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SLAC's Polarized Electron Source Laser System and Minimization of Electron Beam Helicity Correlations for the E-158 Parity Violation Experiment


SLAC $\text{E-158}$ is an experiment designed to make the first measurement of parity violation in M\o ller scattering. $\text{E-158}$ will measure the right-left cross-section asymmetry, $A_{LR}^{M\o ller}$, in the elastic scattering of a 45-GeV polarized electron beam off unpolarized electrons in a liquid hydrogen target. $\text{E-158}$ plans to measure the expected Standard Model asymmetry of $\sim 10^{-7}$ to an accuracy of better than 10$^{-8}$. To make this measurement, the polarized electron source requires for operation 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), $^{\hbox{beam}}{\hbox{A}_{\hbox{LR}}}$'s. This laser beam is produced by a unique SLAC-designed flashlamp-pumped Ti:Sapphire laser and is propagated 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 $^{\hbox{beam}}{\hbox{A}_{\hbox{LR}}}$'s. 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 $^{\hbox{beam}}{\hbox{A}_{\hbox{LR}}}$'s. We also present results on beam performance from engineering and physics runs for $\text{E-158}$.

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