  There is a compelling physics case for a large, xenon-based underground
detector devoted to dark matter and other rare-event searches. A two-phase time
projection chamber as inner detector allows for a good energy resolution, a
three-dimensional position determination of the interaction site and particle
discrimination. To study challenges related to the construction and operation
of a multi-tonne scale detector, we have designed and constructed a vertical,
full-scale demonstrator for the DARWIN experiment at the University of Zurich.
Here we present first results from a several-months run with 343 kg of xenon
and electron drift lifetime and transport measurements with a 53 cm tall purity
monitor immersed in the cryogenic liquid. After 88 days of continuous
purification, the electron lifetime reached a value of 664(23) microseconds. We
measured the drift velocity of electrons for electric fields in the range
(25--75) V/cm, and found values consistent with previous measurements. We also
calculated the longitudinal diffusion constant of the electron cloud in the
same field range, and compared with previous data, as well as with predictions
from an empirical model.
