Continuing study on electron-cloud clearing techniques in high-intensity positron ring: Mitigation by using groove surface in vertical magnetic field

Beam instability caused by an electron cloud is one of the limiting factors in the performance of future advanced positron and proton storage rings. At a wiggler section in the positron ring of the KEK B-factory (KEKB), we installed a test beam chamber with a replaceable insertion and investigated different techniques for the mitigation of electron-cloud effect in a high magnetic field. All techniques were investigated under identical conditions. In this study, the insertions with an isosceles-triangular groove surface and a flat surface were investigated and compared. The groove insertion was composed of grooves running longitudinally along the beam orbit in order to reduce the beam impedance. In this experiment, a large reduction of almost one order of magnitude in the measured electron-cloud current was observed when the groove surface was used instead of the flat one. This is the first experimental demonstration of the concept of the groove surface in a magnetic field. The results are also compared with those obtained in a previous experiment in which a clearing electrode insertion was used.