Measurements of ion mobility and GEM discharge studies for the upgrade of the ALICE time projection chamber

ALICE is one of the four experiments at the Large Hadron Collider (LHC). The quark-gluon plasma, which is predominantly produced in lead-lead collisions at LHC, is of particular interest for ALICE. After the long shut-down 2 (2019-2021) the LHC will provide lead-lead collisions at an increased interaction rate of 50 kHz. In order to examine every event at this interaction rate the ALICE Time Projection Chamber (TPC) needs to be upgraded. The TPC’s ReadOut Chambers (ROCs) are currently multi-wire proportional chambers. To prevent space charge build-up of slow ions, drifting from the ROCs into the TPC, a gating grid is used. The corresponding closure time imposes a dead time on the TPC read out, which prohibits data taking at a readout rate higher than 3 kHz. New ROCs have therefore been designed, relying on stacks of Gas Electron Multiplier (GEM) foils for the gas amplification, allowing for continuous readout. With the new ROCs, a certain fraction of ions will be drifting at all time into the TPC. Knowing the exact ion mobility in the counting gas is thus required in order to determine which amount of ion-back drift is tolerable. In this work we study the ion mobility in Ne-CO2-N2 (90-10-5), which is the gas mixture for the upgraded TPC, as well in several other argon- and neon-based mixtures. During stability studies for the new ROCs the phenomenon of secondary discharges has been observed. We thus also study discharges in GEM stacks and provide a detailed investigation of secondary discharges.