Self-organized formation of a field-reversed high-β plasmoid in a mirror field orthogonal to the axis of counter-injected plasmoids

We report experimental evidence for the self‐organized formation of a field-reversed configuration (FRC)-like high‐β plasmoid inside a magnetic mirror field. The plasmoid forms when two plasmoids counter‐injected along an axis perpendicular to the mirror field axis collide at high speed and then merge. This ‘orthogonal merging’ geometry, which differs from conventional axisymmetric FRC merging along the device axis, can (in principle) preserve both axial ends of the mirror for reactor-relevant components such as divertors and direct energy-conversion stages. In the FRC Amplification via Translation–Collisional Merging device, two deuterium plasmoids with a line-integrated density of (1–1.5) × 1020 m−2 were accelerated to ∼150 km s−1 and merged at a relative speed of ∼300 km s−1, producing a localized high-density region within the mirror. Excluded-flux probes, internal magnetic probes, interferometry, and ion Doppler spectroscopy revealed (i) direct reversed-field signatures during the early post-collision and reformation phases, (ii) a transition from paramagnetic to diamagnetic rotation, and (iii) subsequent spin-up. Representative mid-plane interferometry measurements show that the bulk plasma density persists on a timescale comparable to the decay of the excluded-flux signal, while the longest-lived non-invasive excluded-flux traces indicate a persistence time of approximately 130 μs, substantially longer than the estimated classical diffusion time in a simple mirror (<70 μs with a mirror ratio of R ∼ 2.9). Although the full magnetic topology was not reconstructed non-invasively, the combined observations are consistent with the formation of an FRC-like high-β state containing a closed-field component that reduces parallel losses relative to a simple mirror. These results indicate a feasible pathway toward mirror–FRC hybrid operation. They also broaden the options of FRC formation and high-speed fueling in linear systems while maintaining the availability of the axial ends for reactor-relevant hardware.