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In a previous work we proposed a scheme for polarization control at the LCLSbaseline, which exploited the microbunching from the planar undulator. Afterthe baseline undulator, the electron beam is transported through a drift by aFODO focusing system, and through a short helical radiator. The microbunchingstructure can be preserved, and intense coherent radiation is emitted in thehelical undulator at fundamental harmonic. The driving idea of this proposal isthat the background linearly-polarized radiation from the baseline undulator issuppressed by spatial filtering. Filtering is achieved by letting radiation andelectron beam through Be slits upstream of the helical radiator, where theradiation spot size is about ten times larger than the electron beam transversesize. Several changes considered in the present paper were made to improve theprevious design. Slits are now placed immediately behind the helical radiator.The advantage is that the electron beam can be spoiled by the slits, andnarrower slits width can be used for spatial filtering. Due to this fundamentalreason, the present setup is shorter than the previous one. The helicalradiator is now placed immediately behind the SHAB undulator. It is thussufficient to use the existing FODO focusing system of the SHAB undulator fortransporting the modulated electron beam. This paper presents complete GENESIScode calculations for the new design, starting from the baseline undulatorentrance up to the helical radiator exit including the modulated electron beamtransport by the SHAB FODO focusing system.

Microbunch preserving in-line system for an APPLE II helical radiator at the LCLS baseline