Beam energy measurement at linear colliders using spin precession

Linear collider designs foresee some bends of about 5–10 mrad. The spin precession angle of 1 TeV electrons on a 10 mrad bend is 23.2 rad and it changes proportional to the energy. Measurement of the spin direction using Compton scattering of laser light on electrons before and after the bend allows determining the beam energy with an accuracy of about 10^-5. In this paper the principle of the method, the procedure of the measurement, and possible errors are discussed. Some remarks about the importance of plasma focusing effects in the method of beam energy measurement using Moller scattering are given.