  A tunable plasma-based energy dechirper has been developed at FLASHForward to
remove the correlated energy spread of a 681~MeV electron bunch. Through the
interaction of the bunch with wakefields excited in plasma the projected energy
spread was reduced from a FWHM of 1.31$\%$ to 0.33$\%$ without reducing the
stability of the incoming beam. The experimental results for variable plasma
density are in good agreement with analytic predictions and three-dimensional
simulations. The proof-of-principle dechirping strength of $1.8$~GeV/mm/m
significantly exceeds those demonstrated for competing state-of-the-art
techniques and may be key to future plasma wakefield-based free-electron lasers
and high energy physics facilities, where large intrinsic chirps need to be
removed.
