Cavity Beam Position Monitors

Beam-based alignment and feedback systems are essential for the operation of future linear colliders and free electron lasers. A certain number of beam position monitors with a resolution in the submicron range are needed at selected locations. Most beam position monitors detect the electric or the magnetic eld excited by a beam of charged particles at di erent locations around the beam pipe. In resonant monitors, however, the excitation of special eld con gurations by an o -center beam is detected. These structures o er a large signal per micron displacement. This paper is an attempt to summarize the fundamental characteristics of resonant monitors, their advantages and shortcomings. Emphasis will be on the design of cylindrical cavities, in particular on the estimation of expected signals, of resolution limits and the resulting beam distortion. This includes also a short introduction into numerical methods. Fabrication, tuning, and other practical problems will be reviewed brie y. Finally, some resonant devices used for beam position diagnostics will be discussed and listed.