM behavior to edge stability theory, specifically the peeling-ballooning model that describes the stability boundary of the H-mode pedestal. This work provided the physics framework within which ELM control techniques — including resonant magnetic perturbations, pellet pacing, and operating in ELM-free regimes — have been developed and tested on ASDEX Upgrade and other machines.[2]
Beyond ELMs, Zohm has made significant contributions to the broader understanding of magnetohydrodynamic (MHD) stability in tokamaks. His work on neoclassical tearing modes (NTMs) — magnetic islands that form at rational flux surfaces and degrade confinement — has been particularly influential. NTMs are driven by the bootstrap current and represent a beta limit in many tokamak scenarios, making their understanding essential for high-performance reactor operation.
Zohm and his collaborators demonstrated the stabilization of NTMs using localized electron cyclotron current drive (ECCD), in which microwave beams are aimed at the island location to replace the missing bootstrap current and suppress the instability. This technique, proven on ASDEX Upgrade, has become a standard tool in the tokamak control toolkit and is planned as a key actuator on ITER for NTM control.
ASDEX Upgrade's progressive conversion to a full tungsten first wall, completed under Zohm's tenure, represented a bold experimental choice that proved prescient. Tungsten is the leading candidate material for plasma-facing components in ITER's divertor and in future reactors due to its high melting point and low erosion rate. However, tungsten impurities in the plasma core can cause severe radiative losses, and operating a tokamak with tungsten walls requires careful control of impurity sources and edge plasma conditions.
The ASDEX Upgrade tungsten wall program demonstrated that high-performance H-mode operation is compatible with a full tungsten environment, provided appropriate impurity control techniques are employed. This result, to which Zohm's physics leadership contributed, provided critical confidence for ITER's material choices and for DEMO design concepts that assume tungsten armor.[3]
Zohm has been one of the leading voices in European fusion on the path from ITER to DEMO, the demonstration power plant that would follow ITER in the European fusion roadmap. He has contributed to conceptual DEMO studies that attempt to define the physics and engineering requirements for a fusion device that generates net electricity, breeds its own tritium fuel, and operates with high availability.
His approach to DEMO physics is characteristically systematic: define the operating scenario based on validated physics, identify the gaps between current knowledge and reactor requirements, and design experiments on existing machines to close those gaps. Zohm has argued for an approach in which DEMO design choices are constrained by experimentally demonstrated physics rather than projected improvements, a conservative philosophy that reflects the experimental tradition of the ASDEX Upgrade program.
Zohm is also recognized as an effective communicator of fusion physics, both within the community and to broader audiences. His textbook on magnetohydrodynamic stability of tokamaks has become a standard reference, and his review articles and lectures have helped train a generation of fusion physicists. His ability to synthesize complex experimental results into clear physical narratives has made his contributions influential not only through their technical content but through their clarity of exposition.