Results and problems in the construction phase of the SuperKEKB vacuum system

The SuperKEKB, the upgrade of the KEKB, is an electron–positron collider with asymmetric energies, that is, 7.0 GeV electrons and 4.0 GeV positrons, designed for a luminosity of 8 × 10³⁵cm⁻² s⁻¹. As a key item of the upgrade project, a new vacuum system for the SuperKEKB has been in construction since 2010. Over 1000 beam pipes, vacuum pumps, bellows chambers, and other various vacuum components had almost been fabricated in 2014. All the new beam pipes were baked as a general rule before being installed in the KEKB tunnel. A thin TiN film with a low secondary-electron yield was coated on the inside of the beam pipes for the positron ring as a countermeasure against the electron cloud effect. The performance of the new built-in nonevaporable getter (NEG) pumps and the step-less connection flanges met the expectation. Although several problems such as damage to the bellows chambers due to a large earthquake, cracking of the welding lines of the aluminum beam pipes, and a relatively high air-leak rate at the connection flanges occurred during the construction work, approximately 99% of the beam pipes were successfully installed by the end of October 2015. The activation of the NEG pumps in the tunnel started at the beginning of 2015 and has also been completed in approximately 97% of the ring. The installed beam pipes and the bellows chambers in the tunnel were aligned to their specified positions as the final step of the installation work. The vacuum system was ready in 2015 with the aim of starting the first commissioning in 2016. The various experiences during the construction phase reported here will be a useful reference for the design and construction of other accelerators in the future.