Micromegas chambers for the ATLAS Muon Spectrometer Upgrade

Micromegas (Micro MEsh Gaseous Structure) chambers have been proven along the years to be reliable fast detectors with an excellent spatial resolution. The ATLAS collaboration at LHC has chosen the micromegas technology along with the small-strip Thin Gap Chambers (sTGC) for the high luminosity upgrade of the inner muon station in the high-rapidity region, the so called New Small Wheel (NSW). It employs eight layers of micromegas detectors and eight layers of sTGC. The NSW project requires fully efficient micromegas chambers with spatial resolution down to $100\mu m$ over a total active area of $1200 m^2$ with a rate capability up to $10 kHz/cm^2$ and operation in a moderate magnetic field up to B=0.3 T. The required tracking capability is provided by the intrinsic space resolution combined with a mechanical precision at the level of $30 \mu m$ along the precision coordinate. Moreover together with the precise tracking capability the micromegas chambers should provide a trigger signal. An extensive R&D program is ongoing to find the best configuration to match the requirements. Several tests have been performed on small ($10\times10 cm^2$) and medium ($1\times1 m^2$) size prototypes using high energy hadron beams at CERN, intermediate energy (1-5 GeV) electron beams at DESY, neutron beams at Demokritos (Athens) & Garching (Munich) and cosmic rays. Results obtained with different read-out electronics, either based on the standard analog APV25 chips, or based on a new digital front-end ASIC (VMM) are compared. An overview of the results obtained in the most recent tests concerning all the aspects discussed above is presented.