  Next-generation plasma-based accelerators can push electron beams to GeV
energies within centimetre 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 letter we present 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
$3.8~\mu m$ resulting normalized transverse emittance after the acceleration.
In this article, for the first time to our knowledge, the emittance of the PWFA
beam was directly measured.
