Temporal evolution of longitudinal bunch profile in a laser wakefield accelerator

TL;DR

This paper presents the first high-resolution, single-shot measurements of the evolving longitudinal bunch profile in laser wakefield accelerators, revealing complex multi-bunch structures and dynamics during acceleration.

Contribution

It introduces a novel measurement method capable of detecting complex bunch features without assumptions, and demonstrates the evolution of bunch profiles during acceleration.

Findings

Minimum bunch duration of 4.2 fs achieved

Shot-to-shot fluctuation of 11% rms observed

Transition from laser-driven to particle-driven wakefield identified

Abstract

Due to their ultra-short duration and peak currents in the kA range, laser-wakefield accelerated electron bunches are promising drivers for ultrafast X-ray generation in compact free-electron-lasers (FELs), Thomson-scattering or betatron sources. Here we present the first single-shot, high-resolution measurements of the longitudinal bunch profile obtained without prior assumptions about the bunch shape. Our method allows complex features, such as multi-bunch structures, to be detected. Varying the length of the gas target, and thus the acceleration length, enables an assessment of the bunch profile evolution during the acceleration process. We find a minimum bunch duration of 4.2 fs (full width at half maximum) with shot-to-shot fluctuation of 11% rms. Our results suggest that after depletion of the laser energy, a transition from a laser-driven to a particle-driven wakefield occurs, associated with the injection of a secondary bunch. The resulting double-bunch structure might act as an elegant approach for driver-witness type experiments, i.e. allowing a non-dephasing-limited acceleration of the secondary bunch in a plasma-afterburner stage.