Experimental research began at SLAC in 1966 with the completion of the two-mile-long linear electron accelerator or linac, a machine capable of producing an electron beam with an energy up to 20 GeV (giga-electron volts) or 20 billion volts. Initial experiments directed these electrons onto stationary targets to study the structure of matter. The maximum energy of the linac was increased over the years to 50 GeV as part of an extensive upgrade required for its use in the SLC. The two-mile accelerator continues to generate high-intensity beams of electrons with the highest energy available in the world and serves as the backbone of the SLC.
Three other major research facilities have been built at SLAC, each based on the use of the electron-positron collisions rather than fixed-target electron beam experiments: the 8-GeV SPEAR storage ring (1972), the 30-GeV PEP storage ring (1980), and the 100-GeV Stanford Linear Collider, or SLC (1989). The energy quoted for these electron-positron colliders is the center of mass energy, or twice the beam energy.