Multiphysics Simulation A Virtual Insight into Nuclear Fusion

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The Lawson Machine 26 is General Fusion's first large-scale fusion machine. Controlled by multiphysics simulation instead of trial-and-error. When plasma, metal and magnetic fields collide in milliseconds, only physics counts.

The development of fusion systems presents engineers with extreme technical challenges. With the Lawson Machine 26, General Fusion has successfully commissioned a large-scale fusion demonstration plant for the first time. The development process was continuously supported by a multiphysics simulation, with which highly dynamic, non-linear processes could be realistically mapped and specifically optimized.

The controlled use of fusion energy is considered one of the most challenging tasks of modern energy technology. While many research approaches rely on tokamaks or laser-driven inertial confinement fusion, General Fusion is pursuing a different path with so-called magnetized target fusion. This involves introducing a magnetized plasma into a solid or liquid metal container and then compressing it in order to achieve the temperatures and pressures required for fusion reactions.

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The Lawson Machine 26, LM26 for short, serves as a demonstration facility to clarify key physical and technical issues and reduce risks for future commercial facilities. The focus is on the electromagnetic compression of a lithium liner, which brings the plasma to fusion conditions within a few milliseconds. The resulting forces, deformations and thermal effects are so extreme that conventional development and testing methods quickly reach their limits.

When Physics Cannot Be Considered Separately

The compression of the lithium liner is a prime example of a highly coupled physical system. Electromagnetic forces drive the movement, mechanical stresses lead to plastic deformation, while at the same time material properties change due to an increase in temperature, while the enclosed plasma must be kept stable. Every change in electrical parameters or geometry has a direct effect on several areas of physics.

To map these interactions realistically, General Fusion relied on integrated multiphysics modeling right from the start. Comsol Multiphysics made it possible to couple and consistently calculate electromagnetic fields, structural mechanics, heat transport and external circuits within a uniform, time-dependent model.

From the Material Sample to the System Geometry

A central component of the development process was the realistic modeling of the lithium liner. Under the extreme loads during compression, lithium exhibits a highly non-linear, rate- and temperature-dependent material behavior. Extensive experimental investigations were carried out in order to model this correctly. In collaboration with Veryst Engineering, tensile tests were carried out in which lithium samples were subjected to high strain rates under controlled temperature conditions until failure.

The experimentally obtained data was used to calibrate a Johnson-Cook material model, which was then directly integrated into the multi-physics simulations. On this basis, the engineers were able to reliably predict the trajectories and deformations of the lithium liners during electromagnetic compression.

The simulations themselves were time-dependent and fully coupled. Large deformations, changing contact conditions and extremely short time constants required the use of automatic remeshing methods to ensure numerical stability and accuracy. In order to keep the computational effort manageable, a 2D axisymmetric model was also used, which captures the essential physical effects but can be calculated much more efficiently than a full-resolution 3D model.

A particular focus was on matching the compressor impedance between the power supply and the mechanical dynamics of the liner. A high proportion of the stored electrical energy can only be converted into kinetic energy if the electrical and mechanical time constants are matched. With the help of the simulation, parameters such as coil geometry, number of windings and distance between coils and liner could be specifically varied in order to maximize compression efficiency.

The models were validated by closely interlinking simulation and experiment. A total of 40 lithium liners were electromagnetically compressed in order to verify the numerical results. Measurement methods such as Structured Light Reconstruction and Photon Doppler Velocimetry provided detailed deformation and velocity profiles that showed a high degree of agreement with the simulation results. This iterative approach made it possible to gradually increase the model complexity and at the same time build confidence in the predictions.

Faster Development Cycles Through Parallel Simulation

A significant advantage of the chosen approach was the speed of the development process. With the help of cluster sweep functions, extensive parameter studies could be carried out in parallel on computing clusters. Simulation runs that would previously have taken weeks or months could be completed in less than 24 hours. This enabled the development team to make design decisions based on a much broader database and to define safe operating areas at an early stage.

Relevance for Other High-Tech Applications

Even though the LM26 is an unusual project, the underlying simulation approach can be transferred to many other fields of application. Particularly in power electronics, high-current systems or complex electromagnetic-mechanical actuators, similar couplings between electrical, thermal and structural effects occur. In such systems, integrated multiphysical simulation can make a decisive contribution to reducing development risks and shortening innovation cycles.

Central Development Tool

With the successful commissioning of Lawson Machine 26, General Fusion has shown that multiphysical simulation has long since become a key development tool. It enables extremely complex energy systems to be mastered step by step and sound design decisions to be made. While the plant is now being further optimized, the developed workflow already forms the basis for the next step on the way to the commercial use of fusion energy. (mr)

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