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

Large area Micromegas detectors will be employed for the first time in high-energy physics experiments. A total surface of about $\mathbf{150~m^2}$ of the forward regions of the Muon Spectrometer of the ATLAS detector at LHC will be equipped with 8-layer Micromegas modules. Each layer covers more than $\mathbf{2~m^2}$ for a total active area of $\mathbf{1200~m^2}$. 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$\mathbf{\%}$ transverse momentum resolution for $\mathbf{1~TeV}$ muons, in addition to an excellent intrinsic resolution, the mechanical precision of each plane of the assembled module must be as good as $\mathbf{30~\mu m}$ along the precision coordinate and $\mathbf{80~\mu m}$ perpendicular to the chamber. The design and construction procedure of the Micromegas 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 mechanical precision. Measurements and simulations of deformations created on chamber prototypes as a function of thermal gradients, internal stress (mesh tension and module fixation on supports) and gas over-pressure were essential in the development of the final design. During installation and operation all deformations and relative misalignments will be monitored by an optical alignment system and compensated in the tracking software.