A sudden step change is observed in the vertical position readings from beam position monitors (BPMs) mounted at the several sector vacuum chambers of Pohang Light Source. To study the source of this sudden step change, we measured the RF transmission scattering matrix (S₂₁) through the pickup electrodes of BPMs mounted at the both ends of the sector vacuum chamber. The measured S₂₁ graph of the sector vacuum chambers suffering sudden step change has a peak in the BPM operation frequency bandwidth. Otherwise that of the other sector vacuum chambers doesn’t have a peak. It is shown by the numerical simulation that the peak found in the BPM operation frequency bandwidth corresponds to the longitudinal harmonic of transverse electric resonance mode.

Keywords: Beam position monitor,Transverse electric field mode,Vacuum chamber,Pohang light source,빔위치측정기,가로전장모드,진공용기,포항광가속기