General Fusion’s magnetic 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, which is in operation through February 2025. Initially, COMSOL® was used to model the magnetomechanical compression of small-scale lithium rings and cylinders. These 2D axisymmetric models, which were coupled to nonlinear solid mechanics, magnetic fields, and heat transfer modules, were validated against high-speed imagery and laser diagnostics from those experiments. The validated models were then helpful in defining the LM26 compressor design and operating conditions.
A central challenge is that plasma equilibrium characteristics and lithium liner model parameters need to be adjusted during the compression shot. While some material tests were conducted on lithium samples in tension and compression, the range investigated was insufficient to cover all experimental conditions of the LM26. To overcome this, a Bayesian inference reconstruction procedure was used to solve an inverse problem. The process began by recreating the compression sequence of the lithium liner using a parametric sweep of the COMSOL Multiphysics model, which was constrained by experimental structured light reconstruction (SLR) and photon Doppler velocimetry (PDV) measurements in the LM26. This method enabled General Fusion to provide accurate magnetic flux boundary conditions to internal Grad-Shafranov magnetohydrodynamic (MHD) solvers. In turn, the MHD solver was used to determine the plasma density profile needed to reconstruct the plasma equilibrium and calculate its temperature. This work is important, as General Fusion aims to get LM26 to a plasma temperature of 1 keV, with a future target of 10 keV.
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