Advancing the fusion of magnetized targets by solving an inverse problem with COMSOL Multiphysics
General Fusion’s Magnetized Target Fusion approach involves compressing a spherical tokamak plasma to achieve fusion conditions. This presentation will detail how COMSOL Multiphysics® software was used in the LM26 fusion demonstration, operational since February 2025. Initially, COMSOL® was used to model the magnetomechanical compression of small-scale lithium rings and cylinders. These 2D axisymmetric models, which coupled the nonlinear solid mechanics, magnetic field and heat transfer modules, were validated by high-speed imaging and laser diagnostics from these experiments. The validated models then played a key role in defining the design and operating conditions of the LM26 compressor.
One of the main challenges is that plasma equilibrium characteristics and lithium coating model parameters need to be adjusted during compression firing. Although some material testing was performed on lithium samples in tension and compression, the range studied was insufficient to cover all experimental conditions of the LM26. To overcome this problem, a Bayesian inference reconstruction process was used to solve an inverse problem. This process began by recreating the compression sequence of the lithium coating using a parametric scan of COMSOL Multiphysics models, which were constrained by experimental Structured Light Reconstruction (SLR) and Photon Doppler Velocimetry (PDV) measurements in LM26. This method allowed General Fusion to provide accurate magnetic flux boundary conditions to Grad–Shafranov’s in-house magnetohydrodynamic (MHD) solvers. MHD solvers, in turn, were used to reconstruct the plasma equilibrium and determine the plasma density profiles needed to calculate its temperature. This work is critical, as General Fusion aims for LM26 to reach a plasma temperature of 1 keV, with a future goal of 10 keV.
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