Performance tests of GEM chambers for the CMS detector at CERN

Gas electron multiplier detectors (GEMs) belong to the category of Micro-Pattern Gas Detectors. They are part of the foreseen upgrade of the CMS muon system targeting the high-luminosity phase of the LHC. In the forward region, large-area GEM chambers will be installed in order to be able to handle high particle rates and increase the geometrical acceptance for physics searches. A special challenge concerning the chamber production is the necessary large (square meter) size to be fabricated with high uniformity and efficiency. Before integration into CMS, the performance parameters of the GEM chambers have to be verified by undergoing several tests. One of the sites performing these tests is located at Aachen. This talk gives an introduction of the design of GEM detectors and discusses the status and results of the performance tests.