Stimulated dielectric wake-field accelerator

Abstract

A wake-field accelerator is described based on the use of a waveguide structure in which many modes can participate in wake-field formation, and in which the wake-field period equals the period of a train of drive bunches. A dielectric-lined waveguide is analyzed that is shown to support multimode propagation with all modes having nearly equal phase velocities, equal to the initial velocity of injected charge bunches that drive the wake fields. For this waveguide, the ratio of wake field to drag field for a bare drive bunch is 4.7, as compared to 2.0 for a single-mode waveguide. The composite ${\mathrm{TM}}_{0n}$ wake field of such a structure is shown to include highly peaked axial electric fields localized on each driving bunch in a periodic sequence of bunches. This allows stimulated emission of wake-field energy to occur at a rate that is larger than the coherent spontaneous emission from a single driving bunch of equal charge and energy. This mechanism can make possible the design of a stimulated dielectric wake-field accelerator that has the potential of providing an acceleration gradient for electrons or positrons in the range of 50–100 MV/m, taking a driving bunch charge of a few nC. We present calculations for such wake fields from a bunched sheet beam in a two-dimensional dielectric waveguide. Numerical examples are given, including the acceleration of a 30 MeV test bunch to 155 MeV in a structure 200 cm in length, using ten identical 2 nC/mm drive bunches.