A thin beryllium injection window for CESR-C

The e^+- collider CESR is expanding its beam energy range to below 2 GeV for the planned Charm Physics Program. The original 5 GeV injection beam lines consisted of two 0.025-mm thick titanium windows, separated by a helium line at atmospheric pressure. The beam injection efficiency at low energies is reduced dramatically due to excessive emittance growth from multiple scattering in the Ti windows. The Ti windows were replaced with 0.075-mm thick beryllium windows to restore injection efficiency at low beam energies. This paper describes the design and construction of the Be windows. Finite-element analysis was used to determine the minimum Be window thickness able to support atmospheric pressure across 24-mm diameter aperture. The Be window housing is a cylinder consisting of two stainless steel tubes (SST) vacuum brazed to each end of a copper ring. The Be window is subsequently vacuum brazed to the copper ring at a lower braze temperature. The SST ends of the window assembly provide mechanical strength and protection to the fragile window, and facilitate welding of the window assembly to the injection vacuum chambers. Injection efficiency close to 70% was achieved with the beryllium injection windows in CESR.