An extensive series of experiments has been carried out by researchers from the Institute of Plasma Physics of the CAS and the international ITER project, which aims to start producing clean and almost inexhaustible energy through the so-called tokamak within the next four years. For the first time, scientists have been able to provide experimental evidence of a physical limit to the electric currents flowing between the plasma and the first reactor wall at a time when the plasma is unstable. The new findings, published in the Nuclear Fusion journal, will help improve computer models for future fusion reactors.
Nuclear fusion is a promising route to low-emission and safe power generation. As part of the ITER project, which is the world's largest scientific experiment, scientists plan to commission a tokamak of the same name in southern France in 2026 – the first experimental fusion device in which hot plasma is expected to produce more energy than is put into it. The next step will be to build a DEMO fusion plant prototype.
"In order to ensure continuous and reliable operation, fusion reactors face a number of challenges. One of them is the sudden and unplanned premature termination of plasma discharge in the form of so-called plasma disruption," summarises Radomír Pánek, Director of the Institute of Plasma Physics of the CAS and also a member of the management of the European organization Fusion for Energy as well as a representative of Europe on the ITER Organisation's Scientific Council.
Disruptions occur when the plasma ceases to be stable. "Besides disrupting the smoothness of operation, these phenomena are significant because they can cause extreme pressures on individual components of fusion devices – such as in the ITER tokamak currently under construction," says Jiří Adámek of the Institute of Plasma Physics of the CAS, who led the experimental research and is the first author of the published study.
Physicists part of the international fusion community are therefore working hard to understand these processes, learning to predict them reliably, and developing techniques that can significantly mitigate their impact on the reactor. "This is why we and our ITER colleagues have embarked on a series of experiments in our Prague COMPASS tokamak, which has already produced a number of results in the past that have had major impact on the design and operation of the ITER tokamak," Jiří Adámek explains.
Disruptions can also damage the tokamak
Disruptions in tokamaks cause sudden losses of thermal and magnetic energy stored in the plasma. In large installations such as the ITER tokamak, they can lead to extreme energy flows and mechanical forces impacting the components of the first reactor wall. "Disruption can thus affect or even damage the wear and overall lifetime of these components," Adámek points out.
The main source of mechanical stress during a disruption is the electric currents, the so-called halo currents, circulating between the plasma and the first reactor wall which, in combination with the strong magnetic field in the tokamak, generate enormous forces.
An important physical limit
A series of experiments in the COMPASS tokamak, carried out by researchers in 2020 before the facility ended its operation last year, was designed to determine just how large and how distributed the halo currents are during disruptions.
The main result of these systematic measurements is a comparison of the halo currents, or, their densities (Jhalo), with the plasma ion flux (Jplasma) for different values of the plasma flow (IP) before a disruption.
"This unique comparative measurement demonstrated for the first time that halo currents are in fact limited by the plasma ion flux, which represents an important physical limit," Adámek says. According to him, the COMPASS experiments also confirmed that the total value of the halo currents entering the first wall of the tokamak increases with the electric current flowing through the plasma before a disruption.
This fact, together with the newly discovered physical limit, leads to the conclusion that the total surface area over which the halo currents pass increases with the total plasma current. As a result, the halo currents are thus spread out more widely within the tokamak chamber, thereby reducing the local stresses on the first wall components.