Test of ATLAS Micromegas detectors with ternary gas mixture at the CERN GIF++ facility

The ATLAS collaboration at LHC has chosen the resistive Micromegas technology, along with the small-strip Thin Gap Chambers (sTGC), for the high luminosity upgrade of the first muon station in the high-rapidity region, the New Small Wheel (NSW) project. Four different types of Micromegas quadruplets have been constructed at four construction sites in Italy (SM1), Germany (SM2), France (LM1) and CERN$/$Greece$/$Russia (LM2). At CERN, the final validation and the integration of the modules into Sectors and their commissioning are in progress. The achievement of the requirements for these detectors revealed to be even more challenging than expected. One of the main features being studied is the HV stability of the detectors. Several approaches have been tested in order to enhance the stability, among them the use of different gas mixtures. A ternary Argon-CO$_2$-iC$_4$H$_{10}$ mixture has shown to be effective in dumping discharges and dark currents. It allows the operation of the Micromegas detectors at working points with high cosmic muon detection efficiency. The presence of Isobutane in the mixture required a set of aging studies, ongoing at the GIF++ radiation facility at CERN, where the expected HL LHC background rate is reached by a $^{137}$Cs 14 TBq source of 662 keV photons. Preliminary aging results and effectiveness of the ternary mixture will be shown.