A magnetic mirror is a configuration of magnetic field lines in which a charged particle is reflected from a high density magnetic field to low density magnetic field. This mirror effect will only occur for particles within a limited range of velocity and angle of approach. Magnetic mirrors are made of specialized electromagnets designed to create a highly inhomogeneous field. Large magnetic mirrors have been used experimentally as a means of plasma confinement. One major application being researched is to confine the hot, electrically charged plasma inside a fusion reactor to generate fusion power (see magnetic confinement fusion). A category of experimental fusion reactors called mirror machines confine plasma within a magnetic field between two magnetic mirrors. The largest to be built was the Mirror Fusion Test Facility (MFTF) in 1986.