Laser deflector as a single-shot diagnostic for electron-beam longitudinal phase space

Laser and plasma wakefield accelerators are promising for many applications such as future TeV electron–positron colliders and x-ray free electron lasers (XFELs). These applications require high beam quality in terms of energy spread, emittance, and shot-to-shot stability. To achieve high beam quality, one has to precisely characterize the beam phase space. This is difficult owing to the highly nonlinear acceleration process and the sub-micrometer, sub-femtosecond resolutions required for diagnostics. Here, we propose a new single-shot method for reconstructing the longitudinal phase space of electron beams in laser-driven or particle beam-driven plasma wakefield accelerators. This method is enabled by a laser deflector regime, where an independently controllable laser pulse co-propagating with electrons inside a plasma wakefield modulates the electron motion leading to distinct periodic structures in the angularly resolved spectra. A constrained theoretical model, benchmarked by particle-in-cell simulations, was developed to reproduce the modulated electron spectra and fully reconstruct the longitudinal phase-space distribution of electron beams with a temporal resolution of half a laser period. The reconstruction provides the slice energy spread of the electron beam, which plays a critical role in the performance of applications such as XFELs. We also demonstrate that this laser deflector method can be applied to electron beams from traditional accelerators by periodically modulating the electron spectrum via direct-laser acceleration in vacuum.