Colliding linacs

The radius and the cost of electron-positron colliding rings increase as the square of the beam energy, so that it is generally felt that new approaches are needed for reaching energies definitely larger than the ones attainable with LEP. This article reviews the schemes that have been proposed. They are all based on the use of linear accelerators so that, before describing them, we write down the general conditions that a few beam parameters have to satisfy in order to reach the wanted luminosity at any given energy. The presentation of the various proposed schemes is as complete as possible; but it is in the nature of a futuristic subject, and we can only list the technological problems that have to be solved in the next decade before electron-positron colliding linacs become the reliable tools that storage rings are nowadays