There are many types of accelerator designs, although all have certain features in common. Only charged particles (most commonly protons and electrons, and their antiparticles; less often deuterons, alpha particles, and heavy ions) can be artificially accelerated; therefore, the first stage of any accelerator is an ion source to produce the charged particles from a neutral gas. All accelerators use electric fields (steady, alternating, or induced) to speed up particles; most use magnetic fields to contain and focus the beam. Meson factories (the largest of which is at the Los Alamos, N.Mex., Scientific Laboratory), so called because of their copious pion production by high-current proton beams, operate at conventional energies but produce much more intense beams than previous accelerators; this makes it possible to repeat early experiments much more accurately. In linear accelerators the particle path is a straight line; in other machines, of which the cyclotron is the prototype, a magnetic field is used to bend the particles in a circular or spiral path.