Vacuum chamber with distributed titanium sublimation pumping for the G-line wiggler at Cornell High Energy Synchrotron Source

This article describes a 3-m-long vacuum chamber for the new wiggler magnet at the Cornell Electron Storage Ring (CESR) for the synchrotron light beam line of the Cornell High Energy Synchrotron Source (CHESS). Copper was chosen as the main chamber material for its good electric and thermal conductivities. Proper mechanical design and welding procedure were implemented to meet very tight tolerances to ensure adequate vertical aperture for the stored beams in CESR while allowing the required small wiggler gap. Distributed titanium sublimation pumping is incorporated along the 3 m length of the chamber to provide sufficient pumping speed and capacity for CESR and CHESS operations. The chamber pumping performance was evaluated prior to installation. Linear distributed pumping speeds at the beam line of ∼720 l/s/m for $N2$ and CO and ∼4000 l/s/m for $H2$ were measured. The measured pumping capacities for $N2,$ CO and $H2$ are ∼1.0, ∼2.0 and ∼77 Torr l, respectively, for each titanium sublimation cycle. Measurements also showed that CO molecules adsorb on the $N2$ and $H2$ saturated titanium films with virtually the same initial sticking coefficient as on a fresh titanium film. Analyses indicated very different CO adsorption mechanisms between the $N2$ and $H2$ saturated titanium films. While the replacement of surface $H2$ by CO was observed, little desorption of nitrogen was measured. Operational experience showed excellent vacuum pumping performance over two years after the chamber installation.