Vacuum chambers for Swiss Light Source arcs

The vacuum chambers for diffraction limited storage ring differ from the previous storage rings generation in three main aspects: the cross-section dimension, which is divided by a factor of 2 or more to fit smaller magnet apertures; the material, which includes much more copper for heat dissipation and to limit resistive wakefields; the coating of the inner surface with a nonevaporable getter (NEG) to ensure good pumping level despite low conductance. This paper gives a detailed description of the vacuum chambers’ design used in the arc section of SLS 2.0, starting with conceptual design choices based among others on synchrotron radiation heat and wakefield considerations. The particular case of the main bending magnet vacuum chamber is explained in detail from design to manufacturing, including the development of an appropriate NEG coating procedure. Finally, the overall assembly of a 17 m long arc, its activation to reach pressure in the ${10}^{-11}\mathrm{mbar}$ range followed by transport and installation into the magnets is presented. This validates the choice of ex situ activation for an arc vessel made out of copper, with wall thicknesses as small as 1 mm and with less than 0.5 mm clearance to magnet poles.