A wide range of nucleon and nuclear structure experiments in Jefferson Lab's
Hall A require precise, continuous measurements of the polarization of the
electron beam. In our Compton polarimeter, electrons are scattered off photons
in a Fabry-Perot cavity; by measuring an asymmetry in the integrated signal of
the scattered photons detected in a GSO crystal, we can make non-invasive,
continuous measurements of the beam polarization. Our goal is to achieve 1%
statistical error within two hours of running. We discuss the design and
commissioning of an upgrade to this apparatus, and report preliminary results
for experiments conducted at beam energies from 3.5 to 5.9 GeV and photon rates
from 5 to 100 kHz.Comment: 6 pages, 7 figures. To appear in the Proceedings of the International
Nuclear Physics Conference (INPC 2010), July 4-9 2010, Vancouver, Canada
(Journal of Physics: Conference Series