Wikipedia
A magnetic mirror is a type of magnetic confinement device, used in physics experiments to trap high temperature plasma using magnetic fields. In a magnetic mirror a specially shaped electromagnet creates a configuration of magnetic field lines which reflects charged particles from a high density magnetic field region to a low density magnetic field region. This mirror effect will only occur for particles within a limited range of velocities and angles of approach.
Large experimental magnetic mirror machines have been developed to confine hot deuterium plasma as a possible approach to fusion power, since the plasma is too hot for any solid container. The largest machine that has been built was the Mirror Fusion Test Facility (MFTF) completed in 1986 at Lawrence Livermore Laboratory in the US. Mirror research continues today in countries like Russia.
A charged particle moving within a region of magnetic field experiences a Lorentz force that causes it to move in a helical (corkscrew) path along a magnetic field line. The radius of the circle that the particle describes is called the radius of gyration or gyroradius. If it enters a region of denser magnetic field lines, a field gradient, the combination of the radial component of the fields and the azimuthal motion of the particle results in a force pointed against the gradient, in the direction of lower magnetic field. It is this force that can reflect the particle, causing it to decelerate and reverse direction.