Broadband impedance calculations of the TAN vacuum chamber for the Large Hadron Collider (LHC)

We report research studies on broadband impedance calculations of the TAN vacuum chamber for the Large Hadron Collider (LHC). The TAN is the zero degree neutral particle absorber and contains the transition from a single vacuum beam tube to two beam tubes. Single bunch collective instability induced by broadband impedance may limit the beam intensity of the LHC. The broadband impedance of each component in the machine needs to be studied carefully, and the total impedance be controlled below certain threshold. Due to the complexity of the TAN chamber geometry, the impedance calculation has to be performed using 3D MAFIA in the time domain. To verify numerical simulation results, a few examples with simple geometries have been studied, and compared with the results obtained by analytical formula and a wellknown 2D code ABCI. A 3D model for the TAN chamber is established to estimate the magnitude of longitudinal broadband impedance. Transverse broadband impedance has been studied numerically as well and will be discussed