Status of SuperKEKB Vacuum System during Phase-2 Commissioning

Abstract

SuperKEKB is an electron–positron collider with asymmetric energies at KEK aiming at a high luminosity of 8.0×10³⁵ cm^－2 s^－1. During the Phase-1 commissioning from February to June 2016 and the Phase-2 commissioning from March to July 2018, the vacuum system worked well as a whole, and the increase in pressure per unit beam current decreased steadily. The photodesorption coefficients at arc sections reached approximately 1×10^－6 molecules photon^－1 and 7×10^－8 molecules photon^－1 finally for the positron and electron rings, respectively. The beam lifetime was mainly limited by the Touschek effect in both rings, rather than the beam-gas scattering. The electron cloud effect was observed in the positron ring during Phase-1, but it was suppressed by additional countermeasures applied before Phase-2.