LCLS Injector Straight-Ahead Spectrometer

Abstract

SummaryThe spectrometer design was modified to allow the measurement of uncorrelated energy spread for the nominal lattice. One bunch from every 120 each second would be sent to the straight ahead spectrometer while the transverse cavity is on. The implementation of this "stealing mode" will not be available for the LCLS commissioning and the early stage of operation. However, the spectrometer was redesigned to retain that option. The energy feedback relies independently on the beam position of the beam in the dispersive section of dogleg 1 (DL1).The main modification of the spectrometer design is the Pole face rotation of 7.5 degrees on both entrance and exit faces. The location and range of operation of the 3 quadrupoles remains unchanged relative to those of the earlier design. IGeometry and Dimensions Optimization of optical functionMAD was used to optimize the straight-ahead-spectrometer parameters, because the space charge effects are nearly negligible. In the dispersive region, the rms beam size is the quadratic sum of the emittance term and the dispersive term, as shown in equation (1) 2 2 δ σ βε σwhere β is the betatron function ε the beam emittance D the dispersion function σ δ the rms of the relative energy spread (uncorrelated)To achieve a good resolution of the uncorrelated energy spread σ δ , the emittance term, βε, needs to be smaller than the dispersive term. For our nominal 1nC tuning, at 135MeV, the emittance ε is expected to be 1mm-mrad. To resolve a σ δ = 3keV, this condition is then