Observation and simulation of the nonlinear dependence of vacuum pressures on the positron beam current at the KEKB

A non-linear dependence of vacuum pressure on the beam current has been observed in the KEKB positron ring. The behaviour changes with the bunch fill pattern, the bunch current and the external magnetic field, which is similar to that of the beam size blow up phenomenon. A simple simulation on the non-linear pressure rise is proposed assuming that the behaviour occurs by the electron multipactoring. The simulation well reproduces the observed non-linear pressure behaviours. The results support a suggestion that the beam size blow up is caused by the electron cloud around the positron beam