Energy spread minimization in a beam-driven plasma wakefield accelerator

Next-generation plasma-based accelerators can push elec-tron bunches to gigaelectronvolt energies within centimetre distances. The plasma, excited by a driver pulse, generates large electric fields that can efficiently accelerate a trailing witness bunch, enabling the realization of laboratory-scale applications ranging from high-energy colliders to ultrabright light sources. So far, several experiments have demonstrated large accelerations but the resulting beam quality, par-ticularly the energy spread, is still far from state-of-the-art conventional accelerators. Here we show the results of a beam-driven plasma acceleration experiment where we used an electron bunch as a driver followed by an ultrashort wit-ness bunch. By setting a positive energy chirp on the witness bunch, its longitudinal phase space is rotated during accel-eration, resulting in an ultralow energy spread that is even lower than the spread at the plasma entrance. This result will significantly impact the optimization of the plasma accelera-tion process and its implementation in forthcoming compact machines for user-oriented applications.