Layout of the ELSA U-turn

In order to contain the ELSA [1] project inside an existing building, and to provide the possibility of a magnetic compression of the electron bunches, a U-turn has been designed. Careful studies have been done to prevent as much as possible geometric and chromatic aberrations. Space charge effects in dipoles have been evaluated, and a first-order correction is proposed. Global optimisation is realized by matching the beam with sets of quadrupoles and by careful studies of the fringing fields. As a four-cell achromat could not be installed in the building, a solution with a vertical symmetry plane has been studied.