Micromegas Detectors for the Muon Spectrometer Upgrade of the ATLAS Experiment

Large area Micromegas (MM) detectors will be employed for the Muon Spectrometer upgrade of the ATLAS experiment at the LHC. A total surface of about 150 m2 of the forward regions of the Muon Spectrometer will be equipped with 8 layers of MM modules. Each module covers a surface area of approximately 2 to 3 m$^{2}$ for a total active area of 1200 m$^{2}$. Together with the small-strips 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 planned 2018/19 shutdown. This upgrade will maintain a low pt threshold for single muons and provides excellent tracking capabilities for the HL- LHC phase. The NSW project requires fully efficient MM chambers with spatial resolution down to 100 $ \mu m$, a rate capability up to about 15 kHz/cm$^{2}$ and operation in a moderate (highly inhomogeneous) magnetic field up to B=0.3 T. The required tracking capability is provided by the intrinsic spatial resolution combined with a challenging mechanical precision. The design, recent progress in the construction and results from the substantial R&D phase (with a focus on novel technical solutions) will be presented. In the R&D phase, small and medium size single layer prototypes have been built, along with, more recently, the first two MM quadruplets in a configuration very close to the final one chosen for the NSW. Several tests have been performed on these prototypes at a high-energy test-beam at CERN, to demonstrate that the achieved performances fulfill the requirements. Recent tests applying various configuration and operating conditions, for example magnetic field dependencies, will also be presented.