  Plasma-based electron and positron wakefield acceleration has made great
strides in the past decade. However one major challenge for its applications to
coherent light sources and colliders is the relatively large energy spread of
the accelerated beams, currently at a few percent level. This energy spread is
usually correlated with particle position in the beam arising from the
longitudinal chirp of the wakefield amplitude. Therefore a dechirper is highly
desirable for reducing this spread down to $\sim0.1\%$ level, while at the same
time for maintaining the emittance of the accelerated beam. Here we propose
that a low-density hollow channel plasma can act as a near-ideal dechirper for
both electrons and positrons. We demonstrate the concept through large-scale
three-dimensional particle-in-cell simulations. We show that the initial
positive correlated energy spread (chirp) on the beam exiting a plasma
accelerator can be compensated by the nearly linear self-wake induced by the
beam in the hollow channel from few percent level down to $\leq 0.1\%$.
Meanwhile, the beam emittance can be preserved due to the negligible transverse
field inside the channel. This passive method may significantly improve the
beam quality of plasma-based accelerators, paving the way for their
applications to future compact free electron lasers and colliders.
