Accelerator deflection-mode dampening experiments at Argonne

As the dimensions of accelerating structures become smaller and beam intensities higher, the non-axisymmetric wake fields driven by the beam become quite large even with slight misalignments of the beam from the geometrical axis. These deflection modes then cause bunch-to-bunch beam breakup and intra-bunch head-tail instabilities. An rf show-wave structure that has very low Q for the deflection modes while maintaining a high Q for the accelerating modes would then be desirable since the unwanted modes can be selectively damped. Such a device utilizing dielectric-lined waveguide is shown in this paper where the uniform outer conductor is replaced by axial, closely spaced, insulated wires which allow only axial wall currents to flow at that boundary. This dielectric-based configuration is of specific interest to the Cerenkov wake-field accelerator program under development at Argonne. A configuration consistent with traditional iris-loaded waveguide is given in this paper where the outer conductor is segmented to allow only axial currents and the irises are segmented to allow only radial currents. Severe enough damping in either configuration to eliminate the head-tail instability could be difficult to obtain depending on the bunch length considered, but strong enough damping to combat bunch-to-bunch beam breakup using these schemes appears quite feasible. Presented here will be experimental results of the dielectric-lined waveguide case