Abstract
The ALICE (A Large Ion Collider Experiment at CERN) collaboration plans an upgrade of the detector during the second long shutdown of the LHC, during which the interaction rate will be increased to 50 kHz for Pb-Pb collisions. This demands operation of the Time Projection Chamber (TPC) in an ungated continuous mode. A gating grid can not be used to prevent ions drifting back into the drift volume. Gas Electron Multipliers (GEM) offer intrinsic suppression of the ion backflow. To keep distortions due to space-charge at a manageable level an ion yield of 10 to 20 back drifting ions per incoming electron is required. In this manuscript, we describe the concept of ion backflow suppression with our triple GEM prototype. Within a detailed gas study, we show our lowest ion backflow values for several different gas mixtures that have been considered for the TPC upgrade. These values have been compared with detailed simulations using ANSYS and Garfield++. Within the scope of this study, a large impact of space-charges on the ion backflow has been observed. Systematic measurements over a wide range of charge densities, for different field configurations and different drift gaps have been carried out.
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