A magnetic mirror is a plasma confinement device which uses a cylindrical plasma with strong magnetic coils at the ends. The combination of a relatively weak magnetic field in the center and strong fields at the ends allows most of the plasma to be confined via the mirror effect. Particles whose velocities are not primarily parallel to the field lines are reflected at the ends. This configuration is still under active investigation for nuclear fusion energy.
A charged particle travelling into an increasing magnetic field will (if the field becomes strong enough) reverse direction and be reflected back, provided its velocity perpendicular to the field is sufficiently large relative to its parallel velocity. This magnetic mirror effect is a direct result of the adiabatic invariance of the magnetic moment. Plasmas can be confined by devices which utilize this effect. The mirror effect also occurs in some tokamak plasmas, since the toroidal magnetic field is stronger on the inboard side than on the outboard side; in this case it gives rise to so-called "neoclassical" behavior. The ratio of the maximum and minimum magnetic fields is known as the mirror ratio and determines the fraction of phase space that can be confined.
See also: plasma physics