  A technical description of NEXT-MM and its commissioning and first
performance is reported. Having an active volume of ~35 cm drift $\times$ 28 cm
diameter, it constitutes the largest Micromegas-read TPC operated in Xenon ever
constructed, made by a sectorial arrangement of the 4 largest single wafers
manufactured with the Microbulk technique to date. It is equipped with a
suitably pixelized readout and with a sufficiently large sensitive volume (~23
l) so as to contain long (~20 cm) electron tracks. First results obtained at 1
bar for Xenon and trimethylamine (Xe-(2 %)TMA) mixture are presented. The TPC
can accurately reconstruct extended background tracks. An encouraging
full-width half-maximum of 11.6 % was obtained for ~29 keV gammas without
resorting to any data post-processing.
