Simulation of Electromagnetic Lithium Cylinder Compression for Applications in Magnetized Target Fusion

Abstract General Fusion is developing Magnetized Target Fusion (MTF) as a practical means of producing deuterium-tritium fusion power. Lawson Machine 26 (LM26) is an MTF demonstration that integrates General Fusion’s operational Plasma Injector 3 (PI3) with a solid lithium cylindrical shell compression system. The lithium plasma liner is electromagnetically compressed by a stack of coils via a “theta-pinch”. Prototype Zero (P0), a testbed at 1:4 scale of LM26, was designed and commissioned to de-risk the complex compression process and validate modeling tools in the absence of plasma to ensure accurate predictions for LM26 operations. Prototype Zero comprises 48 coil turns that are arranged to compress a lithium cylinder with an axial height up to 280 mm, outer radius of 218 mm, and thickness ranging from 10 to 20 mm. Capacitors supply up to 1 MJ of energy to the coils, resulting in the cylinders being compressed in 0.7 to 1 ms at radial velocities exceeding 300 m/s. The center shaft of the machine is composed of two cones which form an hourglass shape and serve to further compress the cylinder in the axial direction. A 2D axisymmetric numerical model was developed using ANSYS LS-DYNA to predict the trajectory of the liner. A circuit model was implemented to represent the RLC circuit connected to the driving coils. LS-DYNA predictions of the liner position were compared to experimental measurements obtained using diagnostic equipment mounted within the center shaft cones. Diagnostics included photon Doppler velocimetry (PDV) to measure radial velocity at the mid-plane of the lithium cylinder, and structured light reconstruction (SLR) to track the axial profile of the cylinder during compression. An alternate arrangement employed a single center shaft cone, enabling direct visualization of the compression with a high-speed camera through a window. Simulations were conducted for a selection of Prototype Zero compression shots and compared with experimental measurements. Results confirm the accuracy of the modelling technique in predicting the shape of a cylindrical lithium liner undergoing electromagnetic compression. This study provides critical validation of the modeling tools in advance of the full-scale LM26 plasma compressor.