Studies of dipole field quality for the beta-beam decay ring

The aim of the beta-beams is to produce highly energetic beams of pure electron neutrino and anti-neutrino, coming from beta-decays of the ¹⁸Ne¹⁰⁺ and ⁶He²⁺, both at gamma = 100, directed toward experimental halls situated in the Frejus tunnel. The high intensity ion beams are stored in a ring until the ions decay. The beta-decay products have a magnetic rigidity different from the one of the parent ions and are differently deflected in the 6 T superconducting dipoles. Consequently, all the injected ions are lost anywhere in the ring, generating a high level of irradiation. So, the dipole apertures need to be large enough to avoid the decay products hitting their walls, which may worsen the field quality. A study on its tolerances has been carried out. Since the decay ring has to accept the beam during a large number of turns, the chosen criteria is the size of the dynamic aperture that the multipolar defects in the dipoles may shrink. Tolerances on the systematic errors of these defects have been investigated. In order to relax the tolerances, a routine was written which automatically enlarges the dynamic aperture in presence of field errors.