Effect of Lithium Liner on Stability of Compressing Plasma in LM26 Experiment at General Fusion

LM26 is a magnetized target fusion (MTF) experiment that can compress a plasma directly with poloidal field coils (PFCs) or indirectly by inserting a solid lithium liner inside the coils. A toroidal plasma is generated using coaxial helicity injection and confined either indirectly by a solid lithium liner or directly by a small buffer field from the PFCs. When the PFC currents are increased, the plasma is compressed in 4 ms to a radial compression ratio of 3 by the liner or in 1 ms to a factor of 10 by the fields directly. The plasma must be MHD stable throughout compression to maintain an energy confinement time longer than the compression time, so it will heat to fusion conditions. We have previously found MHD stable trajectories in a simplified compression geometry [1,2].

We determine the liner trajectory by matching COMSOL and OpenFOAM models to measurements of liner motion. The plasma equilibria are reconstructed from plasma measurements using Bayesian methods. Finally, ideal and resistive MHD stability is analyzed using RDCON [3]. Plasma fixed-boundary modes are important in both operating modes, buckling modes are important when there is a liner, and free-boundary modes are important when the plasma is compressed directly by the PFCs. We determine the dominant source of instability in each shot and identify methods to maintain plasma stability in the experiment.

[1] D Brennan et al. 2020 Nucl. Fusion 60, 046027

[2] D Brennan et al. 2021 Nucl. Fusion 61, 046047

[3] A Glasser et al. 2016 Phys. Plasmas 23 112506