Ion backflow and energy resolution in quadruple GEM stack for the ALICE TPC upgrade

After the upgrade of the LHC at CERN the interaction rate of Pb-Pb collisions in RUN3 will be about 50 kHz. Therefore the Time Projection Chamber (TPC) of the ALICE experiment will be upgraded with quadruple stacks of Gas Electron Multipliers (GEMs), to provide operation in continuous mode. A test detector with a quadruple GEM stack has been set up in Frankfurt to investigate the behaviour of GEMs with emphasis on ion backflow (IBF) and energy resolution by varying GEM voltages, transfer fields between GEMs and the pitch between the GEM holes. Furthermore the electron transport properties of GEM foils were studied for different GEM hole pitches.