Traps in which the plasma confinement regionis bounded along the lines of force of the magneticfield are customarily called open traps. The bestknownand most common type of open trap is theso-called 'magnetic-mirror machine' or 'mirror trap',suggested in the early fifties independently byG.I. Budker in the USSR and R.F. Post in the USA.Plasma confinement in the mirror trap is dueto the reflection of particles from the regions ofstrong magnetic field ('mirrors') at the ends of thedevice. These reflect only particles with sufficientlylarge magnetic moment. Particles with small magneticmoment freely fly out of the device so that a conical'hole' without particles, the loss cone, is formed inthe phase space.