A magnetic field configuration that causes a moving charged particle to reverse the direction of its motion. A pair of magnetic mirrors can thus create a magnetic trap in which the charged particles are held. If the magnetic field, located in the direction of motion of the charged particle, becomes stronger (magnetic field lines move closer together), the forward motion of the particle slows down. Eventually, the particle’s motion will change to the opposite and it will start moving from the stronger magnetic field towards the weaker one. Magnetic mirrors can be used to create a magnetic container that holds the hot plasma. This was one of the first approaches to magnetic confinement fusion research. The disadvantage of a magnetic mirror is that it is only able to contain particles with certain velocities and incidence angles and is therefore leaky.