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Beam Stabilization in the SLAC A-line Using a Skew Quadrupole


The E158 experiment at SLAC is a precision measurement of the left-right asymmetry in M\o ller scattering at low $Q^2$ utilizing a high-current long-pulse polarized electron beam scattering off unpolarized electrons in a liquid hydrogen target. Tolerances on beam size and position/angle stability for E158 are extremely tight, but the electron beam is subject to intensity jitter, dispersion, and wakefield effects in the linac which tend to make it unstable. Horizontal emittance growth due to synchrotron radiation in the transport line from the linac to the target (``A-line'') reduces the sensitivity of the horizontal beam parameters at the target to incoming changes, but instability in the vertical plane was observed during the E158 pilot run. A skew quadrupole recently installed in the A-line 90 m upstream of the target has been used to couple the projected transverse emittances, increasing the vertical emittance of the beam and thereby reducing its sensitivity to incoming changes. Simulations of the performance of this skew quadrupole, along with measured beam data with and without the skew quadrupole, will be presented.

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