An electrical device that accelerates charged atomic or subatomic particles to high energies. The particles may be charged either positively or negatively. If subatomic, the particles are usually electrons or protons and, if atomic, they are charged ions of various elements and their isotopes throughout the entire periodic table of the elements. Before the advent of accelerators, the only sources of energetic particles for research were cosmic rays, or the naturally occurring radioactive atoms that emit subatomic particles such as electrons and alpha particles at various energies ranging from kilovolts to over 8 MeV.