Creating Exact Multipolar Fields with Azimuthally Modulated RF Cavities

RF cavities used in modern particle accelerators operate in TM$_{m10}$-like modes composed of a single, dominant multipole of order $m$; $m=0$ modes are used for the longitudinal acceleration of a particle beam and $m\neq0$ modes for controlling transverse beam dynamics. The practical design of the latter, however, can be complex and require extensive analysis through the iteration of both approximate mathematical models and computationally expensive simulations to optimise the performance of the structure. In this paper we present a new, systematic method for designing azimuthally modulated RF cavities that support modes composed of any number and magnitude of user-specified transverse multipoles, either with or without a longitudinally accelerating component. Two case studies are presented of RF cavity designs that support modes composed of a longitudinally accelerating field in addition to a single transverse multipole, and designs that support modes composed of two transverse multipoles. We discuss generalising the discoveries and conclusions from the two case studies to designing cavities that support modes composed of any number of multipoles. The theoretical work is verified with analysis of 3D simulations and experimental measurements are presented of a cavity operating in a 3 GHz mode that simultaneously longitudinally accelerates and transversely focuses a beam.