  Plasma-based accelerators have made impressive progress in recent years.
However, the beam energy spread obtained in these accelerators is still at ~ 1
% level, nearly one order of magnitude larger than what is needed for
challenging applications like coherent light sources or colliders. In plasma
accelerators, the beam energy spread is mainly dominated by its energy chirp
(longitudinally correlated energy spread). Here we demonstrate that when an
initially chirped electron beam from a linac with a proper current profile is
sent through a low-density plasma structure, the self wake of the beam can
significantly reduce its energy chirp and the overall energy spread. The
resolution-limited energy spectrum measurements show at least a threefold
reduction of the beam energy spread from 1.28 % to 0.41 % FWHM with a
dechirping strength of ~ 1 (MV/m)/(mm pC). Refined time-resolved phase space
measurements, combined with high-fidelity three-dimensional particle-in-cell
simulations, further indicate the real energy spread after the dechirper is
only about 0.13 % (FWHM), a factor of 10 reduction of the initial energy
spread.
