Achievements and problems in the first commissioning of superKEKB vacuum system

The first (phase-1) commissioning of SuperKEKB, an asymmetric-energy electron–positron collider at High Energy Accelerator Research Organization, known as KEK, Japan, began in February 2016 and successfully ended in June 2016. Three major tasks of the phase-1 commissioning were as follows: (1) vacuum scrubbing of the new beam pipes in anticipation of a sufficiently long beam lifetime in the next commissioning, (2) checking the stabilities of various new vacuum components at high beam currents of approximately 1 A, and (3) beam tuning to study stability and achieve low emittance. The coefficient of photon stimulated desorption rate, η (molecules photon−1), decreased steadily with increasing photon dose, as expected. The temperature increases of the bellows chambers, connection flanges, etc., were less than 5 °C at 1 A. The effectiveness of an antechamber, a TiN coating, and a clearing electrode to suppress the electron cloud effect (ECE) was confirmed. However, the ECE in the Al-alloy bellows chambers without TiN coating was observed. The use of permanent magnets to create an axial magnetic field successfully suppressed this effect. However, the ECE was still exhibited at higher linear current densities (=bunch current/bunch spacing). Furthermore, the attachment of permanent magnets or solenoids even to the beam pipes with antechambers and TiN coating at drift spaces is under consideration before the next (phase-2) commissioning.