LCLS Bunch Compressor Configuration Study for Soft X-ray Operation

The Linac Coherent Light Source (LCLS) is originally designed and commissioned with two bunch compressors (BC1 and BC2) to achieve the desired bunch length and peak current prior to the undulator line. The dispersive properties of bunch compressors enhance the growth of electron beam micro-bunching, so BC1 and BC2 provide important tuning parameters to suppress micro-bunching instability (MBI). Given the less stringent peak current requirement for soft x-ray operation, we explore new bunch compressor configurations to reduce undesired micro-bunching. New configurations include using BC1 or BC2 only, and different compression distributions between the two bunch compressors. We have developed a simplified theoretical model to characterize the micro-bunching gain under various bunch compressor configurations. Meanwhile, we require reasonable longitudinal phase space and beam current to ensure the applicability of the new schemes. We have experimentally tested the feasibility of the new configurations and obtained promising preliminary data that indicate better MBI suppression for soft x-ray operation.