The Longitudinal Polarimeter at HERA

The design, construction and operation of a Compton back-scattering laser
polarimeter at the HERA storage ring at DESY are described.
The device measures the longitudinal polarization of the electron
beam between the spin rotators at the HERMES
experiment with a fractional systematic uncertainty of 1.6 %. A measurement
of the beam polarization to an absolute statistical
precision of 0.01 requires typically one minute when the device is operated
in the multi-photon mode.
The polarimeter also measures the polarization of each individual
electron bunch to an absolute statistical precision of 0.06 in
approximately five minutes.
It was found that colliding and non-colliding bunches can
have substantially different polarizations.
This information is important to the collider
experiments H1 and ZEUS for their future longitudinally polarized
electron program because those experiments use the colliding
bunches only.