Trapping and Acceleration of Nonideal Injected Electron Bunches in Channel-Guided LWFAs

The standard regime for the laser wakefield accelerator (LWFA) usually requires external injection of MeV electrons. Ideally, the injected electron bunch should be injected into the proper phase of the accelerating wake, have a bunch length that is small compared with the plasma wavelength, and a low emittance and energy spread. This paper reports Hamiltonian analysis and simulation studies of two 'nonideal' injection schemes that demonstrate strong phase bunching and good accelerated beam quality in a channel-guided laser wakefield accelerator. For the case of monoenergetic, unphased (long bunch) injection, there is an optimum range of injection energies for which the LWFA can trap a significant fraction of the injected pulse while producing an ultrashort, high-quality accelerated pulse. Phased and unphased injection in a channel-guided LWFA with a broad injected energy spread has also been simulated. Although the trapping fraction is generally much smaller than in the monoenergetic case, some simulations exhibit final accelerated bunches with remarkably small energy spread. These results suggest that relatively poor quality injection pulses may still be useful in LWFA demonstration experiments. Implications for planned LWFA experiments at NRL are discussed