  The performance of the upgraded solid deuterium ultracold neutron source at
the pulsed reactor TRIGA Mainz is described. The current configuration stage
comprises the installation of a He liquefier to run UCN experiments over
long-term periods, the use of stainless steel neutron guides with improved
transmission as well as sputter-coated non-magnetic $^{58}$NiMo alloy at the
inside walls of the thermal bridge and the converter cup. The UCN yield was
measured in a `standard' UCN storage bottle (stainless steel) with a volume of
32 litres outside the biological shield at the experimental area yielding UCN
densities of 8.5 /cm$^3$; an increase by a factor of 3.5 compared to the former
setup. The measured UCN storage curve is in good agreement with the predictions
from a Monte Carlo simulation developed to model the source. The growth and
formation of the solid deuterium converter during freeze-out are affected by
the ortho/para ratio of the H$_2$ premoderator.

Instrumentation and Detectors;Nuclear Experiment