Fusion research aims to create a climate-friendly power station. Its purpose is similar to the sun: it derives energy from the fusion of atomic nuclei. This fuel is ionized hydrogen gas, which is extremely thin. It’s called plasma. The plasma must be heated to more than 100 million degrees to ignite the fusion flame.
The Asdex Upgrade has been equipped with a diverter by scientists from the Max Planck Institute for Plasma Physics to control the interaction of hot fuel and surrounding walls. This gave the plant its name: Axialsymmetric diverter experiment. The diverter field improves the thermal insulation of the plasma by removing impurities and creating an additional magnetic field.
The Asdex Upgrade is more compatible with the conditions of a later power station than its predecessor Asdex. However, the diverter, important plasma properties, including the density and load on the walls, and the Asdex Upgrade are better adapted to these characteristics. The Asdex Upgrade has a powerful plasma heater and sophisticated measuring equipment to observe the plasma. This allows the Asdex Upgrade to develop operating modes for potential power plants. The plant has already answered vital research questions for the European Joint Experiment Jet, the international experimental reactor Iter, and a planned demonstration power station.
For the plasma vessel, a tungsten wall
The Asdex Upgrade was a significant step in developing a future fusion power station. It allowed the researchers to clad the plasma vessel’s wall with tungsten instead of carbon. Carbon offers many advantages for experimental plants. It is unsuitable for power plant operation because it is too easily eroded by plasma and binds to too much fuel. Due to its high melting temperature, tungsten is well-suited for use as a wall material. The plasma cools quickly due to even the smallest impurities within the tungsten atoms constantly released from the wall. The Asdex upgrade group solved this problem after a lot of experimentation.
This success had direct consequences: The European joint experiment Jet was rebuilt and received a tungsten divertor. Iter, an international experimental reactor team, abandoned the originally planned experiments using a carbon diverter in favor of tungsten. Tungsten will also be the base material for the demonstration power station.
Injecting hydrogen helps to prevent instabilities.
Different disturbances can result from the interaction between the plasma particles and the confining magnet field. These disturbances can include instability at the plasma edge or ELMs (edge-localized modes). The edge plasma loses some of its confinement and occasionally throws plasma particles or energy outwards onto vessel walls. Medium-sized plants like the Asdex Upgrade can handle this. However, large plants like Iter may become overwhelmed by the diverter. Procedures to prevent instabilities have been developed for Asdex Upgrade to address this problem. The plasma vessel is protected by 16 small magnetic coils that entirely suppress fluctuations. The outermost plasma edge is the second option. ELMs can only be created if the plasma shape is correct – using the magnetic field and injecting hydrogen.
Ensure continuous operation
The Tokamak variety’s fusion plants- such as Asdex Upgrade or Jet – ensure continuous operation by creating a magnetic cage with two superimposed magnet fields. These magnetic fields are ring-shaped magnetic fields produced by external magnetic coils and a field generated by plasma. The magnetic field lines are combined so that they surround the plasma. A transformer coil inside the plasma is used to induce the current plasma pulse-wise. The entire system works in pulses, unlike the more complex stellarators. This is a drawback of the tokamaks.
The Max Planck Institute for Plasma Physics investigates different methods to generate plasma current continuously. You could inject high-frequency waves or particle beams to drive additional plasma current. The system was operated almost entirely without a transformer, and the first time a machine had a metallic inner wall. This phase could have been extended if the Asdex Upgrade was not equipped with superconducting magnet coils, as it was for Iter.
What’s next?
The Asdex Upgrade has seen the diverter shape change and optimized many times over the past 30 years. Researchers now plan to test a new concept for a diverter. The plasma vessel’s roof has two additional magnetic coils to fan the diverter field to ensure the plasma’s power is distributed more evenly. The coils will be assembled in the middle of 2022. These expansions will allow future research at Garching tokamak, which will help solve the problems associated with a demonstration power plant. Arne Kallenbach, Project Leader, says that the Asdex Upgrade could be considered a blueprint for a future tokamak-fusion power plant. The 30-year-old sample discharges provide reliable information that can be used to build a power plant.
