LHC Bellows Impedance Calculations

Misha D'yachkov and Francesco Ruggiero

April 1997

Abstract:

To compensate for thermal expansion the LHC ring has to accommodate about 2500 bellows which, together with beam position monitors, are the main contributors to the LHC broad-band impedance budget. In order to reduce this impedance to an acceptable value the bellows have to be shielded. In this paper we compare different designs proposed for the bellows and calculate their transverse and longitudinal wakefields and impedances. Owing to the 3D geometry of the bellows, the code MAFIA was used for the wakefield calculations; when possible the MAFIA results were compared to those obtained with ABCI. The results presented in this paper indicate that two designs for the bellows, in which shielding is provided by using sprung fingers which can slide along the beam screen, both have acceptable impedances. However one design may be preferable to the other for mechanical considerations. Several failure modes, such as missing fingers and imperfect RF contact, have also been studied.

F. Ruggiero
Mon Apr 28 17:59:19 METDST 1997