Study of GEM structures for a TPC readout

Measurements and numerical simulations on the charge transfer in Gas Electron Multipliers (GEM) are presented and their implications for the usage of a GEM in Time Projection Chambers are discussed. A small test chamber has been constructed and operated with up to three GEM foils. The charge transfer parameters are derived from the electrical currents monitored during the irradiation with an 55Fe source. The measurements are performed in magnetic fields up to 5 T. They are compared with numerical calculations using the programs MAXWELL and GARFIELD. The paper presents current results and gives a short outlook on future research.