We report the observation of energy-spread compensation of electron bunches in a laser wakefield accelerator in experiment. The compensation was caused by the gradient wakefield in plasma wake, and the energy spectra of the bunches evolved during the acceleration so that we propose a new method to diagnose the longitudinal length of the ultrashort electron bunch. By analyzing the energy spectra of electron bunches with different acceleration length, the wakefield gradient difference and the wakefield slope of the bunch could be estimated by combining with the slippage between the plasma wave and the electron bunch, thus the electron bunches′ longitudinal length could be estimated. By applying this new method, the longitudinal length of electron bunches with charge of about 40 pC generated from a laser wakefield accelerator was estimated to be (2.4 ± 2.2) μm in experiment, which was in good agreement with three-dimension particle-in-cell simulations.
Zhijun Zhang, "Energy spread minimization in a cascaded laser wakefield accelerator via velocity bunching (Conference Presentation)," Proc. SPIE 10240, Laser Acceleration of Electrons, Protons, and Ions IV, 102400M (Presented at SPIE Optics + Optoelectronics: April 25, 2017; Published: 9 June 2017); https://doi.org/10.1117/12.2264688.5463398616001.
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