A 10 kilo-tonne dual-phase liquid argon TPC is one of the detector options considered
for the Deep Underground Neutrino Experiment (DUNE). The detector technology relies on
amplification of the ionisation charge in ultra-pure argon vapour and offers several advantages
compared to the traditional single-phase liquid argon TPCs. A 4.2 tonne dual-phase liquid argon
TPC prototype, the largest of its kind, with an active volume of 3 × 1 × 1 m3 has been constructed
and operated at CERN. In this paper we describe in detail the experimental setup and detector
components as well as report on the operation experience. We also present the first results on the
achieved charge amplification, prompt scintillation and electroluminescence detection, and purity
of the liquid argon from analyses of a collected sample of cosmic ray muons.peerReviewe