  In this work we present a systematic study of Micromegas detectors in high
pressure gaseous Xenon using trimethylamine (TMA) as quencher gas. Gas gains
and energy resolutions for 22.1 keV X-rays are measured for pressures between 1
and 10 bar and various relative concentrations of TMA from 0.3 % to 15 %. We
observe stable operation at all pressures, and a strongly enhanced gas gain,
suggestive of Penning-like energy-transfer processes. The effect is present at
all pressures and it is strongest at TMA concentrations ranging from 1.5 % to 3
%. Operating in this concentration range, the maximum gain reached values as
high as x10^3 (x10^2) at 1 (10) bar. Besides, the energy resolution achievable
for 22.1 keV X-rays is substantially better than the one previously obtained in
pure Xe, going down to 7.3 % (9.6 %) FWHM for 1 (10) bar. These results are of
interest for calorimetric applications of high pressure gas Xe TPCs, in
particular for the search of the neutrinoless double beta decay of Xe-136. The
resolutions achieved would extrapolate into 0.7 % (0.9 %) FWHM at the Qbb value
of Xe-136 for 1 (10) bar.
