The Principles and Construction of Linear Colliders*

The linear collider in its simplest form consists of two linear accelerators aimed at one another so that their beams collide in the space between them, the interaction region, as shown schematically in Fig. 1. Their beam energies, or more properly their mean beam energies, since each beam has some energy spread, are the same so that the centers-of-mass of the particle-particle collisions are stationary on the average. One of the linacs (linear accelerators) is equipped with a positron source so that the colliding system is an electron and a positron, a more fruitful system to study than two electrons. In order to develop high enough luminosities for highenergy particle physics, the linacs must be far more sophisticated than linacs of the past, and they must have ancillary damping rings to condense their beams to tiny lateral dimensions. We shall discuss the problems posed to the designers and builders of high-energy linear colliders in the following sections, but first a little history will explain why we are studying these new machines. -