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Energy chirp compensation of the electron bunch (e bunch) in a laser wakefield accelerator, which is caused by the phase space rotation in the gradient wakefield, has been applied in many schemes for low energy spread e-bunch generation, but not yet been systematically demonstrated in experiment. We report the experimental observation of energy chirp compensation of the e bunch in a laser wakefield accelerator. By adjusting the acceleration length using a wedge-roof block, the chirp compensation of the accelerated e bunch was observed via an electron spectrometer. Apart from this, some significant parameters for the compensation process, such as the longitudinal dispersion and wakefield slope at the bunch position, were also estimated. A detailed comparison between experiment and simulation shows good agreement of the wakefield and bunch parameters. These results give a clear demonstration of the longitudinal characteristics of the wakefield in plasma as well as the bunch dynamics, which are important for a better control over a compact laser wakefield accelerator.
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