Electron cloud effect and its cures in the SuperKEKB positron ring

Various countermeasures against the electron cloud effect (ECE) were adopted for the positron ring of SuperKEKB. During Phase-1 commissioning from February to June 2016, however, the ECE was observed, such as a blow up of the vertical beam size. The electron clouds at high beam currents were in the beam pipes at drift spaces in the ring, where antechambers and titanium nitride (TiN) film coating were prepared as countermeasures against ECE. Before starting the next commissioning, permanent magnets and solenoids to generate a magnetic field in the beam direction were attached to the beam pipes as additional countermeasures. Consequently, in the experiment at the end of May during Phase-2 commissioning from March 2018, it was found that the threshold current linear density for exciting ECE increased by a factor of at least 1.5 when compared to that during Phase-1 commissioning.