INVESTIGATION OF TECHNIQUES FOR PRECISE COMPTON POLARIMETRY AT ELSA

A Compton polarimeter is currently being installed at the Electron Stretcher Facility ELSA to monitor the degree of polarization of the stored electron beam. For this purpose, circularly polarized light that is emitted by a laser and backscattered off the beam has to be detected. When the polarization of the laser light is switched from left-hand to right-hand circular polarization, the spatial distribution of the backscattered photons is shifted. The extent of this modification is a measure of the beam’s polarization degree. Two different experimental techniques that are suitable for a measurement of the effect were compared and evaluated closer through numerical simulations that will be presented in this contribution. MOTIVATION The fast ramping electron stretcher ring at ELSA is able to supply a spin polarized electron beam of up to 3.5 GeV to hadron physics experiments. The degree of polarization can be measured directly after the 50 keV electron source via a Mott polarimeter as well as at the external beamline by a Moller polarimeter. During the acceleration process, depolarizing resonances in the circular accelerators lead to a polarization loss. In order to minimize this loss, correction measures which require information on the polarization of the stored electron beam have to be undertaken. Since both monitoring methods currently in use require scattering targets, neither of them is appropriate for a parasitic measurement of the polarization degree in the stretcher ring during operation. Compton polarimetry, by contrast, offers a fast and accurate possibility to determine polarization with negligible electron loss and without other significant negative impact on the beam. Figure 1 shows the position of all polarimeters in the stretcher ring.