A linac-driven femtosecond polarized x-ray source

In recent months a novel femtosecond X-ray source, the Sub-Picosecond Photon Sourcxe (SPPS), has been commissioned at SLAC. The source consists of moderate-emittance electron bunches extracted from the SLAC Damping Rings, an acceleration through the SLAC 3km linac up to ~28 GeV, compression through an initial compressor chicane followed by further compression through an existing SLAC dogleg in the Final Focus Test Beam (FFTB) tunnel, and generation of X-ray pulses of about the same temporal length (~80 fs) by an APS-built 28 period transverse undulator. In this note we consider the possibility of extending the capabilities of the SPPS to produce femtosecond pulses of arbitrary polarization, which would enable the impulsive excitation and study of ultrafast magnetic phenomena, a field hitherto inaccessible to experimental science. An analysis of the expected performance of the proposed source together with requirements for linac operation are presented.