PT Journal AU NEXT Collaboration (Alvarez, Vea Carcel, S Cervera-Villanueva, A Diaz, J Ferrario, P Gil, A Gomez-Cadenas, JJ Laing, A Liubarsky, I Lorca, D Martin-Albo, J Martinez, A Monrabal, F Muñoz Vidal, J Nebot-Guinot, M Rodriguez, J Serra, L Simon, A Sorel, M Yahlali, N TI Description and commissioning of NEXT-MM prototype: first results from operation in a Xenon-Trimethylamine gas mixture SO Journal of Instrumentation JI J. Instrum. PY 2014 BP P03010 - 22pp VL 9 DI 10.1088/1748-0221/9/03/P03010 LA English DE Time projection chambers; Particle tracking detectors (Gaseous detectors); Double-beta decay detectors AB A technical description of NEXT-MM and its commissioning and first performance is reported. Having an active volume of similar to 35 cm drift x 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 (similar to 23 l) so as to contain long (similar to 20 cm) electron tracks. First results obtained at 1 bar for Xenon and Trymethylamine (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 similar to 29 keV gammas without resorting to any data post-processing. ER