First emittance measurement of the beam-driven plasma wakefield accelerated electron beam

Next-generation plasma-based accelerators can push electron beams to GeV energies within centimeter distances. The plasma, excited by a driver pulse, is indeed able to sustain huge electric fields that can efficiently accelerate a trailing witness bunch, which was experimentally demonstrated on multiple occasions. Thus, the main focus of the current research is being shifted towards achieving a high quality of the beam after the plasma acceleration. In this paper we present a beam-driven plasma wakefield acceleration experiment, where initially preformed high-quality witness beam was accelerated inside the plasma and characterized. In this experiment the witness beam quality after the acceleration was maintained on high level, with 0.2% final energy spread and 3.8 μm resulting normalized transverse emittance after the acceleration. In this article, for the first time to our knowledge, the emittance of the plasma wakefield accelerated beam was directly measured.