Design of shunt structure to control transverse electric resonance modes in vacuum chamber of storage ring

It is a significant issue to control the unwanted transverse electric (TE) modes in the vacuum chamber for preventing the position reading noise from the beam position monitor (BPM) of the storage ring. We introduce shorting bars, called as shunt structure, in the shade region of synchrotron radiation to control the frequency distribution of TE resonance modes excited in the sector vacuum chambers of the Pohang Light Source II (PLS-II) storage ring. The design of shunt structure is carried out using full three dimensional finite-difference time-domain (FDTD) eigenmode simulation, in order that the TE resonance modes appear on the outside of the BPM operation frequency band of PLS-II storage ring without large reduction of vacuum conductance, increasing of beam impedance and major mechanical modification. It is verified in the measurement of the RF reflection scattering matrix (S11) through the pickup electrode of BPM using a vector network analyzer that the peak frequencies in the S11 graph are found out of the BPM operation frequency band and well matched with the eigenmode frequencies obtained by the FDTD simulation.