Design and Construction of Large Size Micromegas Chambers for the ATLAS Upgrade of the Muon Spectrometer

Large area Micromegas detectors will be employed fo r the first time in high-energy physics experiments. A total surface of about 150 m$^2$ of the forward regions of the Muon Spectrometer of the ATLAS detector at LHC will be equipped with 8-layer Micromegas modules. Each module extends over a surface from 2 to 3 m$^2$ for a total active area of 1200 m$^2$. Together with the small strip Thin Gap Chambers they will compose the two New Small Wheels, which will replace the innermost stations of the ATLAS endcap muon tracking system in the 2018/19 shutdown. In order to achieve a 15% transverse momentum resol ution for 1 TeV muons, in addition to an excellent intrinsic resolution, the mechanical prec ision of each plane of the assembled module must be as good as 30 $\mu$m along the precision coordi nate and 80 $\mu$m perpendicular to the chamber. The design and construction procedure of the microm egas modules will be presented, as well as the design for the assembly of modules onto the New Small Wheel. Emphasis will be on the methods developed to achieve the challenging me chanical precision. Measurements of deformation on chamber prototypes a s a function of thermal gradients, gas over-pressure and internal stress (mesh tension and module fixation on supports) will be shown in comparison to simulation. These tests were essential in the development of the final design in order to minimize the effects of deformat ions. During installation and operation all deformations and relative misalignments will be monitored through an optical alignment system and c ompensated in the tracking software. The optical alignment concept will also be outlined.