  We installed a source for ultracold neutrons at a new, dedicated spallation
target at TRIUMF. The source was originally developed in Japan and uses a
superfluid-helium converter cooled to 0.9$\,$K. During an extensive test
campaign in November 2017, we extracted up to 325000 ultracold neutrons after a
one-minute irradiation of the target, over three times more than previously
achieved with this source. The corresponding ultracold-neutron density in the
whole production and guide volume is 5.3$\,$cm$^{-3}$. The storage lifetime of
ultracold neutrons in the source was initially 37$\,$s and dropped to 24$\,$s
during the eighteen days of operation. During continuous irradiation of the
spallation target, we were able to detect a sustained ultracold-neutron rate of
up to 1500$\,$s$^{-1}$. Simulations of UCN production, UCN transport,
temperature-dependent UCN yield, and temperature-dependent storage lifetime
show excellent agreement with the experimental data and confirm that the
ultracold-neutron-upscattering rate in superfluid helium is proportional to
$T^7$.
