  A fast, hydrodynamic numerical model has been developed on the COMSOL
Multi-physics platform to simulate the evolution and dynamics of charged
particles in gaseous ionization detectors based on the Gaseous Electron
Multipliers (GEM). Effects of using two-dimensional (2D), 2D axisymmetric and
three-dimensional(3D) models of the detectors have been analyzed to choose the
optimum configuration. The chosen model has been used to follow the entire
operating regime of single, double and triple GEM detectors, including
avalanche and streamer mode operations. The accumulation of space charge, its
contribution towards the distortion of the applied electric field and
production of streamers have been investigated in fair detail using the
optimized model.
