Design and Performance Simulations of the Bunch Compressor for the APS LEUTL FEL

A magnetic bunch compressor has been designed and is being commissioned for the APS linac to provide higher peak current for the LEUTL FEL. Of primary concern is control of emittance growth due to coherent synchrotron radiation (CSR). In addition, tolerances must be carefully evaluated in order to choose the most stable operating conditions and ensure that the system can meet operational goals. The computer code ELEGANT was used to design the bunch compressor, set tolerances, and simulate likely performance. This 6-D code includes a fast, simple simulation of CSR effects as well as longitudinal and transverse wakefields. It also is believed to be unique in allowing optimization of actual tracking results, such as bunch length, energy spread, and emittance. Automated setup and analysis of matching and tolerance simulations allowed consideration of numerous configurations without excessive effort. This permitted choosing configurations that reduce sensitivity to errors. Simulations indicate that emittance growth should be tolerable for up to 600 A peak current, for which the normalized emittance will increase from 5 to about 6.8 pi-um. The simulations also provide predictions of emittance variation with chicance parameters, which we hope to verify experimentally. * Work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. W-31-109-ENG-38