Asdex Upgrade: Paving the Way for a Fusion Power Plant

The plasma vessel from Asdex Upgrade. At the bottom you can see the divertor’s baffle plates.
Credit: © Volker Rohde

On March 21, 1991, the Asdex Upgrade experimental facility at the Max Planck Institute for Plasma Physics in Garching generated the first plasma.

For 30 years, the Asdex Upgrade has been paving the way for a fusion power plant that generates climate-neutral energy. The tokamak fusion plant was repeatedly expanded and improved during this time. Not least for this reason, it provides numerous insights that are incorporated into the design and operation of other fusion plants. For example, the Asdex Upgrade team has developed scenarios for the operation of the Jet test plant in the UK and the Iter test plant in France as well as forecasts for a planned demonstration power plant. A conversion planned for mid-2022 is intended to prepare the plant for the future.

The goal of fusion research is to develop a climate- and environment-friendly power plant. Like the sun, its purpose is to derive energy from the fusion of atomic nuclei. The fuel for this is an extremely thin, ionized hydrogen gas – a plasma. To ignite the fusion fire, the plasma must be enclosed in magnetic fields almost without contact and heated to over 100 million degrees.

In order to regulate the interaction between the hot fuel and the surrounding walls, scientists at the Max Planck Institute for Plasma Physics have equipped the Asdex Upgrade with a divertor, which has given the plant its name: Axial symmetric divertor experiment. Through an additional magnetic field, the divertor field removes impurities from the plasma and improves its thermal insulation.

However, in contrast to its predecessor Asdex, the Asdex Upgrade, the divertor and important properties of the plasma, especially the density and the load on the walls, are more closely adapted to the conditions in a later power plant. Equipped with a powerful plasma heater and sophisticated measuring equipment for observing the plasma, the Asdex Upgrade can therefore be used to develop operating modes for a potential power plant. In 38,700 plasma discharges to date, the plant has answered essential research questions for the European joint experiment Jet and the international experimental reactor Iter as well as a planned demonstration power plant.

A tungsten wall for the plasma vessel

With the Asdex Upgrade, the researchers took a significant step towards a future fusion power plant when they clad the wall of the plasma vessel with tungsten instead of carbon. Carbon has considerable advantages for experimental plants. However, it is unsuitable for the operation of a power plant because it is too strongly eroded by the plasma and binds too much fuel to itself. Because of its high melting point, tungsten is well suited as a wall material – at least in principle. But the plasma cools down quickly because of even the smallest impurities in the tungsten atoms that are repeatedly released from the wall. After a lot of experimentation, the Asdex upgrade team has been able to deal with this problem.

Direct consequences of this success: In a major rebuild, the European joint experiment Jet received a tungsten divertor in 2011. The international experimental reactor Iter team decided to forego the initially planned experiments with a carbon divertor and go straight for tungsten. Tungsten is also the reference material for the demonstration power plant.

Injecting hydrogen prevents instabilities

In the interaction of the charged plasma particles with the confining magnetic field, various disturbances of the plasma confinement can occur. These include instabilities at the plasma edge or ELMs (edge localized modes). In the process, the edge plasma briefly loses its confinement and periodically throws plasma particles and energy outwards onto the vessel walls. While medium-sized plants such as the Asdex Upgrade are able to cope with this, the divertor in large plants such as Iter could become overloaded. In order to solve this problem, procedures to prevent instabilities were developed for the Asdex Upgrade. Sixteen small magnetic coils in the plasma vessel completely suppress the instability with their fields. A second method starts at the outermost plasma edge. If the right plasma shape can be set – via the magnetic field – while ensuring a sufficiently high particle density – by injecting hydrogen – ELMs cannot develop.

Ensuring continuous operation

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