  Building on the successful experience in operating the DarkSide-50 detector,
the DarkSide Collaboration is going to construct DarkSide-20k, a direct WIMP
search detector using a two-phase Liquid Argon Time Projection Chamber (LArTPC)
with an active (fiducial) mass of 23 t (20 t). The DarkSide-20k LArTPC will be
deployed within a shield/veto with a spherical Liquid Scintillator Veto (LSV)
inside a cylindrical Water Cherenkov Veto (WCV). Operation of DarkSide-50
demonstrated a major reduction in the dominant $^{39}$Ar background when using
argon extracted from an underground source, before applying pulse shape
analysis. Data from DarkSide-50, in combination with MC simulation and
analytical modeling, shows that a rejection factor for discrimination between
electron and nuclear recoils of $\gt3\times10^9$ is achievable. This, along
with the use of the veto system, is the key to unlocking the path to large
LArTPC detector masses, while maintaining an "instrumental background-free"
experiment, an experiment in which less than 0.1 events (other than
$\nu$-induced nuclear recoils) is expected to occur within the WIMP search
region during the planned exposure. DarkSide-20k will have ultra-low
backgrounds than can be measured in situ. This will give sensitivity to
WIMP-nucleon cross sections of $1.2\times10^{-47}$ cm$^2$ ($1.1\times10^{-46}$
cm$^2$) for WIMPs of $1$ TeV$/c^2$ ($10$ TeV$/c^2$) mass, to be achieved during
a 5 yr run producing an exposure of 100 t yr free from any instrumental
background. DarkSide-20k could then extend its operation to a decade,
increasing the exposure to 200 t yr, reaching a sensitivity of
$7.4\times10^{-48}$ cm$^2$ ($6.9\times10^{-47}$ cm$^2$) for WIMPs of $1$
TeV$/c^2$ ($10$ TeV$/c^2$) mass.
