Paper VT-TuM5
The Vacuum Commissioning and Simulation of Non-Evaporable Getter Dominated Cornell High Energy Synchrotron Source Upgrade.

Tuesday, October 22, 2019, 9:20 am, Room A213

The Cornell High Energy Synchrotron Source Upgrade (CHESS-U) converts the Cornell Electron Storage Ring (CESR) from dual-beam to single-beam operation while significantly reducing the beam emittance, increasing the beam energy to 6 GeV, and improving the facility’s X-Ray beamline brightness.

The CHESS-U vacuum system was completed in the fall of 2018 and the initial beam current and energy targets were met in the spring of 2019. The majority of the CHESS-U vacuum system consists of narrow gap aluminum chambers. With pre-installation 150 C bake followed by in situ 95 C hot-water bake and relying on the high pumping speed of distributed and lumped Non-Evaporable Getters (NEGs), a low 10^-9 Torr base pressure was quickly achieved.

Since the CHESS-U vacuum pumping system is NEG-dominated and NEGs are prone to surface saturation at high synchrotron radiation (SR) induced gas loads, the vacuum conditions during the CHESS-U accelerator commissioning were carefully monitored and periodical vacuum simulations using MolFlow were performed to ascertain the status of the NEGs. The SR-induced vacuum conditioning has proceeded very well, with the dynamic pressure holding in the low 10^-9 range with 100 mA stored positron beam current, after an accumulated beam dose of 20 A-hr. With the moderate initial beam conditioning, a beam lifetime allowing X-ray beam operation to commence has already been achieved. Further gradual improvements in the dynamic pressure and beam lifetime are expected during the course of X-ray user operations.

In this paper, we describe the CHESS-U vacuum system, report on the SR-induced vacuum conditioning status, and detail the computational model developed to accurately simulate the vacuum conditions while taking into account the NEG saturation and the radiation-induced cleaning of the chambers.