RHIC Vacuum Pressure Bump

Abstract

Effects of the beam loss, ion and electron gas desorption on the RHIC vacuum pressure bump have been reviewed. The beam loss and ion desorption are probably not the dominant factors in the RHIC vacuum problem. On the other hand, electron desorption, given the electron multipacting, can cause serious vacuum problem. In general, the electron multipacting was not predicted for the RHIC according to the present understanding of the mechanism. However, beam loss created ions can extend electron lifetime during the bunch gap, and electron multipacting becomes possible at the RHIC. Also, the pressure rise in the RHIC Gold beam run was much worse than the usual electron cloud induced vacuum pressure rise, which was observed in many machines. One possibility is that the beam loss created ions have raised the electron density saturation limit and, therefore, contributed to further pressure rise. Some complications in diagnostics of electron multipacting by coherent tune shift in bunches will be discussed. Electron collection may indicate the coincidence between the electron multipacting and the pressure rise. It also can help to determine the electron density.