Measurement of longitudinal beam polarization by synchrotron radiation

A method to measure the longitudinal polarization of an electron beam using synchrotron radiation is proposed. Quantum theory predicts that a longitudinally polarized electron emits a slightly different number of synchrotron photons into the space above and below the orbit plane. The degree of this asymmetry is proportional to the magnetic field strength and the ratio between photon and electron energy. For CEBAF (electron energy E_e=4 GeV and 100 μA beam current), a dedicated bending magnet with the field strength of B=6.25 T permits us to measure a 100% polarized beam with an accuracy of about 1% in one second when the measurement is performed with photons in the range between E_γmin=6.0 keV and E^γmax=600 keV. The technique for measuring this asymmetry is also discussed