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

Large area Micromegas detectors will be employed for the first time in high-energy physics experiments. A total surface area of about 150 m$^2$ of the forward regions (pseudo-rapidity coverage -- 1.3 $\boldsymbol{< |\eta| <}$ 2.7) 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 (NSW), 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 resolution for 1 TeV muons, in addition to an excellent intrinsic position resolution, the mechanical precision of each plane of the assembled module must be $\boldsymbol{30{\mu}m}$ along the precision coordinate and $\boldsymbol{80{\mu}m}$ perpendicular to the chamber. All readout planes are segmented into strips with a pitch of $\boldsymbol{450{\mu}m}$ for a total of 8192 strips per plane and more than 2 million channels for the NSW. In two of the four planes the strips are inclined by $\boldsymbol{1.5^{\circ}}$ and provide a measurement of the second coordinate. The design and construction procedure of the Micromegas modules are presented, as well as the design for the assembly of modules onto the NSW. Emphasis is given on the methods developed to achieve the challenging mechanical precision. Measurements of deformation on chamber prototypes as a function of thermal gradients, gas over-pressure and internal stress (mesh tension and module fixation on supports) are also shown and compared to simulation. These tests were essential in the development of the final design in order to minimize the effects of deformations. During installation and operation all deformations and relative misalignments will be monitored through an optical alignment system and compensated in the tracking software.