Charged particles in a magnetic field orbit in spirals of the Larmor radius. Ions are heavier than electrons and so have a larger Larmor radius. (Credit: IPP)

The radius of a circle on which a charged particle orbits in a homogeneous magnetic field. The charged particle in a magnetic field is subject to the Lorentz force. It acts as a centripetal force perpendicular to the direction of the magnetic field, so it causes the particle to orbit “around” the magnetic field line. If the particle also has forward velocity, it moves in a spiral along the magnetic field. Lighter particles (e.g. electrons) have a smaller Larmor radius than e.g. ions.