SLACʹs polarized electron source laser system and minimization of electron beam helicity correlations for the E-158 parity violation experiment

SLAC E-158 is an experiment designed to make the first measurement of parity violation in Møller scattering. E-158 will measure the right-left cross-section asymmetry, ALRMøller, in the elastic scattering of a 45-GeV polarized electron beam from unpolarized electrons in a liquid hydrogen target. E-158 plans to measure the expected Standard Model asymmetry of ∼10−7 to an accuracy of better than 10−8. To make this measurement, the photoemission-based polarized electron source requires an intense circularly polarized laser beam and the ability to quickly switch between right- and left-helicity polarization states with minimal right-left helicity-correlated asymmetries in the resulting beam parameters (intensity, position, angle, spot size, and energy), beamALRʹs. This laser beam is produced by a unique SLAC-designed flashlamp-pumped Ti:Sapphire laser and is directed through a carefully designed set of polarization optics. We analyze the transport of nearly circularly polarized light through the optical system and identify several mechanisms that generate beamALRʹs. We show that the dominant effects depend linearly on particular polarization phase shifts in the optical system. We present the laser system design and a discussion of the suppression and control of beamALRʹs. We also present results on beam performance from engineering and physics runs for E-158.