An optimized design of the ELISe spectrometer for the electron/RI collider ex- periment at FAIR*

For electron scattering off radioactive nuclei, the ELISe spectrometer will be constructed as a part of FAIR. The system consists of a Pre-Deflector (PD), a Quadrupole (Q), a Hexapole (H) and a vertically bending dipole Magnet (VM). In the conceptual design of the spectrometer, the proposed iron poles of PD are similar to a clam-shell gap to accept large azimuthal scattering angles [1]. However, this system is rather complicated to build and operate in order to keep and scale homogeneous magnetic fields. Besides, the ion-optics calculations show that the solid angle is too small, particularly at angles smaller than 15 due to the strong edge focusing at the front edge of PD [2]. In order to overcome these difficulties, we propose to change the design of PD to the constant gap [3]. Fig.1 shows the mid-plane of the constant gap PD. The main coils are wound around the iron-poles.