Characteristics of Directional Coupler Beam Position Monitors

Abstract

The purpose of this paper is to review some of the properties of directional coupler (stripline) beam position monitors. These devices are used extensively as non-intercepting beam position monitors (BPM's) around accelerators, and offer certain advantages over the more conventional electrostatic (split plate) type pickups. The general approach will be from the engineering rather than the physics point of view. Although starting from Maxwell's equations would be more precise, it is also more pedantic, and obscures some of the more important engineering features. Due to lack of space, detailed derivations are not included, but references are given where relevant. The discussion here will be limited to cylindrical geometry couplers, primarily because the expressions are usually simpler than rectangular geometry couplers. In many cases, rectangular geometry would be preferred over cylindrical, but the basic engineering features remain the same.