magnetic mirror is a linear confinement concept that reflects charged particles back and forth between regions of strong magnetic field at each end of an open column of 📝plasma.
The mirror is the open-ended member of the 📝magnetic confinement fusion (MCF) family — where the 📝Tokamak and 📝stellarator close their field lines into a torus so particles can never run off the end, a mirror accepts open field lines and plugs the ends with magnetic force. A charged particle spiraling into a strengthening field converts its parallel motion into gyration until it is reflected, the same mechanism that traps particles in Earth's Van Allen belts. The concept's classic defect is the loss cone: particles traveling too nearly parallel to the axis are never reflected and stream out the ends, an open leak that capped mirror confinement.
Mirrors were the main U.S. alternative to the tokamak through the 1970s and 1980s, a program that ended when the MFTF-B facility at Lawrence Livermore National Laboratory was mothballed for budget reasons in 1986 on the day of its dedication. High-field superconducting magnets have since reopened the line: WHAM, the Wisconsin HTS Axisymmetric Mirror, confined its first plasma in 2024 between 17-📝tesla mirror coils — the strongest fields of any operating magnetic 📝fusion experiment — and Realta Fusion is working to commercialize the configuration.