Discharge studies with a double GEM setup

The interaction rate of lead-lead collisions at the LHC will increase to 50 kHz during run 3. Hence an upgrade of the readout chambers of the Time Projection Chamber (TPC) of ALICE is necessary. New readout chambers equipped with stacks of four Gas Electron Multipliers (GEMs) have been developed. These allow for a continuous readout and preserve the momentum and dE/dx resolution of the current TPC. However, the new chambers have a higher risk to be damaged during a discharge than the current wire chambers. It has been confirmed at beam tests that the high voltage (HV) settings of the GEM stacks have a sufficiently low discharge probability. In order to understand the discharge mechanism and to further minimize the discharge probability, studies with small detectors have been carried out and will be presented here. Our detector consists of two GEMs (10 x 10cm²), a drift cathode and a readout plane. Discharges are triggered intentionally by increasing the voltage of one of the GEMs and releasing alpha particles in the gas (Ar-CO₂ 90-10). The potentials on the GEM sides and the signal at the readout plane are recorded to study the influence of different parameters, such as different resistors in the HV supply lines for the GEMs, on the discharge behavior. Results on secondary discharge probabilities are also shown.