  The DARWIN observatory is a proposed next-generation experiment to search for
particle dark matter and other rare interactions. It will operate a 50 t liquid
xenon detector, with 40 t in the time projection chamber (TPC). To inform the
final detector design and technical choices, a series of technological
questions must first be addressed. Here we describe a full-scale demonstrator
in the vertical dimension, Xenoscope, with the main goal of achieving electron
drift over a 2.6 m distance, which is the scale of the DARWIN TPC. We have
designed and constructed the facility infrastructure, including the cryostat,
cryogenic and purification systems, the xenon storage and recuperation system,
as well as the slow control system. We have also designed a xenon purity
monitor and the TPC, with the fabrication of the former nearly complete. In a
first commissioning run of the facility without an inner detector, we
demonstrated the nominal operational reach of Xenoscope and benchmarked the
components of the cryogenic and slow control systems, demonstrating reliable
and continuous operation of all subsystems over 40 days. The infrastructure is
thus ready for the integration of the purity monitor, followed by the TPC.
Further applications of the facility include R&D on the high voltage
feedthrough for DARWIN, measurements of electron cloud diffusion, as well as
measurements of optical properties of liquid xenon. In the future, Xenoscope
will be available as a test platform for the DARWIN collaboration to
characterise new detector technologies.
