Trajectory simulation of multipactoring electrons in an S-band pillbox RF window

The behavior of multipactoring electrons in an S-band pillbox RF window used in the KEK electron linac was investigated. The electric field distributions in the pillbox were calculated using the MAFIA code. The trajectory of emitted electrons to the surface of an alumina window disk in the calculated electric field was simulated. The energy deposition distribution of electrons impinging on the alumina disk was also obtained. The results agree well with the discoloring patterns observed for alumina disks after RF operation. This shows that the discoloring can be explained in terms of the bombardment of single-surface multipactoring electrons. Knowledge concerning the electric field distribution and the behavior of multipactoring electrons in the pillbox RF window will be useful in designing RF windows from an electrical point of view