Design of a Superconducting Longitudinal Gradient Bend Magnet for the SLS Upgrade

The Paul Scherrer Institute is planning a major upgrade of the Swiss Light Source (SLS-2) with the development of a new storage ring lattice providing ultralow emittance while maintaining the locations of the undulator-based beam lines, in order to meet the growing demand of the users for higher photon flux and brilliance. The preliminary design of the lattice features a multibend achromat with seven bending magnets in each of the 12 arcs. This compact low-emittance lattice will include the use of high field longitudinal gradient bends (LGB), whose design and fabrication are particularly challenging. The superconducting dipole will have a hyperbolic longitudinal field variation with a narrow peak (FWHM of about 40-70 mm) to limit energy spread and the radiated power. The total magnet length should not exceed 400 mm with a field integral of around 0.6 Tm. The peak field value at the beam position has to be around 6 T. The LGB has to host a room temperature vacuum chamber and an antichamber to release the synchrotron radiation. This requirement determines the magnet aperture and with it the peak field at the conductor location that drives the choice of the superconducting material for the winding. In this paper, the design of the LGB magnet is presented.