The Magnetic Bottle

If a magnetic field has the correct shape, if the field lines pinch together as shown at the left or the right side of Figure (1), then the magnetic force F_mag on a charged particle has a component that is directed back from the pinch. For charged particles with the correct speed, this back component of the magnetic force can reflect the particle and reverse v_|| . If the magnetic field is pinched at both ends, as in Figure (1) the charged particle can reflect back and forth, trapped as if it were in a magnetic bottle. In the subject of plasma physics, one often deals with hot ionized gasses, particularly in experiments designed to study the possibility of creating controlled fusion reactions. These gases are so hot that they would melt and vaporize any known substance they touch.
The only known way to confine these gases to do experiments on them is either do the experiments so fast that the gas does not have time to escape (inertial confinement), or use magnetic fields and devices like the magnetic bottle shown in Figure (1) (magnetic confinement).

Figure 1: Magnetic bottle. When the magnetic field lines pinch together, the charged particles can be reflected back in a process called magnetic mirroring. (At the two ends of the magnetic bottle above, the magnetic force F_B has a component back into the bottle.)