Micromegas chambers for the ATLAS muon spectrometer upgrade

Micromegas, an abbreviation for Micro MEsh Gaseous Structure, is a robust detector with excellent spatial resolution and high rate capability. An R\& D activity, called Muon ATLAS MicroMegas Activity (MAMMA) which was initiated in 2007 in order to explore the potential of the MM technology for use in the ATLAS experiment. After several years of prototyping and testing, the ATLAS collaboration has chosen the micromegas technology (MM) along with the small-strip Thin Gap Chambers (sTGC) for the upgrade of the inner muon station in the high-rapidity region, the so called New Small Wheel (NSW) upgrade project. It employs eight layers of MM and eight layers of sTGC detectors. The NSW project requires fully efficient micromegas chambers, able to cope with the maximum expected rate of $15\,\mathrm{kHz/cm^2}$ featuring spatial resolution better than $100\,\mu\mathrm{m}$. The MM detectors will cover a total active area of $\sim1200\,\mathrm{m^2}$ and will be operated in a moderate magnetic field with intensity up to $0.4\,\mathrm{T}$. The required track reconstruction accuracy is provided by the intrinsic detector space resolution, accompanied by a mechanical precision at the level of $30\,\mu\mathrm{m}$ along the precision coordinate. Moreover, together with the precise tracking capability the NSW MM chambers will to contribute to the ATLAS Level-1 trigger system. An extensive R&D program is ongoing to determine the best configuration that satisfies these requirements. Several tests have been performed on small ($10\times10\,\mathrm{cm^2}$) and medium ($1\times1\,\mathrm{m^2}$) size prototypes, including also the recently developed MM quadruplet prototype, using medium ($1-5\,\mathrm{GeV/c}$) and high momentum ($120-150\,\mathrm{GeV/c}$) hadron beams at CERN. A brief overview of the results obtained from recent design and performance tests concerning the aspects discussed above is presented.