A near-ideal dechirper for plasma based electron and positron acceleration using a hollow channel plasma

Plasma based electron and positron wakefield acceleration has made great strides in the past decade. However one major challenge for its applications on coherent light sources and colliders is its relatively large energy spread, currently at the few percent level. This energy spread is usually correlated with particle position in the beam arising from the longitudinal chirp of the wakefield. Therefore an ideal dechiper 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 and demonstrate the concept through large scale three dimensional particle-in-cell simulations. We show that the initial energy chirp on the beam exiting a plasma accelerator can be systematically reduced 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 is also well 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.