Obtaining the Wakefield Due to Cell-to-Cell Misalignments in a Linear Accelerator Structure

Abstract

The authors are interested in obtaining the long-range, dipole wakefield of a linac structure with internal misalignments. The NLC linac structure is composed of a collection of cups that are brazed together, and such a calculation, for example, is important in setting the straightness tolerance for the composite structure. The authors derivation, presented here, is technically applicable only to structures for which all modes are trapped. The modes will be trapped at least at the ends of the structure, if the connecting beam tubes have sufficiently small radii and the dipole modes do not couple to the fundamental mode couplers in the end cells. For detuned structures (DS), like those in the injector linacs of the JLC/NLC, most modes are trapped internally within a structure, and those that do extend to the ends couple only weakly to the beam; for such structures the results here can also be applied, even if the conditions on the beam tube radii and the fundamental mode coupler do not hold. The authors believe that even for the damped, detuned structures (DDS) of the main linac of the JLC/NLC, which are similar, though they have manifolds to add weak damping to the wakefield, a resultmore » very similar to that presented here applies.« less