  For the future neutrino oscillation experiment DUNE, liquid argon time
projections chambers with a fiducial mass of 10 kton each are foreseen. The
dual phase concept is one of the two implementations considered, wherein
electrons produced by ionization in the liquid are extracted to a gaseous
region above the liquid where they are amplified. For the amplification, large
electron multipliers will be used. The technology was tested in various
prototypes, most recently with a 3 x 1 x 1 m$^3$ large setup. An even larger
prototype of 6 x 6 x 6 m$^3$ is currently being constructed and will start
operation in 2019. An intensive R\&D program was carried out with the focus on
achieving an effective gain of at least 20. In the simulation study here
presented for the first time not only the electron signal is considered but
also the ion backflow and the expected production of secondary scintillation
light is studied, because the latter might limit the capability of the detector
to trigger on low energetic no-beam physics. It is found that the ion backflow
and the light yield can be expected to be very large. The results for the
effective gain show a discrepancy with experimental data, both in size and
shape of the gain curve. Based on literature studies, it is argued that photon
feedback contributes to the gain in detectors filled with pure noble gases,
especially in the case of pure argon.
