Design and fabrication of crotch absorbers with beryllium diffuser for Cornell high energy synchrotron source upgrade

Abstract

With the CHESS-U upgrade in 2018, the south arc region of Cornell Electron Storage Ring (CESR) has been replaced with new magnets and vacuum chambers, and CESR will operate with higher beam energy and beam current at lower beam emittance. A new crotch absorber has been designed for the upgrade, based on previous CHESS crotch absorbers. The main body of the crotch absorber is a water-cooled hollow cylinder made of oxygen-free high-conductivity copper, and it has a 5-mm thick beryllium band brazed on the outside in the middle section to reduce the power density it absorbs on the surface. In this paper, we describe the mechanical design, present finite element thermal and structural analysis results with a focus on the region with high temperature and high thermal stress, and discuss the fatigue lifetime of the absorber. For typical operation with 6 GeV beam energy and a 200 mA beam current, the highest temperature on the crotch is approximately 300 °C based on the thermal analysis. Correspondingly, the fatigue lifetime is conservatively estimated to be around 4 × 104 cycles. We also show some details in the fabrication of the crotch assembly, especially about the vacuum furnace brazing joint between the beryllium band and the copper main body, including X-ray fluorescence inspection of the brazing joint. In addition, in order to have a relevant comparison in an effort to estimate the fatigue lifetime, we carry out finite element analyses (FEA) for the low cycle fatigue experiment with beryllium tiles using electron-beam heating of Watson et al. [Fusion Eng. Des. 37, 553 (1997)] in 1990s and provide new understanding for some of their experimental observations. These analyses assure the effectiveness of the design.