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Part 2 of 2 Parts (Please read Part 1 first)
In the interaction of the charged plasma particles with the confining magnetic field, various disturbances of the plasma confinement can happen. These disturbances include instabilities at the plasma edge referred to as edge localized modes (ELMs). In this process, the edge plasma briefly loses its confinement and throws plasma particles and energy outwards onto the walls of the plasma containment vessel. While medium-sized plants such as the ASDEX Upgrade are able to deal with this problem, the divertor in large fusion reactors such as ITER could easily become overloaded. In order to solve this problem, procedures to prevent such instabilities have been developed for the ASDEX Upgrade. Sixteen small magnetic coils built into the plasma containment vessel are able to completely suppress the instabilities in the confinement fields. A second method for dealing with instabilities starts at the outermost edge of the plasma. If the right plasma shape can be achieved by the magnetic confinement fields while a sufficiently high particle density is ensured by the injection of hydrogen, then ELMs cannot develop.
Continuous operation is guaranteed by tokamak type fusion reactors such as the ASDEX Upgrade, the JET or the ITER which construct a magnetic cage with two superimposed magnetic fields. One field is ring-shaped and is generated by external magnetic coils and another field that is generated by a current flowing in the plasma. The combination of the two fields results in the field lines being twisted in a way that encloses the plasma. The current that flow through the plasma is normally induced in pulses generated by a transformer coil in the plasma. Unlike the more complicated stellarator fusion reactor design, the tokamaks entire system operates in pulses which is a problem of the tokamak design.
Scientists at the MPIPP are investigating various methods for continuously generating the current in the plasma. For example, the injection of high-frequency waves or particle beams can drive an additional current in the plasma. They have almost been able to operate the system without the need for a transformer. This was done for the first time in a machine with a metallic inner wall. If the ASDEX Upgrade had not been equipped with normally conducting copper coils but had rather been constructed with superconducting magnetic coils, this phase could have been extended for much longer. It could have made continuous operation possible,
During the thirty years that have been dedicated to the ASDEX Upgrade, the MPIPP has changed and optimized the divertor shape several times. The researchers now intend to test a new divertor concept. Two additional magnetic coils on the roof of the plasma containment vessel are intended to fan out the divertor field so that the power from the plasma is distributed over a larger area. Assembly of the coils is scheduled to begin in the middle of 2022. These expansions will also enable future investigation at the Garching tokamak to solve the problems expected in a future demonstration power plant.
Arne Kallenbach is the Project Leader for the ASDEX Upgrade. He said, “In many ways, the ASDEX Upgrade can be seen as a blueprint for a tokamak fusion power plant. Together with newly developed computer codes, the sample discharges developed over 30 years provide reliable information for a power plant.”