Microwave Beam Position Monitors at SLAC

The design and performance of three types of nonintercepting microwave beam position monitors are described. The monitors locate the transverse centroid of the bunched beam. Monitors of the first two types each consist of two orthogonal position-sensing cavities and one reference cavity. The microwave signals induced by the beam in these cavities are combined in two phase bridges and linearly detected. The video signals are normalized with respect to beam current and transmitted to two control buildings where they are displayed. Thirty-three monitors along the linear accelerator have 0.8-inch apertures, and six special monitors at the end of the machine have 2.0-inch apertures. A third type of monitor has been installed in one experimental area. Waveglide couplers on a 3-inch diameter beam drift tube feed signals to a phase bridge. The bridge sum and difference signals are amplified, detected, and differentially displayed on an oscilloscope. The device can detect 0.004-inch changes in position of a 0.01 mA beam with a response time of 50 ns.