When a charged particle enters a magnetic field, it feels a force given by
where
is the charge on the particle,
is the velocity and
is the magnetic field. The force will be perpendicular to both the magnetic field and the velocity of the particle.
If the velocity of the particle is perpendicular to the field the particle will travel in a circle, but generally it will have a component of velocity parallel to the field. This component will remain constant in a uniform field and the particle will describe a helix.
The magnetic field can be shaped however using coils as shown below so that the particle experiences components of force towards the axis and to the midpoint between the coils. The particle will slow down as it approaches the coils and can be made to move repeatedly back and forth between the coils. Bottles such as this can be used to confine plasmas and are used in fusion research. Unfortunately present designs are leaky and a practical fusion reactor is many years away.
>