Multistaging in free-space laser particle accelerators

Abstract

One important issue with regard to acceleration of electrons in free space using intense laser beams is the phase slippage of the electrons relative to the electromagnetic field. This arises from a phase velocity mismatch between the electron and light wave. Left uncontrolled this slippage can result in degradation of the e-beam characteristics (e.g., emittance) and dispersion of the electron bunches. By a method similar to microwave accelerators, multistaging offers a means to control the e-beam evolution by tuning each successive stage. The phase of the electron bunches relative to the laser field at the entrance of each section determines the acceleration and/or focusing that ensues in that section; therefore, the entrance phase is a natural tuning parameter. It is shown that by controlling the entrance phase it is possible to preserve the e-beam quality, both transverse (emittance) and longitudinal (bunching, energy spread). Calculations of the longitudinal and transverse beam dynamics are performed to determine the evolution of a finite-emittance e beam from stage to stage. By this method the conditions on entrance phase that allow successful e-beam trapping are found. It is also shown that conditions that assure e-beam trapping automatically preserve the overall beam quality.