Ion backflow studies for the ALICE TPC upgrade with GEMs

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