  The aim of this short note is to present an option for a source of ultracold
neutrons (UCNs), which could profit from the pulse time-structure of the future
ESS spallation neutron source in Lund, and thus which could produce a very high
UCN density and a rather high UCN flux simultaneously. In order to realize this
idea one has to install a relatively thin solid-deuterium UCN source in a close
vicinity to the spallation target and to couple it with an extraction UCN guide
with an entrance membrane window, which is moving periodically and
synchronously with the operation cycle of the spallation source, as explained
in the text below. This proposal profits from the fact that all characteristic
parameters of the problem, such as the pulse duration of the ESS spallation
source, the typical thickness of solid deuterium source that could be easily
realized, the typical time of generation of UCNs in solid deuterium, the length
and diameter of the extraction neutron guide and the time diagram of the
membrane motion that is still realistic, they all fit nicely to optimum desired
parameters. The UCN density produced in such a way could approach 10^6 UCN/cm3.
