Comments on the high-frequency behavior of the coupling of an accelerator beam to its environment

The short bunch lengths and the associated high frequencies found in the latest designs of linear colliders, superconducting linacs, FEL drivers, damping rings, and synchrotron light sources have heightened the importance of understanding the high-frequency behavior of the interaction of an accelerator beam with its environment. This parametric domain is at the limits of both the numerical and analytical tools which have been developed to date, and consequently there exists some question as to the correct asymptotic frequency dependence. The resulting uncertainty in the coupling of a particle beam to vacuum chamber discontinuities has hindered evaluation of bunch lengthening in storage rings and transverse beam blowup in linacs, and limits confidence in assessments of beam quality in proposed designs. Another high-frequency phenomenon, which is of particular concern in damping and storage rings, is the synchrotron radiation process in the presence of conductive boundaries. Estimates have indicated that this effect can provide the dominant limit on peak beam current in small, smooth-walled machines, but this earlier work does not take into account fully the complex, finite-Q resonance structure which is present. The charge to the August 1987 Impedance Beyond Cutoff Workshop at Lawrence Berkeley Laboratory was to investigate these issues in some depth and to provide clarification of the main features of the beam coupling impedance at frequencies well above the lowest propagation frequency of the beam pipe. In this note the motivation for this effort, an overview of the progress made, and a few remarks on remaining questions are offered. 12 refs