LCLS STRIPLINE BPM SYSTEM COMMISSIONING *

The Linac Coherent Light Source (LCLS) began x-ray laser operation this year. Stripline BPM system requirements include position resolution better than 5 microns for single-bunch beam charge of 200 pC. We describe the processing scheme, system specifications, commissioning experience, and performance measurements. INTRODUCTION The LCLS requires better BPM resolution at lower beam charge than did previous users of the SLAC linac. We estimated that the existing BPM striplines could be used with a redesigned BPM processor to achieve the required resolution (Table 1) with improved systematics. We chose to implement the processor by developing a four-channel analog front-end (AFE) board to shape, amplify, and provide calibration of the stripline BPM signals for digitization on a 4-channel waveform digitizer board developed for LCLS low-level RF. Table 1: LCLS BPM System Requirements Parameter Value Comments Dynamic Range 0.08 – 8 nC 40 dB Resolution 5 microns @ 0.2 nC Stability 5 microns Over 1 hour Requirements and Simulation The BPM processor shapes and digitizes the stripline signals. The signal acquisition is expected to perform over a 0.08 8 nC dynamic range with a resolution better than 5 microns at 200 pC and stability of less than 5 microns offset drift over an hour. Calculations in MathCAD and Matlab, and simulation in SystemView showed that the processor should have a few low-noise RF amplifiers, an overall gain about 34 dB, a relatively broadband front filter and a narrow band backend filter. Programmable attenuators, distributed between amplifiers, are adjusted for optimal gain while keeping amplifiers out of saturation [1].