Aperture and Field Constraints for the Vacuum System in the LHC Injection Septa

Each beam arriving from the SPS has to pass through five injection septum magnets before being kicked onto the LHC orbit. The injection layout implies that the vacuum chambers for the two circulating beams pass through the septum magnet yokes at a flange distance from the chamber of the beam to be injected. Specially designed vacuum chambers and interconnections provide the required straightness and alignment precision, thus optimising the aperture for both the circulating and injected beams, without affecting the quality of the magnetic dipole field seen by the injected beam. The circulating beams are shielded against the magnetic stray field by using μ-metal chambers with a thickness of 0.9 mm to avoid saturation of the μ-metal (0.8 T), coated with copper (0.4 mm) for impedance reasons and NEG for pumping and electron cloud purposes. A 2 mm gap between the iron yoke and the μ-metal chamber allows an in-situ bake-out at 200° C, based on a polyimide/stainless steel/polyimide sandwich structure with an overall thickness of 0.2 mm. The constraints are described and the resulting vacuum system design, the apertures and the residual stray field are presented in this paper.