Verification and remaining problems of the countermeasures against the electron cloud effect in the SuperKEKB positron ring

The electron cloud effect (ECE) was observed in the Phase-1 commissioning (February – June, 2016) of the SuperKEKB positron ring. The electron cloud was considered to be formed in the beam pipes at drift spaces, which have antechambers and TiN coating as the countermeasures against ECE. Before starting the Phase-2 commissioning (March – July, 2018), permanent magnets or solenoids, which make axial magnetic fields of several ten gausses, were attached along the beam pipes as additional countermeasures. The effect of the magnets was verified in the Phase-2 commissioning. At the same time, the maximum secondary electron yield (SEY) of the TiN coating and the photoelectron suppression effect of the antechambers were evaluated from the simulations and the experiments using beams during the Phase-2 commissioning. (author)