Open Access

Ernst Hellbär

• The Time Projection Chamber (TPC), a large gaseous detector, is the main particle identification device of the ALICE experiment at the CERN LHC. The desired performance of the TPC defines the requirements for the gas mixture used in the detector. The active volume was filled with either Ne-CO2 (90-10) or Ne-CO2-N2 (90-10-5) during the first LHC running period. For LHC Run 2 the gas mixture is changed to Ar-CO2. Calculations of relevant gas properties are performed for Ar-based gas mixtures and compared to Ne-based gas mixtures to identify the most suitable Ar mixture. The drift velocity of ions in Ar is lower than in Ne. The closing time of the gating grid has to be adjusted accordingly to avoid drift field distortions due to back-drifting ions. The drift times of ions in the TPC readout chambers are calculated for the respective gas mixtures to determine the time to collect all ions from the amplification region. For LHC Run 3 the TPC readout chambers will be upgraded. The Multiwire Proportional Chambers (MWPCs) will be replaced by readout chambers based on Gas Electron Multipliers (GEMs) which are operated in continuous mode. As a consequence an ion backflow of the order of 1% causes significant space-charge distortions in the TPC drift volume. Similar distortions are expected in data taken specifically for the study of space-charge effects at the end of Run 1. The gating grid of the MWPCs is operated in the open state allowing the ions from the amplification region to enter the drift volume. The magnitude of the distortions in this data is measured and compared to the expectations for the TPC upgrade and results from current simulations.