Measurement of a weak polarization sensitivity to the beam orbit of the CEBAF accelerator

An accelerator-based experiment was performed using the CEBAF accelerator of the Thomas Jefferson National Accelerator Facility to investigate a predicted sensitivity of the beam polarization to the vertical betatron orbit in the recirculation arcs. This is the first measurement of any such effect at CEBAF, and provides information about the polarized beam delivery performance of the accelerator. A brief description of the accelerator is given, followed by the experimental methods used and the relevant issues involved in measuring a small (∼10−2) change in the beam polarization. Results of measurements of the polarization sensitivity parameters and the machine energy by polarization transport techniques are presented. The parameters were obtained by measurement of the strength of the effect as a function of orbit amplitude and spin orientation, to confirm the predicted coupling between the spin orientation and the quadrupole fields in the beam transport system. This experiment included characterizing the injector spin manipulation system and 5 MeV Mott polarimeter, modeling of the polarization transport of the accelerator, installation of magnets to create a modulated orbit perturbation in a single recirculation arc, and detailed studies of the Hall C M&phis;ller polarimeter.