Detailed experimental results for high-trapping efficiency and narrow energy spread in a laser-driven accelerator

Presented are details of the staged electron laser acceleration (STELLA) experiment, which demonstrated high-trapping efficiency and narrow energy spread in a staged laser-driven accelerator. Trapping efficiencies of up to 80% and energy spreads down to 0.36% (1σ ) were demonstrated. The experiment validated an approach that may be suitable for the basic design of a laser-driven accelerator system. In this approach, a laser-driven modulator together with a chicane creates a train of microbunches spaced apart by the laser wavelength. These microbunches are sent into a second laser-driven accelerator designed to efficiently trap the microbunches in the ponderomotive potential well of the laser electric field while maintaining a narrow energy spread. The STELLA scientific apparatus and procedures are described in detail. In-depth comparisons between the data and model are given including the predicted energy spectrum, energy-phase plot, and microbunch length profile. Data and model comparisons as a function of the phase delay between the microbunches and the accelerating wave are presented. The model is exercised to reveal how the high-trapping efficiency process evolves during the acceleration process.