SuperKEKB main ring vacuum system status until the end of 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. The main ring (MR) consists of two rings: a 4 GeV positron ring (low-energy ring, LER) and a 7 GeV electron ring (high-energy ring, HER). During the Phase-1 commissioning of the MR from February to June 2016 and the Phase-2 commissioning from March to July 2018, the vacuum system worked well overall. By the end of Phase-2, the total beam doses (integrated beam currents) were 1113 and 1002 Ah, and the maximum beam currents were approximately 1010 and 870 mA for the LER and HER, respectively. The increase in pressure per unit beam current decreased steadily. The photodesorption coefficients reached approximately 1 × 10−6 and 7 × 10−8 molecules photon−1 with photon doses of 5.88 × 1024 and 9.27 × 1024 photons m−1 for the LER and HER, respectively. The beam lifetime was limited by the Touschek effect in both rings. Various new vacuum components, including those installed after the Phase-1 commissioning, such as the beam pipes for the Belle II particle detector, beam collimators, and so on, have been functioning well so far. The electron cloud effect was observed in the LER during Phase-1, but it was suppressed by additional countermeasures applied before Phase-2. Regarding the pressure bursts accompanied with beam losses, which were frequently observed during Phase-1, the frequency of the pressure bursts drastically decreased during Phase-2, but the longer operation time with lower beam currents during Phase-2 than Phase-1 should be taken into account. Several new problems, such as the heating of stainless-steel beam pipes due to the synchrotron radiation emitted from the final focusing quadrupole magnets at the collision point, were found during Phase-2, and countermeasures are being prepared for the Phase-3 commissioning, which will commence in March 2019. SuperKEKB has been accumulating various data and information that will facilitate the design and operation of the vacuum systems in future high-current accelerators.