II.

Linac

The linear accelerator, or linac, was first conceived in the late 1920s. It uses large alternating voltages to push particles along in a straight line. Particles pass through a line of hollow metal tubes enclosed in an evacuated cylinder. Within a hollow conductor there is no electric field (see Electricity), so a charged particle travels at constant speed inside each of the tubes. Between one tube and the next there is a potential difference which varies in size and direction as an AC voltage is applied to the series of tubes. Bunches of charged particles are accelerated from tube to tube, moving with the voltage wave as it travels along the linac.

Theoretically, a linac of any energy can be built. The largest linac in the world, at Stanford University, is 3.2 km (2 mi) long. It is capable of accelerating electrons to an energy of 50 GeV (50 gigaelectronvolts, or 50 billion eV). Stanford’s linac is designed to collide two beams of particles, accelerated in turn by the linac and temporarily kept in storage rings (see Storage Ring Colliders, below).