Measurement of Small Electron Beam Spots

Measurements of transverse beam size are tremendously important to the performance of ee linear colliders. In this paper we review the traditional technologies used to make such measurements, such as pro le monitors and wire scanners, and the limitations on same. We then introduce a new tool for electron beam size measurement: Compton-scattered laser light, which may be used as an unbreakable \wire" or in the form of an interferometer beam size monitor. We describe the use of such an interferometer BSM, noting both the general issues related to its design and operation and the speci c experiences with such an interferometer at the Final Focus Test Beam (FFTB) at SLAC. We conclude by considering the ultimate limits of the laser-interferometer BSM. Submitted to Annual Review of Nuclear and Particle Science. Work supported by the Department of Energy, Contract DE-AC03-76SF00515.