Lcls Resonant Cavity Beam Position Monitors *

The Linac Coherent Light Source (LCLS) is a freeelectron laser (FEL) at SLAC producing coherent 1.5 Å x-rays. This requires precise, stable alignment of the electron and photon beams in the undulator. We describe construction and operational experience of the beam position monitor (BPM) system which allows the required alignment to be established and maintained. Each X-band cavity BPM employs a TM010 monopole reference cavity and a single TM110 dipole cavity detecting both horizontal and vertical beam position. The processing electronics feature low-noise single-stage three-channel heterodyne receivers with selectable gain and a phase-locked local oscillator. Sub-micron position resolution is required for a single-bunch beam of 200 pC. We discuss the specifications, commissioning and performance of 36 installed BPMs. Single shot resolutions have been measured to be about 200 nm rms at a beam charge of 200 pC.