In accelerators, collimators are used to shape the beam and protect the machine against damage. They have however an adverse effect on the beam quality. We present a trapped-mode analysis of the new collimators for PETRA IV and their impact on beam dynamics. Wakefield and eigenmode simulations are used to determine the beam-coupling impedance of the collimators. From that, the quality factor, shunt impedance and normalised shunt impedances are extracted. The results show that higher-order modes make a significant contribution to the beam-coupling impedance of the structure. It is shown that geometric modifications can effectively reduce the beam-coupling impedance. Based on the found modes, beam stability calculations show stable behaviour.

In accelerators, collimators are used to shape the beam and protect the machine against damage. They have however an adverse effect on the beam quality. We present a trapped-mode analysis of the new collimators for PETRA IV and their impact on beam dynamics. Wakefield and eigenmode simulations are used to determine the beam-coupling impedance of the collimators. From that, the quality factor, shunt impedance and normalised shunt impedances are extracted. The results show that higher-order modes make a significant contribution to the beam-coupling impedance of the structure. It is shown that geometric modifications can effectively reduce the beam-coupling impedance. Based on the found modes, beam stability calculations show stable behaviour.