CROSSING OF DEPOLARIZING RESONANCES IN CIRCULAR ELECTRON ACCELERATORS

Inflat electronstoragerings, onlytheverticalcomponent of the beam polarization is preserved. During acceleration, the crossing of several depolarizing resonances may cause severe beam depolarization. Even in case of fast ramping speeds of upto 4 GeV/s, first ordereffects like imperfection and intrinsic resonances have to be compensated by dedicated measures. At the accelerator facility ELSA, schemes like fast tune jumping and harmonic orbit correction are successfully applied on the fast energy ramp up to 2.4 GeV. Characteristics of the setup as well as the optimization efforts to improve the resonance compensation will be reported in detail. MOTIVATION Currently, experiments using ultrarelativistic polarized electrons become more and more important to explore the fundamental physics in the femtoscopic scales. Offering multi-GeV electrons with a high degree of polarization for those experiments is still a demanding task for designing and operating accelerators. In circular accelerators, the particles pass the magnetic element lattice periodically and thus occuring depolarizing resonances do not only have to be taken into account but they also have to be corrected for. Polarized electrons in circular accelerators can be obtained by storing electrons and waiting for self polarization according to Sokolov and Ternov. The synchrotron light emission can cause spin flips, in which the flip towards spin-up state is preferred. In case of equilibrium a mean degree of polarization of max. 92.4 % can be obtained. As just one in approx. 10 10 emitted photons causes a spin flip, the self polarization time for lower GeV-range electrons typically exeeds several hours depending on the bending radius. The more efficient way is to generate a polarized ensemble of electrons in a dedicated source and to further accelerate it using cavities till the particles reach the desired energy. In a flat ring and according to the Thomas-BMT equation, the spin tune aγ(t) is given by the particle energy. Correspondingly,the spin precessionfrequency aγ(t)f0 varies in time depending on the gyromagnetic anomaly a, the commonly linear increase of the Lorentz factor γ(t) and the revolution frequency f0. If the spin precesses in the vertical plane in phase with arising horizontal magnetic fields,