MHD stability analysis of the Helios stellarator fusion power plant

Details on the magnetohydrodynamic (“MHD”) stability and evolution of Thea Energy Inc.’s “Helios” stellarator equilibrium (Swanson, 2025) are discussed in this paper. Helios is a two-field-period, quasi-axisymmetric (“QA”) stellarator configuration with R = 8 m major radius, aspect ratio A = 4.5 , and on-axis magnetic field B = 6 T . Linear MHD stability analysis is presented including Mercier criterion, ideal ballooning, finite- n , kink, and vertical stability. Nonlinear MHD evolution of Helios is simulated using the M3D-C1 code, which has been recently enhanced to accommodate stellarator geometries (Zhou et al., 2021) and incorporates self-consistent calculations of the bootstrap current (Saxena et al., 2025). Despite the development of a slightly chaotic edge region in the outer region of the plasma, the pressure profile remains robust through 2000 Alfvén times. Additionally, ballooning modes are observed nonlinearly but are found to saturate at low levels. These results indicate that not only is Helios stable to ideal modes, but that confinement properties are preserved despite a chaotic edge region.