Optimization of the HV scheme for GEM-based detectors

Gas Electron Multiplier (GEM) - based detectors are widely used in many experiments (COMPASS, LHCb, TOTEM) and future upgrades (ALICE, CMS, sPHENIX). Electrical discharges that may occur during operation of those detectors are possibly harmful to hardware and electronics and can damage it permanently in form of increased leakage currents or electric short circuits that render the detector effectively blind. Initial discharges, caused by high charge densities obtained in a single GEM hole may trigger a secondary discharge between two GEMs in a stack or between the last GEM and the readout anode. The latter is especially dangerous - as the front-end electronics can be severely affected by high energy released in a discharge event. The behaviour of the electric field in the gap between GEM foils or a GEM foil and the readout anode after an initial spark cannot explain the appearance of the secondary discharges which nature is still not fully understood. However the thorough optimization of the HV scheme, in terms of its RC characteristics, allows to minimize the propagation probability. A set of recommendations has been compiled which will be employed in the optimization procedure for the HV scheme of the upcoming ALICE TPC Upgrade. This research was supported by BMBF, HGF and the DFG cluster of excellence Origin and Structure of the Universe.