  A double-phase argon Time Projection Chamber (TPC), with an active mass of
185 g, has been designed and constructed for the Recoil Directionality (ReD)
experiment. The aim of the ReD project is to investigate the directional
sensitivity of argon-based TPCs via columnar recombination to nuclear recoils
in the energy range of interest (20-200 keV$_{nr}$) for direct dark matter
searches. The key novel feature of the ReD TPC is a readout system based on
cryogenic Silicon Photomultipliers, which are employed and operated
continuously for the first time in an argon TPC. Over the course of six months,
the ReD TPC was commissioned and characterised under various operating
conditions using $\gamma$-ray and neutron sources, demonstrating remarkable
stability of the optical sensors and reproducibility of the results. The
scintillation gain and ionisation amplification of the TPC were measured to be
$g_1 = (0.194 \pm 0.013)$ PE/photon and $g_2 = (20.0 \pm 0.9)$ PE/electron,
respectively. The ratio of the ionisation to scintillation signals (S2/S1),
instrumental for the positive identification of a candidate directional signal
induced by WIMPs, has been investigated for both nuclear and electron recoils.
At a drift field of 183 V/cm, an S2/S1 dispersion of 12% was measured for
nuclear recoils of approximately 60-90 keV$_{nr}$, as compared to 18% for
electron recoils depositing 60 keV of energy. The detector performance reported
here meets the requirements needed to achieve the principal scientific goals of
the ReD experiment in the search for a directional effect due to columnar
recombination. A phenomenological parameterisation of the recombination
probability in LAr is presented and employed for modeling the dependence of
scintillation quenching and charge yield on the drift field for electron
recoils between 50-500 keV and fields up to 1000 V/cm.
