Optimisation of gas composition for Micromegas of the ATLAS experiment

Resistive Micromegas detectors will be used for the upgrade of the ATLAS muon system foreseen in 2021. The gas mixture so far considered for Micromegas operation is Ar:CO2 93:7 which is widely used for other ATLAS Muon detectors, thus requiring only little changes of the present gas system of the experiment. After the observation of high voltage instabilities on the first detectors, measurement campaigns with different mixtures have been performed with the aim of increasing the stability region, defined as the HV difference between the start of the efficiency plateau and the start of the instability. The gas selection for the ATLAS Micromegas should however fulfil a number of constraints, as non-flammability and good ageing properties as well as a drift velocity above 2.5 cm/us and optimal transverse diffusion. Different measurements have been carried out at CERN and LMU on medium and small-size resistive Micromegas and at Saclay and LMU on ATLAS Micromegas detectors. The tests have been focused on Argon-based gas mixtures with different CO2 concentration and with the addiction of small fraction of isobutane. A systematic review of the results obtained from laboratory tests with radioactive source and cosmic rays will be presented together with the performance recently measured at CERN on pion and muon beam. Conclusions about the current understanding of the gas optimisation for the ATLAS Micromegas will also be discussed.