Nuclear Fusion Research

Fraunhofer EMI has developed experimentally validated simulation models that help control critical plasma events in fusion reactors.

Global demand for primary energy and electricity is rising. At the same time, CO₂ emissions must be drastically reduced to address the challenges of climate change. In addition to renewable energy sources such as wind and solar power, nuclear fusion could therefore also contribute to climate-friendly electricity generation and help meet base-load demand in the future.

However, there is still some way to go before the first commercially viable fusion power plant is built. And the large-scale international research project ITER (International Thermonuclear Experimental Reactor) is, quite literally, on this path. The world’s largest fusion reactor to date is currently under construction in Cadarache, in southern France. ITER is intended to demonstrate that a significant net energy gain can be achieved technically through the fusion of hydrogen nuclei into helium—a process that occurs in a similar way in the sun. The hydrogen isotopes deuterium and tritium serve as fuels.

For the positively charged atomic nuclei to fuse, their mutual repulsive forces must be overcome. To do this, the fuel is heated to temperatures well over 100 million degrees Celsius. Since hydrogen no longer exists as a gas at these temperatures but as a plasma, it can be manipulated using magnetic fields and thus confined within a ring-shaped magnetic field cage (tokamak principle). If this plasma is sufficiently dense and confined at high temperatures for a long enough time (Lawson criterion), the fusion processes begin and release enormous amounts of energy.

In a future power plant, this energy could be harnessed to generate electricity via a cooling system, a heat exchanger, and a turbine.

Controlled Mitigation of Plasma Disruptions

A key challenge is maintaining the plasma in a controlled state over the long term. So-called plasma disruptions can arise suddenly from plasma instabilities, leading to a loss of plasma confinement and a rapid decay of the plasma current. In the process, large thermal and mechanical loads are released within a very short time. These can damage reactor components and result in additional maintenance costs as well as longer downtimes.

To mitigate the consequences of such events, ITER is being equipped with a Disruption Mitigation System (DMS). This system involves injecting frozen hydrogen or neon pellets at high speed into the fusion reactor—albeit indirectly. Just before reaching the plasma, the pellets strike an impact plate—the so-called Shattering Unit—positioned at an angle to their flight path, at speeds of up to 500 meters per second. The impact breaks them into fragments of varying sizes, which penetrate deep into the plasma, cooling it and increasing its density. This reduces the plasma’s energy before it can strike the wall. At the same time, it prevents the formation of high-energy relativistic electrons, whose enormous energy could severely damage the vacuum vessel.

The effectiveness of the DMS depends largely on the size and velocity distribution of the generated fragments. Therefore, it is essential to understand how pellet material properties, impact velocity and angle, and other boundary conditions affect the fragmentation process.

Simulation Models and New Diagnostic Methods from Fraunhofer EMI

This is where Fraunhofer EMI’s research came in. On behalf of the ITER Organization, the researchers developed numerical models and software that can be used to simulate and analyze the complex fragmentation process. Experiments were used to calibrate and validate the models. With the help of the validated simulations, the experimental database can be significantly expanded. In addition, fragment sizes and velocities can be predicted for different materials and impact conditions. 

A particular challenge was reliably characterizing the very small and fast fragments produced after the impact. These fragments cannot be easily collected and subsequently measured. For this reason, Fraunhofer EMI developed a dedicated synthetic diagnostic tool that employs a fragment detection and tracking algorithm, enabling the automated comparison of experimentally measured and simulation-based fragment statistics. 

The result is a robust, experimentally validated tool that supports the further development and optimization of the DMS. It can be used to improve the design of the Shattering Unit and identify impact conditions that produce the desired fragment distributions. In addition, the fragment-plume characteristics predicted by the validated model can serve as input for subsequent magnetohydrodynamic simulations of tokamak plasmas using codes such as JOREK. In this way, the research helps reduce peak loads on the reactor wall, minimize damage, and shorten downtime—key prerequisites for safe and economically viable fusion power plants in the long term.

Or to put it more colloquially: Researchers at Fraunhofer EMI are investigating how to break the ice most cleverly.

Fraunhofer EMI carried out this work under contract IO/21/CT/4300002337 with the ITER Organization from 2021 to 2026.