First attempt of the measurement of the beam polarization at an accelerator with the optical electron polarimeter POLO

The conventional methods for measuring the polarization of electron beams are either time consuming, invasive or accurate only to a few percent. We developped a method to measure electron beam polarization by observing the light emitted by argon atoms following their excitation by the impact of polarized electrons. The degree of circular polarization of the emitted fluorescence is directly related to the electron polarization. We tested the polarimeter on a test GaAs source available at the MAMI electron accelerator in Mainz, Germany. The polarimeter determines the polarization of a 50 keV electron beam decelerated to a few eV and interacting with an effusive argon gas jet. The resulting decay of the excited states produces the emission of a circularly polarized radiation line at 811.5 nm which is observed and analyzed.