  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 oers 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 3x1x1 $m^3$ 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.
