When it comes to energies of more than a few MeV, accelerators for ions are different from those for electrons. The reason for this is the large mass difference between the particles. Electrons are already close to the speed of light, the absolute speed limit, at a few MeV; with further acceleration, as described by relativistic mechanics, almost only their energy and momentum increase. On the other hand, with ions of this energy range, the speed also increases significantly due to further acceleration.