Indirect Integration of Longitudinal and Transverse Wake Potentials for Unequal Beam Pipes and Arbitrary Beam Velocity

Abstract

Indirect integration replaces the long uniform beam-pipe parts of a wakefield calculation by field problems in the pipe cross sections. Earlier ultrarelativistic methods were developed mainly for the longitudinal wake, whereas the transverse wake was usually obtained from the Panofsky--Wenzel theorem. We derive indirect formulas that complete a transverse Lorentz-force integral already accumulated in a time-domain calculation. At $\beta =1$, each semi-infinite tail is found from a Dirichlet Poisson problem driven by $E_z$ and a Neumann Poisson problem driven by $c{B}_z$, describing the TM and TE contributions, respectively. We obtain both a fixed-time moving-window representation and a fixed-plane time-history representation for equal or unequal input and output pipes. The method is then extended to a rigid bunch moving with constant velocity $0<\beta c<c$. The longitudinal correction satisfies an anisotropic elliptic equation in $(x,y,s)$ and provides an additional source for the transverse TM problem. In a two-port finite-reference convention, the complete fields, including space charge, are integrated directly between two fixed planes, while the semi-infinite tails are calculated after subtraction of the stationary field in each pipe. The resulting Panofsky--Wenzel relation contains the difference of the stationary transverse electric fields at the two ports. Numerical tests for an unequal rectangular step-out at $\beta =1$ and $\beta =0.8$ confirm the transverse indirect integration and the unequal-pipe boundary term.

14 pages, 4 figures