Resistive-wall impedance of an infinitely long multi-layer cylindrical beam pipe

• Published in: 2007 IEEE Particle Accelerator Conference (PAC) pp. 4216 - 4218
• Main Authors: Metral, E., Zotter, B., Salvant, B.
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01.06.2007
• Subjects: Colliding beam devices; Collimators; Frequency; Impedance; Large Hadron Collider; Maxwell equations; Particle beams; Protons; Skin; Superconducting magnets
• ISBN: 1424409160; 9781424409167
• ISSN: 1944-4680
• DOI: 10.1109/PAC.2007.4439984

The resistive-wall impedance of cylindrical vacuum chambers was first calculated more than forty years ago under some approximations. Since then many papers have been published to extend its range of validity. In the last few years, the interest in this subject has again been revived for the LHC graphite collimators, for which a new physical regime is predicted. The first unstable betatron line in the LHC is at 8 kHz, where the skin depth for graphite is 1.8 cm, which is smaller than the collimator thickness of 2.5 cm. Hence one could think that the resistive thick-wall formula would be about right. It is found that it is not, and that the resistive impedance is about two orders of magnitude lower at this frequency, which is explained by the fact that the skin depth is much larger than the beam pipe radius. Starting from the Maxwell equations and using field matching, a consistent derivation of both longitudinal and transverse resistive- wall impedances of an infinitely long cylindrical beam pipe is presented in this paper. The results, which should be valid for any number of layers, beam velocity, frequency, conductivity, permittivity and permeability, have been compared to previous ones.