Comparison of Stripline and Cavity Beam Position Monitors

Operational principles of stripline and pill-box cavity beam position monitors (BPMs) are discussed. Their theoretically best achievable position and time resolutions are computed. Numbers are estimated using design values of the proposed e+e− linear collider TESLA. Introduction Important characteristics of a beam position monitor (BPM) are position and time resolutions. Position resolution is the smallest deflection of the beam which a BPM can sense. Time resolution is the time which a BPM needs to be ready for the next bunch detection. For example, if bunch to bunch position measurements should be performed, the BPM time resolution should be shorter than the distance between bunches. Different types of BPMs were developed. Which type to choose depends strongly on the particular task. In the next sections most often used BPM types, stripline and cavity BPMs, are discussed. 1 Stripline BPM A displaced beam produces non-equal current distributions on stripline electrodes. Comparing these non-equal signals allows to detect the bunch position transverse to the beam direction. The electrode of a stripline monitor consists of a transmission line of length L and width 2(φ− θ) which is mounted into the beam pipe, see fig.s