Proton beam emittance growth at RHIC

With significant beam intensity improvement in RHIC polarized proton runs in 2005 and 2006, the emittance growth becomes a luminosity limiting factor. The beam emittance growth has a dependence on the dynamic pressure rise, which in RHIC proton runs is mainly caused by the electron cloud. The beam instability is usually absent, and the emittance growth rate is much slower than the ones caused by the head-tail instability. It is suspected that the emittance growth is caused by the electron cloud below the instability threshold.