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ASDEX Upgrade

ASDEX Upgrade is a medium-sized tokamak at the Max Planck Institute for Plasma Physics in Garching, Germany. It is a leading facility for studying divertor physics, plasma-wall interactions with an all-tungsten wall, and developing operational scenarios for ITER and future fusion power plants.

Overview

The Axially Symmetric Divertor Experiment Upgrade, or ASDEX Upgrade (AUG), is a tokamak located at the Max Planck Institute for Plasma Physics (IPP) in Garching, Germany. It is a central device within the EUROfusion research program and serves as a primary support facility for the International Thermonuclear Experimental Reactor (ITER). ASDEX Upgrade's key mission is to investigate solutions for the core challenges of a future fusion power plant, particularly in the areas of plasma-wall interaction, heat exhaust, and steady-state operation. Its most distinguishing feature is its fully tungsten-clad plasma-facing components, making it a unique testbed for the material chosen for the ITER divertor. The device's flexible heating and diagnostic systems enable the exploration of advanced plasma confinement regimes and control strategies essential for next-generation fusion reactors.

Physics and Mechanism

ASDEX Upgrade is a D-shaped tokamak designed to operate with a lower single-null divertor configuration, which is geometrically similar to that of ITER. This design is crucial for handling the immense heat and particle fluxes exhausted from the core plasma. The machine's key parameters—a major radius (R) of 1.65 m, minor radius (a) of 0.5 m, and a toroidal magnetic field (B_t) up to 3.1 T—allow it to achieve plasma conditions relevant to reactor-scale devices, particularly in dimensionless parameters like the normalized plasma pressure (beta) and collisionality.

The device is equipped with a powerful and versatile plasma heating and current drive system totaling over 20 MW. This includes:

  • Neutral Beam Injection (NBI): Two injectors provide up to 20 MW of heating power with deuterium atoms at energies of 60 and 93 keV.
  • Ion Cyclotron Resonance Heating (ICRH): Four antennas deliver up to 6 MW to heat plasma ions.
  • Electron Cyclotron Resonance Heating (ECRH): Eight gyrotrons provide up to 6 MW of steerable, localized heating and current drive, which is critical for controlling magnetohydrodynamic (MHD) instabilities like neoclassical tearing modes (NTMs).

A significant feature of ASDEX Upgrade is its all-tungsten wall. Unlike carbon, which was previously common in tokamaks, tungsten has a high melting point and low tritium retention but is a high-Z (high atomic number) material. If tungsten impurities penetrate the core plasma, they can cause significant radiative energy losses, leading to plasma cooling and disruption. A central research theme at AUG is therefore the development of operational scenarios that minimize tungsten erosion and prevent its accumulation in the plasma core, a critical task for ensuring ITER's success.

Historical Development

ASDEX Upgrade was conceived as the successor to the original ASDEX experiment (1980–1990), which discovered the High-Confinement Mode (H-Mode) in 1982. While ASDEX had a complex internal divertor coil system, ASDEX Upgrade was designed with external poloidal field coils to create a divertor configuration more suitable for a reactor. Construction began in 1982, and the machine achieved its first plasma on March 21, 1991.

Key milestones in its operational history include:

  • 1991: First plasma and initial operations with a carbon wall.
  • 1996–2007: Phased transition to tungsten-coated plasma-facing components. The divertor was the first major component to be clad in tungsten, providing early data on its performance under high heat flux conditions.
  • 2007: Completion of the transition to a fully tungsten-clad first wall, making ASDEX Upgrade the first major tokamak to operate with an all-metal interior, a configuration now referred to as an "ITER-like wall." This transition was a landmark decision, providing invaluable data on operating with high-Z materials.
  • 2010s: Extensive research into mitigating Edge-Localized Modes (ELMs), large instabilities that expel bursts of energy and particles from the plasma edge. AUG pioneered techniques like resonant magnetic perturbations (RMPs) and pellet pacing for ELM control.
  • 2020s: Focus on developing integrated operational scenarios for DEMO, the planned demonstration power plant, including scenarios with high plasma density, low impurity content, and stable, long-pulse operation.

Throughout its history, ASDEX Upgrade has been instrumental in shaping the physics basis for ITER, from validating energy confinement scaling laws to developing control schemes for plasma instabilities.

Current Status (as of 2026)

As of 2026, ASDEX Upgrade remains at the forefront of fusion research under the EUROfusion consortium. Its experimental campaigns are tightly integrated with the needs of ITER construction and future operation, as well as the design of the European DEMO. The facility continues to operate with its all-tungsten wall, providing a unique and indispensable platform for studying plasma-wall interactions in a reactor-relevant environment.

Current research priorities include:

  • Divertor physics: Investigating advanced divertor configurations and detachment control to manage heat loads, a critical challenge for any fusion power plant.
  • Tungsten transport and control: Understanding the mechanisms that govern tungsten erosion from the wall and its transport into the plasma core. Experiments focus on using central ECRH to create a temperature gradient that pushes heavy impurities out of the core.
  • ELM mitigation: Refining ELM control techniques to ensure they are robust and effective for ITER and DEMO. This includes optimizing the use of RMPs and real-time control of pellet injection.
  • Scenario development: Developing and optimizing stationary, high-performance plasma scenarios that meet the requirements for a fusion power plant, including achieving a high tritium breeding ratio and efficient current drive.

Notable Implementations

ASDEX Upgrade is operated by the Max Planck Institute for Plasma Physics (IPP), a leading global center for fusion research. It is the flagship experimental device of the institute's Garching campus. The research program is not conducted in isolation; it is a collaborative effort involving hundreds of scientists from across Europe and internationally through the EUROfusion framework. This collaborative structure ensures that the experimental results and operational experience are directly fed into the design and research plans for ITER and the conceptual design of DEMO. The device's operational team and scientific staff are highly regarded for their expertise in tokamak operation, plasma diagnostics, and theoretical modeling.

Open Challenges

Despite its successes, ASDEX Upgrade continues to address several fundamental scientific and engineering challenges for fusion energy:

  • Tungsten Sputtering and Core Contamination: While manageable in many scenarios, tungsten erosion and subsequent core plasma contamination remain a risk, especially during transient events or in specific plasma regimes. Preventing accumulation without compromising core performance is a persistent challenge that requires sophisticated control methods.
  • Heat Flux Management in the Divertor: Even with advanced divertor geometries, managing the extreme, narrow heat flux channel (the scrape-off layer width) remains a primary concern. Achieving stable and complete divertor detachment across a wide range of operational conditions is not yet fully resolved and is a major focus of research.
  • ELM Control Compatibility: While several ELM mitigation techniques have been demonstrated, ensuring their compatibility with other reactor requirements—such as high core confinement, low impurity levels, and high plasma density—is an ongoing area of integrated scenario development. The ideal solution must be robust and not degrade overall plasma performance.
  • Disruption Prediction and Mitigation: Like all tokamaks, ASDEX Upgrade is susceptible to plasma disruptions. Improving the physics understanding, early prediction, and effective mitigation of these events is critical for protecting the machine and is a high-priority research topic for ITER.

Outlook

In the coming 5-15 years, ASDEX Upgrade is expected to continue its role as a key facility for de-risking ITER operation and paving the way for DEMO. The forward-looking plan includes several hardware upgrades and research campaigns. A major focus will be on testing novel divertor concepts, potentially including upgrades to the divertor geometry to better handle heat exhaust. The machine will also be crucial for testing and validating advanced control algorithms that integrate real-time feedback from multiple diagnostics to maintain stable plasma performance.

As ITER begins its operational phases, ASDEX Upgrade will serve as a flexible testbed to rapidly investigate physics questions that arise from ITER experiments, providing solutions and optimizing operational strategies. Its ability to operate with an all-tungsten wall and ITER-like geometry ensures its continued relevance well into the 2030s, bridging the gap between today's experiments and tomorrow's fusion power plants.

References

  1. The ASDEX Upgrade Programme — Fusion Science and Technology (2015)
  2. Overview of ASDEX Upgrade results — Nuclear Fusion (2017)
  3. Tungsten as a plasma-facing material in fusion devices — Journal of Nuclear Materials (2011)
  4. Plasma-wall interaction in all metal-wall devices — Plasma Physics and Controlled Fusion (2015)
  5. Physics and control of the plasma-wall transition: a tutorial — Plasma Physics and Controlled Fusion (2020)
  6. ELM control in ASDEX Upgrade and extrapolation to ITER — Nuclear Fusion (2015)
  7. Divertor heat load reduction and detachment in ASDEX Upgrade — Nuclear Fusion (2022)
  8. ASDEX Upgrade—a tokamak for ITER and DEMO — Fusion Engineering and Design (2018)