Impedance of a Rectangular Beam Tube with Small Corrugations
Abstract
We consider the impedance of a structure with rectangular, periodic corrugations on two opposing sides of a rectangular beam tube. Using the method of field matching, we find the modes in such a structure. We then limit ourselves to the the case of small corrugations, but where the depth of corrugation is not small compared to the period. For such a structure we generate analytical approximate solutions for the wave number k, group velocity v_g, and loss factor \kappa for the lowest (the dominant) mode which, when compared with the results of the complete numerical solution, agreed well. We find: if w\sim a, where w is the beam pipe width and a is the beam pipe half-height, then one mode dominates the impedance, with k\sim1/\sqrt{w\delta} (\delta is the depth of corrugation), (1-v_g/c)\sim\delta, and \kappa\sim1/(aw), which (when replacing w by a) is the same scaling as was found for small corrugations in a {\it round} beam pipe. Our results disagree in an important way with a recent paper of Mostacci {\it et al.} [A. Mostacci {\it et al.}, Phys. Rev. ST-AB, {\bf 5}, 044401 (2002)], where, for the rectangular structure, the authors obtained a synchronous mode with the same frequency k, but with \kappa\sim\delta. Finally, we find that if w is large compared to a then many nearby modes contribute to the impedance, resulting in a wakefield that Landau damps.