Gas and irradiation studies for the Micromegas detectors of the ATLAS New Small Wheel

The ATLAS collaboration has chosen the resistive Micromegas technology for the high luminosity upgrade of the first forward muon station, the New Small Wheel (NSW). One of the main features being studied is the HV stability of the detectors, mainly related to the resistivity pattern of the strips. Among the several approaches to enhance the stability of the detectors, the use of different gas mixtures are being studied. A ternary argon-CO2-isobutane mixture has shown to be effective in dumping discharges and dark currents. The presence of isobutane in the mixture required a set of ageing studies, ongoing at the GIF++ radiation facility at CERN. A summary of the results obtained up to now will be shown, as well as the upcoming test plans, mainly focused on O(1 year equivalent) time scale ageing effects.