Chirping the LCLS Electron Beam

We explore scenarios for generating a linear time-correlated energy spread in the LCLS electron bunch, prior to the undulator, that is needed for optical (x-ray) pulse compression. The correlated energy spread (‘chirp’) is formed by generating an energy gradient along the length of the electron bunch using RF phasing and/or longitudinal wakefields of the accelerating structures. The sign of the correlation is an important limitation. Excluding a complete re-design of the compression systems, the best possibility is to use ‘over-compression’ to effect the required energy chirp. This is easily done with only a slight strength increase (~10 %) in the chicane bends of the second compressor. In this case, the bend-plane emittance dilution associated with the increased coherent synchrotron radiation (CSR) in the bunch compressor may, however, significantly compromise the electron beam density. The CSR calculations for the momentary extremely short (~1 m) electron bunch during over-compression are quite subtle and an adequate confidence level may not be achievable. A practical limit in this short-pulse scenario may be to use spontaneous rather than FEL radiation. Ignoring the potential emittance growth, a FWHM electron energy spread of 2 % is possible.